Structural Safety in Tall Buildings

IStructE Safety in tall buildings and other buildings with large occupancy

July 2002

Safety in
tall buildings

and other buildings with large occupancy

Prepared by an international working group convened by

The Institution of Structural Engineers

IStructE Safety in tall buildings and other buildings with large occupancy

Published by The Institution of Structural Engineers
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The Institution of Structural Engineers and those who served on the Working Group which produced
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IStructE Safety in tall buildings and other buildings with large occupancy

Constitution of Working Group

John M Roberts, FREng BEng(Hons) PhD CEng FIStructE FICE, Babtie Group, Chairman
John Ahern, AADIP RIBA, J Ahern Associates, representing The Royal Institution of British Architects
Stuart J Alexander, MA CEng FIStructE FICE MIMgt, WSP Group
W Gene Corley, PhD CEng FIStructE, CTL Group
Keith J Eaton, BSc(Eng) PhD CEng FIStructE MIM, The Institution of Structural Engineers
Paul FEverall, MA(Cantab) CEng MICE Hon RICS HonFIBC Hon FB Eng, Department for

Transport, Local Government and the Regions, Observer

Max Fordham, OBE FREng MA FCIBSE FConsE Hon FRIBA, Max Fordham LLP, representing The

Chartered Institution of Building Services Engineers

Craig Gibbons, BEng(Hons) PhD CEng MICE MHKIE, Arup, Hong Kong
Peter R Head, OBE FREng CEng FIStructE FICE, FaberMaunsell
Martin Kealy, BSc(Hons) CEng FIFireE MSFPE MCIBSE, WSP Fire, representing The Institution of

Fire Engineers

Gordon G T Masterton, BSc BA MSc DIC CEng FIStructE FICE FIES MCIWEM, Babtie Group,

representing The Institution of Civil Engineers

David McCullogh, FRICS PPIBC, Hartlepool Building Control, representing The Royal Institution of

Chartered Surveyors

John B Menzies, FREng BSc(Eng) PhD CEng FIStructE Dip CU, Standing Committee on Structural

Safety

David B Moore, BTech PhD CEng MIStructE, BRE Ltd
H K Ng, MSc CEng FIStructE, J M K Consulting Engineers Hong Kong
Alan Parnell, FRIBA PPIFireE, Fire Check Consultants, representing The Royal Institution of British

Architects

Ysrael A Seinuk, PE CEng FACI FICE FASCE, Cantor Seinuk Group Inc
Faith H Wainwright, BA CEng FIStructE, Arup, London

Corresponding members
David I Blockley, FREng BEng PhD DSc CEng FIStructE FICE, University of Bristol
Thomas J P Byrne, BSc MSc, WSP Group
Charles Clifton, BE(Hons) ME(Civil) FIPENZ FNZSEE, HERA
Ian P Firth, BSc MSc DIC CEng FIStructE FICE, Flint & Neill Partnership
Graeme Forrest-Brown, MSc CEng MICE, Maunsell Structural Consultants Ltd
Norman Glover, BSc CE FIStructE, Aegis Institute, Architectural Institute ASCE
Mick G Green, BE CEng MIStructE MICE, Buro Happold
John A Hill, FREng BSc CEng FIStructE FICE, Doran Consulting
Alan P Jeary, PhD DSc CEng FIStructE MIEE FCIOB, University of Western Sydney
Bob A McKittrick, BSc CEng FIStructE FICE, Scott Wilson
Brian S Neale, AGCT CEng FIStructE MICE FIDE, Health and Safety Executive
Graham Owens, MSc PhD CEng FIStructE MICE MWeldI, Steel Construction Institute
Roger J Plank, BSc(Eng) PhD CEng MIStructE MICE, University of Sheffield
Ian Thirwall, CEng FIStructE MICE, Cameron Taylor Bedford
Martin J Wyatt, BSc MSc PhD CEng MIStructE MBCS, BRE Ltd

Secretary
Susan M Doran, BSc(Eng) AKC PhD CEng MICE ACIS, The Institution of Structural Engineers

IStructE Safety in tall buildings and other buildings with large occupancy

Contents

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Definitions for the purposes of this Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

2 Objectives of the Working Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

3 The World Trade Center towers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

4 The collapses of the World Trade Center towers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

5 Safety issues raised by the collapses of the World Trade Center towers . . . . . . . . . . . . . . . .18

5.1 Major safety issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
5.2 Vulnerability to progressive collapse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
5.3 Passive and active fire protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

5.3.1 Passive fire protection, including compartmentation . . . . . . . . . . . . . . . . . . . . . . . . . . .20
5.3.2 Active fire protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

5.4 Escape, its management and the emergency services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

5.4.1 Escape routes and emergency services access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
5.4.2 Management of escape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
5.4.3 Interaction with emergency services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

5.5 Other issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

5.5.1 Security and safety of cladding, including glazing . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
5.5.2 Security and safety of building services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
5.5.3 Security against unauthorised entry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
5.5.4 Implementation of design and construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

6 The new situation post 11 September 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

7 Initial recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
7.2 Vulnerability to progressive collapse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
7.3 Passive and active fire protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

7.3.1 Passive fire protection, including compartmentation . . . . . . . . . . . . . . . . . . . . . . . . . . .32
7.3.2 Active fire protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

7.4 Escape, its management and the emergency services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

7.4.1 Escape routes and emergency services access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
7.4.2 Management of escape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
7.4.3 Interaction with emergency services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

7.5 Other issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

7.5.1 Security and safety of cladding, including glazing . . . . . . . . . . . . . . . . . . . . . . . . . . . .36
7.5.2 Security and safety of building services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36
7.5.3 Security against unauthorised entry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
7.5.4 Implementation of design and construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38

8 Development and research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40

9 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42

10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43

IStructE Safety in tall buildings and other buildings with large occupancy

Appendix A: Recent extreme event damage to tall/large buildings . . . . . . . . . . . . . . . . . . . . . .44

Appendix B: Regulations and Codes of Practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

Appendix C: Use of risk management processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

IStructE Safety in tall buildings and other buildings with large occupancy

Foreword

The reality of threats to the safety of tall and large buildings was starkly demonstrated by the
unprecedented events at the World Trade Center in New York on 11 September 2001. Had these events
not occurred, the World Trade Center would no doubt have continued to give many years of excellent
service. The buildings were not unsafe by any criterion hitherto regarded as being credible in peacetime.

This Report examines what can be learned from the extreme events of 11 September 2001 for the future
design of new buildings and the appraisal of existing ones. The purpose is to assist owners and operators
of tall/large buildings and their professional advisers to play their part in reacting to the new threats to
the safety of building occupants. The Report presents therefore initial recommendations by the Working
Group on ‘Safety in Tall Buildings’ following review of damage by extreme events to tall/large buildings
at the World Trade Center and elsewhere world wide.

The Working Group has concentrated initially on gaining an overview of the safety issues arising from
the events of 11 September 2001. The aim has been to point to directions for improving future provisions
for occupant safety in tall/large buildings. The resulting initial recommendations are in no way a panacea
for dealing with threats to the building infrastructure. Rather they indicate possibilities that require
consideration and study.

There are many ways to inflict heavy blows of death and destruction in cities. For society as a whole,
the most effective measures that can be taken following the events of 11 September 2001 are those
related to improving security in cities (especially around high ‘profile’ tall/large buildings, landmarks
and infrastructure), preventing terrorists from gaining control of means to make attacks, and the deeper
resolution of conflicts that breed resentment and create the environment in which terrorism flourishes.

The solutions to reducing the probability of a recurrence of extreme events, such as occurred on
11 September 2001, do not lie within the gift of building owners and construction professionals.
This Report, nevertheless, seeks to contribute to public safety by providing recommendations to
assist building owners and their professional advisers to provide buildings and infrastructure better
able to sustain any future malicious attacks with a reduced risk of loss of life. Much further work
and international collaboration amongst construction professionals and others is needed to assist
building owners and their professional advisers to optimise occupant safety in extreme events.

I would like to thank members of the Working Group and others, around the world, who have
collaborated and contributed generously to the preparation of this Report. I would also particularly like
to thank John Menzies for preparing drafts of the report for the Working Group.

John Roberts
Chairman
July 2002

IStructE Safety in tall buildings and other buildings with large occupancy

Definitions for the purposes of this Report

Tall/large building

A building of many storeys or of large size that may be occupied by significant numbers of people.

Hazard

Anything that has potential to cause loss or damage (harm).

Hazard (or emergency) scenario

The total circumstances within or around a tall/large building arising due to an event that may place
occupant health and safety in jeopardy.

Risk

The combination of the likelihood of occurrence of a particular hazard and the consequences thereof.

Incident

An abnormal event within or outside a tall/large building that requires investigation by the building
management and may give rise to an emergency.

Extreme event

A man-made or naturally-occurring abnormal event that may cause a major emergency in a tall/large
building.

Emergency

An incident outside or within a tall/large building that requires investigation or action by emergency
services.

Major emergency

An emergency caused by an extreme event outside or within a tall/large building that may place the
safety of all occupants in jeopardy either by causing loss of stability of the whole building or by the
environment in part or the whole of the building becoming harmful to health and safety due to fire gases
or contaminants in the air, water or food supply.

Multi-occupancy

The occupancy of a tall/large building by more than one organisation.

Robustness

The ability of an engineered structure or system that enables it to survive a potentially damaging incident
or extreme event without disproportionate loss of function.

Redundant structure

A structure that possesses more load paths than required for equilibrium.

Fire compartment

A part of a building, comprising one or more rooms, spaces or storeys, constructed to prevent the spread
of fire to or from another part of the same building.

Ductility

The ability of a structural material or element to deform without fracturing.

IStructE Safety in tall buildings and other buildings with large occupancy

Summary

Following the extreme events at the World Trade Center in New York on 11 September 2001, the
Institution of Structural Engineers convened a Working Group on ‘Safety in Tall Buildings’, with the
support of fellow professional bodies, industry and the United Kingdom government, to review and
report on the safety issues. The objective was to provide guidance and advice on the implications that
follow the structural collapses and loss of life at the World Trade Center.

At the outset it was decided the Working Group would not undertake any independent investigation of
the extreme events on 11 September 2001. Rather it would consider all relevant available information,
in particular the papers submitted to the Group by its members and others and the large number of other
papers recently published elsewhere. The scope included buildings of large occupancy generally, since
it was anticipated that the guidance produced would also be relevant to them.

The Group considered not only the collapses and damage to buildings at the World Trade Center, but
also recent collapses and damage to other tall/large buildings due to extreme events in other parts of the
world.

Review of available information on the collapse of the World Trade Center (WTC) towers identified
several major safety questions:

•

What can be done to reduce the vulnerability of a tall/large building to collapsing
progressively and totally?

•

Should provisions for the protection of occupants and the building itself in the event of fire
be set at a higher standard?

•

Could escape routes and evacuation of building occupants and the linkage with the
emergency services be better provided and managed to help save lives?

Consideration of these questions focussed attention on key safety issues related to vulnerability to
progressive collapse, to passive and active fire protection, and to escape, its management and the
emergency services. Other safety issues, i.e. security and safety of cladding, security and safety of
building services, security against unauthorised entry, and implementation of design and construction,
were also found to be relevant. The key issues as a whole are multi-disciplinary and strongly interrelated.

There was recognition that extreme man-made events that may cause a major emergency in a tall/large
building can take many different forms. Their nature and scale cannot be predicted precisely. There was
consensus that loss of life and damage caused can be limited in many extreme events by the use of
broadly-based strategies involving design, construction and management of the building.

Vulnerability to progressive collapse

•

The redundancy of the structure and available alternative load paths.

•

The strength, ductility and hence the energy absorption capacity of the structure (i.e. structural
elements and particularly the connections between them).

•

The retention of structural integrity in fire.

Key safety issues

IStructE Safety in tall buildings and other buildings with large occupancy

Passive and active fire protection

•

The real performance of buildings in fire compared to data from standard fire tests on
components.

•

The robustness of passive fire protection not only in extreme events but also over time in
service.

•

The effectiveness of compartments to prevent spread of fire and smoke.

•

The survivability and functionality of active fire protection systems in extreme events.

•

The desirability of a building being able to survive a full burn-out of its contents without
collapse.

Escape, its management and the emergency services

•

The physical robustness, size and safety of escape routes and the diversity of vertical escape
options.

•

The use of occupant access/egress lifts and emergency services’ lifts for evacuation.

•

Timely access for effective fire fighting and rescue, and provision of protected water mains.

•

Provision for simultaneous evacuation in addition to phased evacuation.

•

Management/emergency response plans for the evacuation of occupants depending on the
nature and severity of the extreme event.

•

Provision and use of communications and information systems during emergencies.

•

Training of building management, emergency services and occupants in emergency
management and response.

•

Procedures for gathering relevant information when an extreme event occurs and for
communication between building management, emergency services and occupants.

Other safety issues

Security and safety of cladding, including glazing

•

Propensity to cause injury in the event of explosion, impact or fire outside or within the
building.

Security and safety of building services

•

Design of services systems for robustness, redundancy, and with isolation provisions.

•

Protection and sealing of systems.

•

Security against unauthorised access to building services equipment, plant and control rooms.

Security against unauthorised entry

•

Prevention of approach and entry with malicious intent.

•

Management and emergency services plans for response to potential extreme event scenarios.

Implementation of design and construction

•

Assurance of adequacy, including durability, of safety-critical elements.

•

Quality of components and workmanship in life-safety installations.

IStructE Safety in tall buildings and other buildings with large occupancy

The Working Group concluded that the events of 11 September 2001 have created a new situation in
which a reappraisal of provisions for safety is required.

Whilst the events were unprecedented in scale, they were not wholly unique. An aircraft impact into a
tall building had occurred previously, although it was not a deliberate act. Indeed, some seven months
after the World Trade Center events, an aircraft struck the 32 storey Pirelli building in Milan. For the
future, it has to be assumed that there may be more severe and different extreme events in
tall/large buildings than have occurred to date. Limitation of damage for all eventualities to that
which is tolerable or practicable has to be the working aim. There is, however, no single or precise
answer to the safety issues of designing tall/large buildings and their operating and management systems
against the wide range of possible extreme man-made events that may occur.

Decisions need to be made by owners, operators, designers and building managers based on an
understanding of all the issues. There are strong interactions between the building structure, fire
protection, building services systems and the building management and emergency services. Overall
strategies involving the design of the building, its management and the relationships with emergency
services are required in order to maximise protection of building occupants for a wide range of possible
extreme events.

Identification of the key safety issues led the Working Group to develop initial recommendations for
consideration by owners, operators, designers, builders, and building and emergency services managers.
The recommendations necessarily at this early stage indicate only possible directions for actions relating
to the key safety issues. Provisions in these directions would help to improve the safety of occupants
when extreme events occur in a new or existing tall/large building. Decisions on their adoption and the
standards to use in any particular case would depend on wider considerations. For that purpose in-depth
technical and economic studies together with consideration of policies on safety of people in tall/large
buildings may be needed. The Report gives a preliminary list of needs for such work.

Vulnerability to progressive collapse

•

Raise the ‘trigger’ threshold, i.e. increase the capability of the structure to limit damage and to
bridge over damaged parts by provision of alternative load paths. For this purpose, use
structural elements with robust, ductile and energy absorbing properties and tie them together
with strong ductile connections, recognising the directions of potential extreme event forces.

•

Give specific consideration to elements that are fundamental to the survival of the structure.

Passive and active fire protection

•

Provide robust, resilient and durable passive fire protection.

•

Treat active fire protection, e.g. sprinklers, as an addition to, and not a substitute for, passive
fire protection, and do not consider it for extreme events.

•

Ensure compartments are gas tight and seals are sound on building completion by inspection,
testing and certification.

•

Provide protection to compartments and mitigate spread of smoke.

•

Design building to survive complete ‘burn out’ of contents.

•

Require independent approval, as a part of licensing and periodic third-party audit of life-safety
systems, of modifications to passive and active fire protection.

Recommendations for consideration

IStructE Safety in tall buildings and other buildings with large occupancy

Escape, its management and the emergency services

•

Provide protection to escape routes from ingress of smoke.

•

Protect vulnerable parts of building services systems and incorporate redundancy.

•

Provide separate stand-by power for vital building services and for lighting of escape routes.

•

Provide robust adequately-sized escape routes and diverse locations for them and provide
protection for final exit routes.

•

In addition to phased evacuation for emergencies, plan for timely simultaneous evacuation of
a large proportion of floors in major emergencies, including use of lifts as well as staircases.

•

Be prepared for extreme event emergencies through development and trial use of emergency
response strategies that guide decisions on evacuation, communication with occupants and the
emergency services.

•

As part of preparedness, make sure that: plans of the building are deposited in a remote
accessible location; engineering advice can be obtained quickly during an extreme event;
communication systems with floors, stairwells and lifts are in place and functioning; training
for the management team, emergency services and occupants is given; and evacuation
procedures are practised at regular intervals.

•

Require independent approval, as a part of licensing and periodic audit of life-safety systems,
of modifications to escape routes.

Other issues

Safety of cladding, including glazing

•

Use laminated and/or toughened glass with fixings designed to take account of potential
explosion loading/impact/fire.

Security and safety of building services

•

Use a broadly-based strategy involving design and building management to reduce the risks.

Security against unauthorised entry

•

Reduce the probability of occurrence of extreme man-made events with potential to cause
progressive collapse, where practicable. For this purpose, use incident prevention or limitation
measures, e.g. provide barriers to protect the base of the building from vehicle impact or
explosion, and provide security against unauthorised entry.

•

Use both design and management provisions to deter and protect against extreme man-made
events taking place in or near the building.

Inspection of design and construction

•

Reduce the risk of the building performance being compromised during the design and
construction processes by appropriate use of independent third-party inspection, testing and
certification of safety-related structure and systems.

IStructE Safety in tall buildings and other buildings with large occupancy

1 Introduction

1.1

The collapse of the World Trade Center (WTC) towers in New York on Tuesday
11 September 2001 resulted in a great loss of life. The WTC towers were designed and
built using good practice of the time, in the mid-1960s/early 1970s. They had performed
well for three decades and indeed stood for some time following the immediate damage
caused by the attack. They had also performed well in a major bomb attack (1993) and a
relatively large fire (1975). For construction professionals, e.g. engineers, architects and
construction and facilities managers, involved in the design, construction and
management of tall buildings and other buildings that house large numbers of people,
questions about the safety of such buildings and their occupants came to mind
immediately following the collapses. One overall question was at the forefront: Can
tall/large buildings be made and managed so that they will be more resistant to damage
by extreme events caused by malicious acts and so that occupants are better protected and
have more time and opportunity to escape?

1.2

The Institution of Structural Engineers therefore convened the Working Group on ‘Safety
in Tall Buildings’ with the support of fellow professional bodies, industry and the United
Kingdom government to review the issues and report. The Working Group was made up
of professional engineers and other professionals with wide and international experience
of safety issues in buildings.

1.3

The solutions to reducing the probability of a recurrence of extreme events such as
occurred on 11 September 2001 do not lie within the gift of building owners, operators
and construction professionals. However, this Report seeks to assist them to provide safer
tall/large buildings, both new and existing, affording better protection to people in
extreme events.

1.4

This Report has been prepared by the Working Group taking into account international
feedback and practices. Safety issues are outlined in Section 5 and initial recommen-
dations for new and existing tall/large buildings are included in Section 7. At this early
stage, the recommendations are for consideration recognising that in-depth studies,
development and research will be needed in many cases to determine application. Areas
for development and research are therefore also identified.

IStructE Safety in tall buildings and other buildings with large occupancy

2 Objectives of the Working Group

2.1

The Working Group, known as the ‘Working Group on Safety in Tall Buildings’, was set
up by the Institution of Structural Engineers in October 2001. The Working Group operated
in a collaborative way and included representatives from across the disciplines concerned
with design, construction and management of tall/large buildings. It had the backing of the
Construction Industry Council (CIC) and invited members included representatives of the
Institution of Civil Engineers (ICE), the Royal Institution of Chartered Surveyors (RICS),
the Chartered Institution of Building Services Engineers (CIBSE), the Royal Institution of
British Architects (RIBA) and the Institution of Fire Engineers (IFE). Members also
included professional engineers from New York and Hong Kong as well as from the United
Kingdom experienced in the design of tall/large buildings and in safety and risk
management. A number of other experts provided assistance to the Working Group by
correspondence. The UK Department of Transport, Local Government and the Regions
(DTLR) nominated an observer.

2.2

The objective of the Working Group was to provide guidance and advice on the
implications that follow the collapses and the subsequent loss of life at the World Trade
Center in New York on 11 September 2001.

2.3

At the outset it was decided the Working Group would not undertake any independent
investigation of what happened at the World Trade Center. It would, however, consider all
relevant available information, in particular the papers submitted to the Group by its
members and others and the large number of papers published elsewhere since 11
September 2001. The aim has been to develop thinking so that the Group could provide
guidance on safety issues in tall buildings. It was anticipated that the guidance produced
would also be relevant to other buildings and structures that may be occupied by large
numbers of people. The Group considered not only the collapses of the WTC towers but
also collapses and damage to other tall/large buildings nearby and to other tall/large
buildings in other parts of the world due to extreme events in recent years.

2.4

The activities of the Working Group have focussed primarily on the safety of people
(occupiers/users/workers) in and around tall/large buildings rather than the safety or
protection of the building itself.

2.5

The Working Group did not consider hazards that a tall/large building conceivably may
pose to other buildings and infrastructure nearby. In particular, foundation movement
disrupting nearby infrastructure and, in the extreme, progressive collapse of the building
causing casualties and damage to buildings nearby were not examined. The likelihood of
this latter hazard arising has generally been assumed to be negligible and thus acceptable
until the collapse of the WTC towers on 11 September 2001. Avoidance of these potential
hazards is not likely to be possible in the crowded centres of major cities. This Report
assumes that the construction of tall/large buildings in close proximity to other buildings
will continue to be permitted in cities.

IStructE Safety in tall buildings and other buildings with large occupancy

3 The World Trade Center towers

3.1

Construction of the two 110-storey WTC towers of the World Trade Center began in August
1966. They were officially opened in April 1973. Each tower was 411m high above ground
level, 63.5 x 63.5m. square on plan, with a central core, 24 x 42m on plan, containing lifts,
staircases and service shafts. Their design and construction are described elsewhere

(1, 2, 3)

, but

briefly were as follows.

3.2

The towers were examples of a form of building generally referred to as a tube-tower
structure. The whole building façade was used as a structural member, each face
comprising a frame made up of 59 box-section steel columns at 1.02m centres connected
together by deep spandrel beams. Shear connection between the two faces at each corner
of the building was provided so that the frames, together with the floors, formed a
torsionally-rigid framed tube fixed to the foundations. This framed tube was designed as a
simple free-standing cantilever structure to carry all lateral loads.

3.3

The core, consisting of 44 steel columns, was designed and detailed to carry vertical load
only. The floors spanned, without intermediate columns, from the facade to the core. The
floor system comprised 900mm-deep lightweight steel primary trusses at 2.04m centres,
braced by secondary trusses and spanning between the perimeter frames and the core. The
secondary trusses supported a profiled steel deck with a 100mm lightweight concrete slab
on top and connected compositely to the primary trusses. There were three independent
emergency fire exit staircases in the core of each building

(2)

. The staircases did not run in

continuous vertical shafts from the top to bottom of each building. Occupants using the
stairs had to transfer from one vertical shaft to another via a transfer corridor at several floor
levels as they descended. There were also 99 separate passenger lifts in the core of each
tower with several serving each floor in two groups operated on different power supplies.

3.4

Passive fire protection was provided to the external box columns by spray-applied mineral
fibre of varying thickness, faced with aluminium pre-formed sheet externally. The
undersides of the floor systems were not protected by a fire-rated suspended ceiling but the
steel trusses were protected by a spray-applied mineral fibre. It has been reported that in the
north tower (WTC1), the fire protection to the trusses in the vicinity of the aircraft impact
had been upgraded

(2)

. A series of structural improvements had also been made in this tower

that may have helped it to remain standing longer after the aircraft impact.
Compartmentation was notionally horizontal by the floor construction, the floor slabs being
cast flush against the spandrel beams. At stair and lift shafts, separation was provided by
walls constructed of metal studs with two layers of gypsum board on the exterior and one
layer on the interior. Vertical separating walls varied, some spanning from slab to slab and
others extending only up to the suspended ceiling. The effectiveness of the compartmen-
tation is likely to have been progressively reduced over the years by the installation of IT
and communications systems. No pressurisation or other smoke control system was used
for the stairways, lift shafts or lift lobbies.

3.5

Active fire protection in the form of sprinklers had been retrofitted in the towers subsequent
to their construction. Standpipes supplying water for hose lines were located in each of
three stair shafts.

3.6

Overall the WTC towers were light open structures, engineered very efficiently to meet
design serviceability and ultimate limit state conditions for normal dead, imposed and wind
loads. They were also designed to withstand as a whole the forces caused by the horizontal
impact of a large commercial aircraft of the time, a Boeing 707. The overturning effect of
this postulated event would not be particularly severe, being of the same order as the wind
load effect.

IStructE Safety in tall buildings and other buildings with large occupancy

4 The collapses of the World Trade Center towers

4.1

The WTC towers collapsed following, in each case, deliberate impact by a Boeing 767
aircraft. Information on the attack and subsequent events leading to the collapse of the
towers is given in detail elsewhere

(2)

. The events are summarised briefly below.

4.2

Each tower remained standing immediately after it was hit. Although the structure was
weakened by the impact, the immediate damage to it, it may be said, was not disproportionate
in the circumstances. There was however a substantial amount of local damage to the
structures and to the passive and active fire protection. On impact, the aviation fuel from the
aircraft caught fire and an immediate conflagration of fuel, aircraft and building contents
developed in the vicinity. Gas temperatures as high as 900–1100oC locally in some areas and
400–800oC in others have been estimated

(2)

. After about 1 hour and 43 minutes in the case of

the north tower (WTC1) and about 56 minutes in the case of the south tower (WTC2), the
heat from the widespread fires had penetrated the remaining structure. The increase in
temperature of the structure weakened it further in the vicinity of the crash location. As a
result, it was unable to continue to support the section of the building above the crash site.
This structure then failed allowing the building above to fall under gravity onto the section
of building below. The descending section of building gained momentum as potential energy
was released and converted to kinetic energy. A progressive collapse of the whole of each
building followed, the increasing kinetic energy being sufficient to cause catastrophic
damage to propagate downwards through the essentially undamaged lower storeys.

4.3

The aircraft impacted on the WTC1 tower almost centrally on the north face and the vertical
axis of the building between the 94th and 98th floor. It caused substantial damage to the
north face. For the WTC2 tower, the aircraft impacted on the south face between the 78th
and 84th floor to one side of the central axis of the building. In this case substantial damage
was apparent to the south face in the zone of impact. The aircraft impact nearer to one
corner of the WTC2 tower appeared eventually to result in the upper section of that building
tilting over to some extent from the vertical as it collapsed.

4.4

Prior to the collapses, several fire compartments of the buildings in the locality of the
impacts had probably been breached. In addition, the lightweight fire protection to the
nearby steel external columns, core columns and floor trusses was friable and would not
have withstood the impact and subsequent fires in the breached compartments sufficiently
to prevent the affected steelwork from heating up to temperatures at which load-bearing
capacity was severely reduced. The column failures initiating the progressive collapses may
have been somewhat different in the two cases because of differences in the impacts and
fire damage. However, the cause, in generic terms, and the end result was the same. Both
buildings suffered complete, catastrophic progressive collapse.

4.5

After the aircraft impacts, emergency services despatched to the towers concentrated on
evacuating and rescuing people. Instructions to occupants of the towers appear to have
differed depending on location. In some cases people were advised to leave the building, in
others to remain. The reports of witnesses indicate that there was no expectation that the
towers might collapse. As the gravity of the situation became more apparent, more people
tried to leave the towers. Most of those below the impact-damaged floors managed to
escape. For all but a very few people at or above the point of impact, escape was impossible
because the stairs were impassable and the lifts unusable. The other effort of the emergency
services was to fight the fires, but with lifts no longer working, access to the fire locations
required an arduous climb. The effort was to no avail. In hindsight, it can be concluded that
the circumstances made it impossible to put out the fires before the towers collapsed. The
task was impossible not only because of the difficulty of access, but also because of the
formidable obstacles to providing sufficient water at the fire location. In addition,
destruction of the fire protection had greatly increased the vulnerability of the structure to
the fire. Sadly many fire fighters were in the towers and also perished when they collapsed.
There were more than 3000 fatalities amongst the occupants of the buildings and the
aircraft, fire fighters, police and other emergency services personnel.

IStructE Safety in tall buildings and other buildings with large occupancy

4.6

There was also substantial damage to buildings adjacent to the WTC towers

(2)

. Seven World

Trade Center (WTC7) caught fire and subsequently collapsed, see Appendix A. The
Marriott Hotel (WTC3), a building with 22 storeys above grade and 6 storeys below,
collapsed under the impact of falling debris from both WTC tower collapses but it did not
collapse progressively. In total, partial or complete collapse of 10 major buildings occurred
and more than 50 buildings were damaged. Some issues raised by the individual
performance of these buildings are incorporated into the discussion in this Report.

IStructE Safety in tall buildings and other buildings with large occupancy

5 Safety issues raised by the collapses of the World Trade Center towers

5.1 Major safety issues

5.1.1 Tall/large buildings should provide a safe environment for people within them and in their

vicinity, and they should enable people to escape to safety as far as is practicable following an
extreme man-made or naturally-occurring event. The provisions made have been tested in recent
years not only at the World Trade Center but also elsewhere in the world, see Appendix A.

5.1.2 The aim of consideration of an extreme event in the design of a tall/large building is to

accept that some damage to the building is likely to be inevitable and to design so that the
damage is localised and still allows occupants the best chance of escape. Prevention of
extreme man-made events through national and international security is a priority.
However, for the future, it has to be assumed that there may be more severe and
different extreme events in tall/large buildings than have occurred to date. Limitation
of damage for all eventualities to that which is tolerable or practicable has to be the
working aim.

5.1.3 In this context, several key questions arise from the experience of the WTC tower

collapses:

•

What can be done to reduce the vulnerability of a tall/large building to collapsing
progressively and totally?

•

Should provisions for the protection of occupants and the building itself in the
event of fire be set at a higher standard?

•

Could escape routes and evacuation of building occupants and the linkage with
the emergency services be better provided and managed to help save lives?

5.1.4 The above questions and others lead to the need to review how designers and

owners/operators determine an appropriate level of protection against extreme events
which have remote probabilities of occurrence but which potentially have very severe
consequences. The questions focus attention on safety issues that are multi-disciplinary and
strongly interrelated. They are discussed below.

5.2 Vulnerability to progressive collapse

5.2.1 The concept of disproportionate damage relates to the cause of the damage. It is

generally expected and required that ‘small’ events that may damage man-made
artefacts and organisations should only cause relatively ‘small’ damage. Similarly
‘large’ events (in comparison to the artefact’s size and purpose or to the organisation)
causing ‘large’ damage may be accepted, particularly if the event is rare or totally
unforeseen. There is an expectation that damage will be resisted to a practical extent in
the operating environment. This expectation applies to all artefacts, including tall/large
buildings. In the case of the damage on 11 September 2001 to the WTC towers, the
initial impact damage caused by the aircraft was not disproportionate in the
circumstances. The subsequent situation where many fatalities resulted from inability of
occupants to escape from locations above the points of impact and from the eventual
fire-induced progressive collapses may be less acceptable if it could be prevented by
practicable means. The challenge now is to determine if and how such situations can be
avoided in the future.

5.2.2 Progressive collapse is a term well understood by structural engineers to refer to a

spreading of collapse through a considerable part or the whole of a structure following local
damage to a relatively small structural part. The event causing the initial local damage does
not generally provide the energy required to cause collapse to propagate progressively. In
most cases of progressive collapse in building and civil engineering structures, the energy
is derived from potential energy released as parts of the structure fall under gravity.
Depending on the form of the structure, progressive collapse may progress vertically or
horizontally. For tall buildings, vertical progression is usually the main concern

(4)

IStructE Safety in tall buildings and other buildings with large occupancy

5.2.3 The essential features of the progressive collapse phenomenon in buildings are therefore

that it requires a local damage event to ‘trigger’ it and, for propagation, it requires growth
of kinetic energy (usually derived from release of potential energy) to exceed the energy
needed to collapse the structure beyond the ‘trigger point’. The size of trigger event needed
and the vulnerability to collapse propagation depends mainly on the redundancy of the
structure, and the strength, ductility and hence the energy-absorption capacity of the
structure, that is of the vertical and horizontal structural elements and, most importantly, the
connections between them.

5.2.4 In general terms, the more redundant the structure (i.e. the more alternative load paths) and

the stronger and more ductile the elements and connections, the bigger the ‘trigger’ needed
to initiate progressive collapse and the less will be the vulnerability to collapse propagation.
Reduction of vulnerability requires the structure to be made more robust so that the
threshold for initiating progressive collapse is raised. As a result the probability of it
occurring due to an extreme event can be made smaller. There is a need to be aware that
some potential trigger events, such as explosions, may load the structure, e.g. floors, in
opposite directions to the forces due to normal loads.

5.2.5 Structures with high vulnerability to progressive collapse are those where release of

potential energy occurs when the initial local damage (the ‘primary’ damage) is a relatively
minor event in the vicinity, such as a local accident or weakening of a critical structural
element. In contrast to cases where collapse is driven by external energy (e.g. explosion),
gravity-driven progressive collapse, as occurred at the WTC towers, results in damage that
is usually perceived as being disproportionate to the original event. Structures that are
highly resistant to progressive collapse are generally termed ‘robust’ structures and are
those where a more severe accident or extensive weakening is needed in order to make
collapse progressive. In recent times there have been several incidents of partial or
complete progressive building collapse, the best known being the partial collapse of Ronan
Point in London in 1968. Large losses of life resulted in some cases, see Appendix A.

5.2.6 For each of the WTC towers, the damage to the vertical load-carrying columns and floors

from the aircraft impacts was followed by further weakening caused by the ensuing fires.
The total weakening was then sufficient to allow the potential energy of the part of the
building above to be released and converted to kinetic energy as that part fell under gravity.
This kinetic energy was sufficient to commence destruction of the building floors below the

IStructE Safety in tall buildings and other buildings with large occupancy

fire locations. These floors then began to add to the kinetic energy of the falling
construction, adding more ‘fuel’ to the destruction of the building and bringing the rate of
collapse similar to that of free fall.

5.3 Passive and active fire protection

5.3.1 Passive fire protection, including compartmentation

5.3.1.1

The purpose of passive fire protection of structural elements in buildings is to prevent
or delay temperature rise in the elements so that they are not weakened to the extent that
they can no longer fulfil their load-carrying role before people have left the building and
surrounding areas and, if possible, the fire is brought under control or burns out. For
steel-framed buildings, the fire protection of columns is particularly critical. Protection
is by means of a non-combustible material encasing and in contact with the element
and/or of an insulating casing that prevents fire gases reaching the element directly. For
this purpose the protection needs to have strength and stability in fire conditions as well
as heat insulating properties and a surface finish with low spread of flame properties.

5.3.1.2

Passive fire protection is usually given a time rating over which it will remain effective,
based on standard laboratory tests, i.e. the BS 476 test in the United Kingdom and the
ASTM E119 test in the United States

(5)

. Laboratory furnaces are relatively small and

therefore cannot mimic the real behaviour of a structure in fire, only the performance of
an individual element. The time rating does not bear any relation to the time the building
will survive in a ‘real’ fire. The standard laboratory tests are comparative and not
intended to be predictive of behaviour in fire in a building.

5.3.1.3

The aircraft impacts and fires were very onerous challenges to the fire resistance
provisions in the WTC towers. Gas temperatures as high as 900–1100oC have been
estimated to have developed locally in the fires

(2)

. Much of the passive fire protection

would have been destroyed in the vicinity of the aircraft impacts by the impacts
themselves and the fire of aviation fuel, followed by ignition and burning of the
aircraft and building contents. Additionally, there is the possibility that the overall
integrity of the passive fire protection systems was weak prior to the damage on 11
September 2001. Passive fire protection should be the correct thickness, durable and
remain firmly in place during the life of the building. It should not flake or fall off. It
should also be resistant to removal by building movement and vibration and by ‘wear
and tear’ by occupants and building maintenance personnel.

5.3.1.4

Compartmentation is used as a passive protection in buildings to prevent (or at least
delay) the spread of fire and smoke from its initial location. Separate compartments may
be created from a floor, part of a floor, escape staircase or lift shaft so that people outside
the compartment on fire are safe until rescued, or have a safe escape route. Compartment
effectiveness may be reduced over time by poorly managed building operations, for
example installations of IT and communications systems. Inadequately supervised
cabling installations often leave holes where fire and smoke can pass through.

5.3.1.5

Generally passive fire protection and compartmentation have protected people to a
considerable degree from conventional fires in buildings, i.e. fires involving the burning
of the contents generally found in offices, residential buildings and the like. Flame
damage is usually concentrated close to the origin of the fire indicating the effectiveness
of compartmentation. However, compartmentation is often less effective at controlling
smoke spread. When extreme damage is inflicted, such as in the WTC towers, compart-

Key issues: Vulnerability to progressive collapse

•

The redundancy of the structure and available alternative load paths.

•

The strength, ductility and hence the energy absorption capacity of the structure (i.e. the
structural elements and particularly the connections between them).

•

The retention of structural integrity in fire.

IStructE Safety in tall buildings and other buildings with large occupancy

mentation measures will clearly have limited effectiveness in containing a major fire
emergency. Compartmentation and control of air has a wider importance arising from
possibilities of extreme man-made events that use air contamination as the instrument
of damage, see Section 5.5.2.

5.3.1.6

An option that could be considered is the design for survival of the structure until the
compartment contents have burned out completely. A fire engineering approach based
on a natural fire exposure would be needed if this design criterion were adopted. In
recent years two building fires in the United Kingdom, at Broadgate

(6)

and at Churchill

Plaza, Basingstoke, have provided an opportunity to observe how modern buildings
perform in fire. In both cases most of the combustible materials were involved in the
combustion process. Structural collapse did not occur. Similarly, large-scale
experimental fire tests on steel, timber and concrete buildings undertaken by BRE

(7)

Cardington and by BHP

(8)

in Australia led to complete ‘burn out’ of all the fire load. Such

practical evidence suggests that buildings, if suitably designed, may be able to survive
complete ‘burn out’ without collapse.

5.3.2 Active fire protection

5.3.2.1

The partial effectiveness of passive protection to keep occupants safe when fires occur
in buildings has led to the increasing use of active fire protection in addition to passive
protection. Active protection is usually provided in tall/large buildings by sprinklers that
operate automatically as a fire develops, dousing the fire with water. Their role is to
catch a fire when it is still small and put it out or inhibit its spread. For the WTC towers,
the active fire protection by sprinklers was effective in dousing small accidental fires
that occurred prior to 11 September 2001. On 11 September 2001, the sprinkler system
would have been overwhelmed by the fires, even supposing the sprinkler and water
supply systems were still operative. Active fire protection using sprinklers is not able to
stop fully developed fires and also is vulnerable separately to loss of water pressure due
to extreme event damage. Design for fire needs to consider the likelihood and
consequences of failure of the sprinkler system.

5.3.2.2

Fire fighting, of course, is an important means of active fire protection through the use
by occupants of equipment provided in the building to fight small fires and the use by
fire fighting services of their more powerful equipment. However, in tall/large
buildings, there are limits to the size of fire that a team of fire fighters can bring under
control. The limits can be severely reduced, as was seen at the WTC towers, by the

IStructE Safety in tall buildings and other buildings with large occupancy

impracticability of timely access to the fire when it is at height and lifts are inoperable.
Dedicated lift shafts for emergency services use can preserve access for fire fighting.
However, extreme event damage can lead to loss of fire fighting lifts thereby preventing
ready access by the emergency services to fire at height. Loss of water supply due to
extreme event damage can also render fire fighting impractical. The provision of robust
shafts and water supplies increases the possibility of effective fire fighting.

5.3.2.3

The heating, ventilation and air conditioning (HVAC) systems can play an important
role in preventing the spread of smoke in tall buildings. However, such capabilities are
not required in UK codes (except Section 20 buildings in inner London). Pressurisation
of staircases (or natural ventilation) is only intended to keep escape and fire access stairs
reasonably clear of smoke – it is not intended to keep floors clear. For tall buildings
compartmentation is intended to reduce smoke spread from floor to floor. The
effectiveness of compartmentation can be undermined by services penetrations, lift
shafts and poor construction. HVAC systems can be designed to depressurise the fire
floor and positively pressurise the adjacent floors, further reducing the spread of smoke
– this is commoly referred to as ‘sandwich pressurisation’ or ‘opposed airflow’. These
systems are commmonly used in the United States and design methods are given in
CIBSE Guide E

(9)

and NFPA 92A

(10)

5.4 Escape, its management and the emergency services

5.4.1 Escape routes and emergency services access

5.4.1.1

The possibility of escape by people from a building in an emergency depends on
whether safe evacuation routes are available. Questions arise on the size of escape
routes and safe areas, on the dependability of services to escape routes and protected
areas, in particular clean air supply and light, and on the extent to which escape routes
and protected areas can be made resistant to extreme event damage.

5.4.1.2

Protected areas can be within or outside of the building. It is necessary for escape routes
to protected areas and finally to places of safety to be adequate in size and negotiable
safely by occupants. Where practical, designated ‘safe refuge’ areas within the building
can be provided for some eventualities.

5.4.1.3

In some modern buildings, cores, escape routes and protected areas may be sealed and
pressurised by air conditioning equipment, e.g. Petronas Towers in Kuala Lumpur.
These systems need to be made robust so that they are unlikely to be made ineffective
by physical damage or contamination in extreme events that are likely to occur. Smoke
contamination of escape stairs has been reported in a number of major fire incidents in
the United Kingdom, United States and Canada, sometimes with fatal consequences.
Contamination may arise due to failure of containment barriers – doors being left open
or inadequate sealing – or from lack of ventilation, pressurisation or purging provision
for contaminated air.

5.4.1.4

In the United Kingdom, increased levels of protection for escape stairs with building
height are required by codes relating to fires. For buildings over 18m high or when

Key issues: Passive and active fire protection

•

The real performance of buildings in fire compared to data from standard fire tests on
components.

•

The robustness of passive fire protection not only in extreme events but also over time in
service.

•

The effectiveness of compartments to prevent spread of fire and smoke.

•

The survivability of effective active fire protection systems in extreme events.

•

The desirability of a building being able to survive a full burn-out of its contents without
collapse.

IStructE Safety in tall buildings and other buildings with large occupancy

phased evacuation is planned, a protected lobby to each stair or a smoke control system
is required. For buildings over 30m high, the building also requires protection by an
automatic sprinkler system, which makes serious smoke contamination of escape routes
less likely, at least for ‘conventional’ fires.

5.4.1.5

Tall/large buildings in the United Kingdom are generally designed with a limited phased
evacuation, e.g. evacuating the ‘fire floor’ and the floor above, as the main response to
an emergency. Post 11 September 2001, large numbers of building occupants are likely
to wish to evacuate over short periods in response to real or perceived emergencies.
Effective communications between building management and occupants is clearly an
important factor in maintaining the safety of occupants during emergencies.

5.4.1.6

Similarly, issues need to be examined relating to the use of lifts for evacuation (and for
access by emergency services’ personnel). In a major emergency a proportion of
occupants are likely to try to use the lifts to escape whatever warnings are given about
not doing so. Evacuation by the lifts used by occupants for normal access/egress may be
possible safely in some emergencies. Where there is a warning of an event, the use of
lifts for evacuation will minimise evacuation time. It may be possible also to use
dedicated fire fighting lifts for evacuation, perhaps of disabled occupants, in some
emergency circumstances. However their use could inhibit emergency services
operations. Evacuation by lift is an established strategy in a few special structures.

5.4.1.7

The use of lifts is more likely to be safe if the shaft is of robust construction and the lift
system and its power supply is robust and protected. An important factor in determining
whether use of a lift in an emergency will be safe is the state of knowledge at the time
of functionality of the shaft and the lift system. Monitoring of the power system and the
air condition is needed to enable the building management to decide whether there is
low(acceptable) risk to occupants, given the emergency in hand, in allowing use of
particular lifts for evacuation. However, a major risk to occupants is that they may be
overwhelmed by smoke as they wait for the lift. Consideration also needs to be given to
providing emergency ‘break-out’ arrangements so that passengers in a lift can be rapidly
rescued (or can rescue themselves) if the lift stops functioning during use.

5.4.1.8In some countries, including the United Kingdom, the provision of dedicated fire

fighting shafts is required. Such shafts include stairs, lift and lobby in a protected
enclosure. They can be an effective facility for enabling emergency services personnel
to reach the incident location quickly. A further advantage is that escape stairs are kept
free for use by occupants evacuating the building. Dedicated fire fighting shafts can also
provide protection for water mains and communications links.

5.4.1.9

Diversity in vertical escape options by lifts or stairs is likely to increase the chance of
successful evacuations. Options might be increased by placing stair entrances on
different sides of a central core. Options might also be increased by dispersing stairs and
lifts in separate shafts instead of placing them in a central core. The balance of
advantage and disadvantage for the safety of occupants is not clear. A central core is
usually large and can be built of robust construction to give good protection. Separate
cores would individually be smaller and placed nearer to the outside of the building
where they would tend to be more vulnerable to extreme events. Whatever the
arrangement, the stairways need to be independently robust so that damage affecting
one stair is less likely to affect nearby stairways.

5.4.2 Management of escape

5.4.2.1

Occupants of tall/large buildings have tended to feel secure in normal circumstances and
to be intent on pursuing their usual day-to-day activities. They have tended not to be
particularly receptive to fire drills and to be reluctant to evacuate. The logistics and
expense of completely evacuating a tall/large building and the hazards involved are
considerable even as a fire drill in non-emergency circumstances. For most
emergencies, simultaneous evacuation may be considered inappropriate.

5.4.2.2

The safety of occupants in major emergencies can usefully be distinguished from their

IStructE Safety in tall buildings and other buildings with large occupancy

safety in emergencies, i.e. what might be considered to be ‘conventional’ accidental events,
e.g. a local fire. In the latter case, there is generally no need for early complete evacuation
of the building, although phased evacuation of several floors may be needed. Normal
practice would involve a pre-alarm whereby only security personnel would be informed of
the early stages of an incident. An alarm or evacuation request would be relayed to
occupants by the building management after the incident has been investigated and only if
it is considered to be sufficiently serious to merit a partial or complete evacuation. This
approach reduces the risk of false alarm but inevitably results in delay in starting
evacuation. However, where a fire can be seen, heard or felt, evacuation is likely to begin
earlier. For all incidents, the building management (and at a later stage the emergency
services) need access to sufficient accurate information to formulate an appropriate
emergency response. Given the wide range of possible emergencies, reliable systems are
required to enable management to obtain the relevant information about the incident and
resulting conditions in the building so that an appropriate response can be determined.
Likewise, reliable systems are needed to enable implementation of the response.

5.4.2.3

The preliminary incident alarm to occupants may be followed by some form of partial
evacuation in which occupants are moved away from the affected area. The remainder
of the occupants are not disturbed or, perhaps more usually, are informed of the
situation, placed on standby, and requested to continue normal activities unless
otherwise instructed as the incident progresses.

5.4.2.4

Such procedures of phased evacuation may be considered adequate for most normal
hazard scenarios, particularly accidental fires associated with the specific occupancy.
Such fires usually have small beginnings and can be confined to an area or to one floor
of a building, for an extended period. The challenge for building management in such
situations is usually to ensure a timely and calm response by the occupants, with a rapid
and efficient evacuation of the affected area or floor.

5.4.2.5

The management of a major emergency in a tall/large building arising from an extreme
event can be crucial to the safe escape of the building occupants. The right decisions are not
easily determined, since any major emergency will be a unique event. Decisions have to be
made quickly bearing in mind the whole building and not just the location of the incident.

5.4.2.6

There have been a number of recent incidents in tall/large buildings of sufficient
magnitude to involve several floors at once, to threaten the whole building structure and
to alarm the building occupants as a whole, e.g. the aircraft strikes on the WTC towers,
and the bomb explosions at St Mary Axe, The Murrah building and the WTC1 tower, see
Appendix A. Dealing with such major emergencies requires integration of building design
and emergency management strategies. The incidents have highlighted inefficiencies and
difficulties of ensuring efficient, rapid and well-managed evacuation of tall/large
buildings. Current prescriptive design has been developed with fire emergencies primarily
in mind and emphasises the provision of horizontal and vertical means of escape.
However, reports of occupant behaviour during the recent incidents show that these
provisions are often inappropriately or inefficiently used. In some cases evacuation times
have been long because occupants have been slow to respond to requests to evacuate and
have then tended to crowd some routes whilst others are underused. In other cases,
occupants have all tried to leave at once, clogging escape routes designed even for
simultaneous evacuation, let alone those designed for phased evacuation. Efficient
evacuation depends upon the implementation of an effective emergency management
strategy, making the best use of warning systems, security staff and escape routes.

5.4.2.7

Where it is decided in a major emergency that the best strategy is to maintain occupants in
place with progressive phased evacuation of affected parts of the building, then particular
consideration needs to be given to the advice to occupants to remain or leave as required.

5.4.2.8

Where it is decided that complete evacuation, or evacuation of large numbers of people
from a number of floors simultaneously, is required, then escape routes must have sufficient
capacity and be a practical option for the majority of occupants. Not all occupants with the
normal range of physical capabilities are likely to be able to walk down 50–100 storeys of

IStructE Safety in tall buildings and other buildings with large occupancy

stairs. Situations where there is an acceptable risk to using lifts to evacuate disabled or large
numbers of able-bodied occupants need to be identified in escape strategies. The
communications systems provided for delivering advice to occupants are crucial.

5.4.2.9

Escape depends on the arrangements for managing the evacuation, including advice
to occupants on when and how to evacuate the building, the routes to take, and the
assistance by emergency services. Simultaneous complete evacuation of a building by
all occupants without use of lifts may not be a practical possibility in existing
tall/large buildings if this was not a design criterion. Provisions for phased evacuation
only, as at the WTC towers, are usually included in designs. However, the experience
of 11 September 2001 indicates that simultaneous evacuation of a substantial
proportion of all floors, is a key requirement in certain major emergencies and should
be planned for. Even where existing tall/large buildings have been designed only for
phased evacuation, there is need to plan for simultaneous evacuation.

5.4.2.10 Irrespective of whether evacuation is phased or simultaneous, there clearly needs to be

good communication with the building occupants on when to leave and the routes to
take. The experience at the WTC towers raises a range of issues about occupant
response and management of evacuation in tall/large buildings.

5.4.2.11 Consideration is needed concerning what information and requests/instructions should

be relayed to occupants for different emergency scenarios and also concerning how the
information is relayed. Several issues arise:

• Following the World Trade Center events, very large numbers of occupants may decide

to evacuate during the early stages of an incident in a tall/large building. Crowding of
escape routes may then occur, possibly preventing occupants from evacuating affected
floors. Depending upon the effectiveness of compartmentation and ventilation or
control of contaminated air, various parts of escape routes may become contaminated.
Occupants attempting to evacuate the building may then be in more danger than those
remaining in situ. On the other hand, if the emergency is serious, occupants delaying
evacuation may become trapped and compromise their own survival.

• The information and requests/instructions given to occupants are therefore crucial.

Occupants need to have a confident understanding of the situation and the evacuation
strategy being used. Information and requests/instructions should be based on accurate
knowledge by the building management of the conditions in all areas of the building
and of the likely changes in each area as the emergency develops. A position often
taken is that occupants should be given as little information as possible. More
considered opinion is that occupants should be kept fully informed. They will then be
in a position to make rational decisions on the action they should take. It may also be
argued that occupants have a right to expect to be kept fully informed. It is generally
agreed that people do not ‘panic’ when an emergency first comes to notice. They tend
to try to assess the situation and, as a result, may delay their escape. A common
procedure is to reassure occupants that they are not in danger but, following the
extreme events at the World Trade Center, occupants of tall/large buildings may not
accept such reassurance without tangible evidence.

• Good communication systems are needed to enable the effective management of

emergencies. The nature and type of communication and alarm systems have a major
influence on the response of occupants. Current guidance, e.g. the CIBSE Guide in the
United Kingdom

(9)

, assumes ‘conventional’ fire emergencies but can be used to model

the effects of extreme events.

5.4.2.12 Occupant familiarisation and ‘training’ for possible emergencies can assist them to

remain calm and respond optimally to standby or evacuation requests. Training of
occupants can greatly increase their ability to escape quickly and provide valuable
reassurance about how quickly they can get out.

5.4.3 Interaction with emergency services

5.4.3.1

The collapses of the WTC towers have raised new concerns relating to the operational

IStructE Safety in tall buildings and other buildings with large occupancy

response and safety of emergency services personnel (fire service, ambulance service,
police and those who are required to disable and remove dangerous devices and
substances) in major emergencies. As for building occupants, the steps taken by
emergency services should be based on an assessment of the nature and effects of the
emergency, including judgment of the likelihood that part or all of the building may
become structurally unsafe. For this purpose, informed assessment from the local
Building Control authority or structural engineers is essential. Its provision needs to be
included in the preparations for major emergencies. Informed assessment is also needed
to identify areas within the building that are already or may become dangerous or
harmful to the health and safety of occupants. Finally contingency plans are needed for
situations where a significant number of emergency services personnel are injured or
otherwise made unable to operate.

5.4.3.2

Preparations for major emergencies should automatically include plans for
attendance in the shortest possible time, for access to the building, and for the
location of emergency services vehicles. They will also include the setting up, in
association with the building management, of command and control procedures so
that the response can be managed efficiently. Up-to-date information on the building
and the incident should be to hand. Communication systems are very important in
achieving an effective response. Reported shortfalls in communications in some
tall/large building emergencies need to be avoided by initial planning, provision of
effective equipment and training.

5.4.3.3

Emergency services teams cannot be effective in all possible emergency situations. For
example, there are limits to what a team of fire fighters can do. For some major
emergencies, there are some circumstances where no benefit would be gained by
allowing emergency services personnel to enter the building, e.g. if dedicated fire
fighting lifts are inoperable. Such considerations need to be taken into account in
emergency response plans.

5.4.3.4

Optimum judgements on deployment of emergency services personnel are only likely to
be made if emergency services managers have relevant knowledge of their capabilities
and good information on the event. These managers need to be informed by the building
management about the building itself and about the nature of the event, its location and
scale, and the possible implications for the safety of the building and the health and
safety of its occupants. Such information is crucial to decisions on whether emergency
services should enter the building, the equipment they should carry, and their purpose.
These requirements need to be met through the development of emergency management
strategies and response plans for each tall/large building.

Key issues: Escape, its management and the emergency services

•

The physical robustness, size and safety of escape routes and diversity of vertical escape
options.

•

The use of occupant access/egress lifts and emergency services’ lifts for evacuation.

•

Timely access for effective fire fighting and rescue, and provision of protected water
mains.

•

Provision for simultaneous evacuation in addition to phased evacuation.

•

Management/emergency response plans for the evacuation of occupants depending on
the nature and severity of the extreme event.

•

Provision and use of communication and information systems during emergencies.

•

Training of building management, emergency services and occupants in emergency
management and response.

•

Procedures for gathering relevant information when an extreme event occurs and for
communication between building management, emergency services and occupants.

IStructE Safety in tall buildings and other buildings with large occupancy

5.5 Other issues

5.5.1 Security and safety of cladding, including glazing

5.5.1.1

Cladding, especially glazing, can become a hazard when certain extreme events occur
within or outside of a building, e.g. explosion or fire. For example, glass fell onto fire
fighters at the First Interstate Bank Building fire in Los Angeles from the 12th to 16th
floor levels and spread out up to 100 feet from the building, see Appendix A.

5.5.1.2

People in the vicinity at ground level near to a tall/large building can also be at risk from
falling glazing when an explosion occurs nearby. Another well-recognised hazard to people
in the vicinity of tall/large buildings is the detachment of cladding panels. The explosion
in the centre of Manchester in 1996, although it caused no fatalities, led to over 800 injuries
almost all of which were due to falling cladding debris, mostly glass, see Appendix A.

5.5.2 Security and safety of building services

5.5.2.1

Although not demonstrated in the attacks on the WTC towers, a wide range of extreme
events could place occupant health and safety in jeopardy by interfering with building
services (ventilation, air conditioning, water supply, heating/cooling and electricity
supplies, waste disposal and catering services). In some cases, but not all, similar
measures are needed to protect occupants to those measures that are used for protection
from the hazard of smoke.

5.5.2.2

Physical damage to services systems can cause loss of functionality or make them
unsafe. Contamination by chemical, radioactive or biological substances can have rapid
and widespread harmful effects on occupants. Such hazardous substances may be
delivered by airborne release outside the building at high or low level, or by airborne
release within the building. They may also be delivered by contamination of water
supplies, by spreading contamination around inside the building through people or
materials that are moved around or by contamination of food or catering equipment.

5.5.3 Security against unauthorised entry

5.5.3.1

Tall/large buildings usually have a high ‘profile’. They are likely to attract the curious
and those with malicious intent because they are landmarks, often occupied by high
profile organisations. They represent a concentration of commercial value. They contain
large numbers of people and are often multi-functional and have multi-occupancy.

5.5.3.2

Threats may arise from commonly-occurring criminal acts, or from malicious actions
that pose a widespread hazard to the building and its occupants as a whole. Conceivably
they may also arise from the use of sophisticated devices, based on widely available
electronic and information technology, placed in a building, possibly connected to the
buildings’ systems and activated when placed, or automatically, or remotely at a later
time. In addition, the latter type of threat may be through contamination using chemical,
radioactive or biological agents. Whilst the risks of commonly-occurring criminal acts
can be reduced through currently-available entrance design and security systems,
malicious acts causing widespread hazards require decisions based on a thorough
appraisal of the design and management of the building that establishes how
vulnerability can be reduced and controlled.

5.5.4 Implementation of design and construction

5.5.4.1

The safety of a tall /large building may be compromised by active and/or latent errors
by those involved in design, construction and management in use, including responses
to emergencies. The underdesign found after the construction of the Citicorp Center
provides a salutary lesson

(11)

. The human error subsequently found in the design of a tall

building in Rio de Janeiro after its progressive collapse in 1998 causing 8 fatalities
provides another example

(12)

. There have also been cases of defective construction or

maintenance leading to serious damage, e.g. façade failures

(13)

. Actions to minimise

these risks to tall/large buildings in the past have followed current practice for buildings
generally, although frequently to a higher standard of attainment on the basis of the
perceived importance of the building in question.

IStructE Safety in tall buildings and other buildings with large occupancy

5.5.4.2

Buildings generally, including tall/large buildings, invariably have defects at the time
they are handed over after construction. Current experience in the United Kingdom, for
example, indicates that ‘zero defects’ at handover rarely occurs. Defects are often
present in life safety installations such as security and fire alarms, passive fire protection
and active fire fighting systems, e.g. sprinklers. Commercial pressures can result in sub-
standard building products being on the market. Many products in use now have not
undergone third party evaluation and a large proportion fail to meet fitness for purpose
standards on first testing. The quality of installed sprinkler systems is not always
ensured by installation inspections. Installed life-safety systems are not always
commissioned and tested, or subsequently maintained in working condition.

5.5.4.3

An important feature of protective products and systems is that they are durable over
time in service. They should not degrade significantly and undermine the protective
capabilities of the systems. New installations or upgrades of existing installations should
be made with long-term robustness in mind.

Key issues: Other issues

Security and safety of cladding, including glazing

•

Propensity to cause injury in the event of explosion, impact or fire outside or within the
building.

Security and safety of building services

•

Design of services systems for robustness, redundancy, and with isolation provisions.

•

Protection and sealing of systems.

•

Security against unauthorised access to building services equipment, plant and control
rooms.

Security against unauthorised entry

•

Prevention of approach and entry with malicious intent.

•

Management and emergency services plans for response to potential extreme event
scenarios.

Implementation of design and construction

•

Assurance of adequacy, including durability, of safety-critical elements.

•

Quality of components and workmanship in life-safety installations.

IStructE Safety in tall buildings and other buildings with large occupancy

6 The new situation post 11 September 2001

6.1

The extreme events that destroyed the WTC towers on 11 September 2001 were
unprecedented in scale, but they were not wholly unique. Aircraft impacts into tall
buildings had occurred previously, although not as a deliberate act. Indeed, a further
landmark building, the 130m high Pirelli building in Milan, was hit by a relatively light
aircraft at the 26th floor level on 18 April 2002

(14)

. Several accidents have also occurred

where an aircraft has crashed in a built-up area of a city. In addition, damage to other
tall/large buildings caused by accidental or malicious acts, particularly explosions and
fire, has occurred over recent years, see Appendix A.

6.2

The explosion damage events referred to in Appendix A are those causing most loss of
life and damage to tall/large buildings in recent years (excluding war zones). Other less
serious events have occurred. In general, reinforced concrete and steel-framed buildings
with well-detailed connections are usually able to withstand nearby explosions without
sustaining extensive permanent damage. Structural damage is likely to be confined to a
zone close to the seat of the explosion. Collapse of an entire building is rare. However,
building communication and services systems are often rendered inoperable. Most
injuries and fatalities are caused by falling glass, blast-propelled debris, or by smoke
inhalation. World wide, the incidence of substantial damage to tall/large buildings by
explosions, accidental or deliberate, is infrequent but possibly increasing. Given the
potential large consequences, explosion damage from small or large devices remains a
major hazard to the occupants of tall/large buildings.

6.3

The fire damage incidents referred to in Appendix A are those that have caused
substantial losses of life and damage to tall/large buildings in recent years. World wide,
substantial fire damage, whether by accident or arson, causing large losses of life is quite
rare although fire incidents in high-rise buildings are common, with the numbers of
fatalities usually being relatively low. However the potential for large life losses exists
and fire is one of the greatest risks for tall/large buildings. Fire must therefore continue
to be considered a major hazard to the safety of occupants of tall/large buildings.

6.4

Extreme event damage to tall/large buildings caused by malicious acts is not therefore
a new problem. However, the events of 11 September 2001 have challenged perceptions
of the safety of tall/large buildings. They have shown that malicious acts can cause the
total destruction of a tall/large building and result in large loss of life. A new situation
has been created in which a reappraisal of provisions for safety is required.

6.5

Further incidents of extreme damage caused by malicious acts can be expected in the
future. Whilst the risk of such events for most cities and buildings is very low, an
ongoing risk exists of large explosions, fire or other form of extreme man-made event
aimed at harming occupants and causing damage to tall/large buildings. Other extreme
events that can be expected arising from malicious acts are those that may make
building services (ventilation, air conditioning, water supply, heating and electricity
supplies and catering services) unsafe.

6.6

Natural disasters, such as earthquakes and hurricanes, may also threaten the safety of
occupants of tall/large buildings and the buildings themselves. Such extreme events are
prevalent in some regions of the world. Tall/large buildings in these regions are
designed to a practical extent to resist these events and protect occupants. Examples of
such events are not given here. Design philosophy and practice are well developed in
many regions of the world relating to common natural disasters. Most buildings that are
properly ‘engineered’ survive well and generally do not collapse during these events.
Design routines continue to be improved as learning from events occurs. The design and
management provisions made for the survival and safety of occupants in buildings in
such disasters provide a basis for learning in order to improve the safety of buildings in
extreme man-made events such as explosions, fire and impact.

6.7

It is clear that there is no single or precise answer to the issues of designing tall/large
buildings and their operating and management systems against the wide range of

IStructE Safety in tall buildings and other buildings with large occupancy

possible extreme man-made events that may occur. There may be many options for
enhancing provisions for the safety of building occupants. Decisions need to be made
by owners, operators, designers and building managers based on an understanding of all
the issues. They need to be based on rational consideration of the ‘profile’ of the
building and the risks to safety during its intended life. There are strong links relating
to safety between the building structure, fire protection, services systems and the
building management and emergency services. Multi-disciplinary effort is essential to
optimise safety. Overall strategies involving the design and construction of the building,
its management and the relationships with emergency services are required in order to
maximise protection of building occupants.

IStructE Safety in tall buildings and other buildings with large occupancy

7 Initial recommendations

7.1 Introduction

7.1.1 The extreme events of 11 September 2002 raise the question of what improvements in the

design of new tall/large buildings, or the appraisal of existing ones, can practically be made
for the protection of people when a major emergency occurs. Extreme events that may
cause a major emergency take many different forms. Although many can be foreseen, some
cannot and, in any event, their nature cannot be predicted precisely. Nevertheless the risks
to occupants and the damage they cause can be decreased by the provision of more robust
structures, services systems, fire and other protection, and means of escape, and by the use
of emergency response plans. Enhanced provisions in these areas can give more effective
protection against many potential extreme events.

7.1.2 The design of buildings, and especially tall/large buildings, is a complex process of

evaluating uncertainties and balancing risks and costs. For extreme events, risk management
techniques are available that can assist identification and evaluation of potential hazard
scenarios and choices of design and management provisions, see Appendix C. The design of
fire protection systems, means of escape, emergency access and management is every bit
as important as the design of suitably robust structures and building services systems. There
are strong interactions between all of these elements and the management of a major
emergency that have a marked influence on the safety of building occupants.

7.1.3 The Working Group has reviewed therefore the issues identified in Section 5 and makes the

following initial recommendations of matters for consideration by owners, operators,
designers, and building and emergency services managers of tall/large buildings. To assist
consideration, in-depth technical and economic studies together with review of policies on
the safety of people in tall/large buildings are needed. Recommendations are also given
therefore on needs for supporting development and research, see Section 8.

7.2 Vulnerability to progressive collapse

7.2.1 The location, direction and magnitude of the forces that extreme events may exert on a

tall/large building cannot usually be predicted accurately. In these circumstances, the main
protection against them initiating progressive collapse is to provide a robust structure that
will remain stable even if a number of structural elements are damaged, i.e. suffer ‘primary’
damage. Robustness is achieved by use of structural redundancy and structural elements
that are strong and ductile and capable of absorbing a high amount of energy as they deform
under extreme loads. The elements need to be joined by connections with similarly
adequate strength and ductility properties so that alternative load paths are present in the
structure. It is insufficient merely to tie structural elements together. Tying alone does not
inherently provide a ductile structure or one with good energy absorption capability. Fully
tied structures made up of strong elements and connections with good ductility (to
maximise their ability to deform under load before they break) have inherent residual
strength and therefore low vulnerability to progressive collapse. Provision of strength and
ductility needs to recognise that the potential directions of extreme event forces may be
opposite to the forces due to normal loads, e.g. uplift due to explosions. In addition, there
are some structural situations where weak tying or no connection between parts of a
structure can protect against the whole structure becoming involved in a progressive
collapse. Knowledge of vulnerability of building structures to progressive collapse is
incomplete and research is needed to improve understanding of the phenomenon.

7.2.2 Redundant structures have alternative load paths for carrying the loads around parts where

local structural damage may occur. Where a structural element is fundamental to the
survival of the whole structure, its design clearly should be given specific consideration.
Such elements need to be robust in themselves and, if possible, protected from potential
exposure to hazards, e.g. where they are necessarily located near fuel storage that in some
extreme event scenarios might catch fire.

IStructE Safety in tall buildings and other buildings with large occupancy

7.2.3 Whilst extreme events are largely unpredictable, the occurrence of many amongst the large

number of possibilities can be foreseen, e.g. severe earthquake in some parts of the world,
accidental or deliberate road vehicle/aircraft impact, explosion, fire, or impact followed by
fire. Generally designers have considered single extreme events and not combined events,
such as occurred at the WTC towers where impact was followed by fire. Given a generally
robust structure, the protection provided against extreme events can usually be made more
effective by using an implicit or explicit risk identification process to determine the
extreme events and combinations of circumstances most likely to occur. It may then be
possible for the design to be adjusted to provide reduction of specific risks using risk
management approaches as discussed in Appendix C. Such reductions may be identified in
particular for combinations of potential impact, explosion and fire events.

7.3 Passive and active fire protection

7.3.1 Passive fire protection, including compartmentation

7.3.1.1

To be effective in extreme events, passive fire resistance materials require greater
capability for resisting removal by impact, explosion, fire, or by degradation over time
due to vibration or ‘wear and tear’ by occupants and maintenance. A greater capability
for protecting structures in fires than provided for by current standards is also needed.
The use of hydrocarbon-grade fire protection that has capability for withstanding rapid
temperature rise and temperatures up to 1200oC, might be considered. Practical research
is needed on the resilience of passive fire protection materials to blast, impact and large
deformations of the structure underneath, and the associated robustness criteria for these
materials.

7.3.1.2

Modern tall/large buildings tend to contain considerable amounts of combustibles that
can also cause relatively high temperature fires, especially if there is a through draught.
Given these circumstances and the potential range of extreme events, it is suggested fire
compartmentation should be more effectively provided and maintained in tall/large
buildings. Pressurisation and smoke control should be a part of the design. Design fires
should perhaps be assumed to last to ‘burn out’ with design based on the performance
of the whole structure in real fires, as opposed to using the indications from standard
comparative tests on building elements. The protection and compartmentation around
key sources of fuel energy, such as oil storage, should be made to a high standard.
Compromises to the integrity and effectiveness of compartmentation, for example as a
result of installation of new IT and communications systems during building use, should
be controlled by appropriate approval processes.

7.3.2 Active fire protection

7.3.2.1

Incidents of fire in buildings generally, and in tall/large buildings in particular, suggest
that sprinklers, which commence in operation when the fire is small, are very effective
in limiting the scale of fire losses, see Appendix A. However, sprinklers are usually not
capable of extinguishing a large or fully developed fire as may arise nearly instanta-
neously from some man-made extreme events. Sprinklers remain a valuable protection
in cases where relatively small fires are the initiating event. It is desirable to increase the
effectiveness of sprinkler operation by providing redundancy in water supply systems
and protection of water supply routes.

Recommendations for consideration: Vulnerability to progressive collapse

•

Raise the ‘trigger’ threshold, i.e. increase the capability of the structure to limit damage
and to bridge over damaged parts by provision of alternative load paths. For this purpose,
use structural elements with robust, ductile and energy absorbing properties and tie them
together with strong ductile connections, recognising the directions of potential extreme
event forces.

•

Give specific consideration to elements that are fundamental to the survival of the
structure.

IStructE Safety in tall buildings and other buildings with large occupancy

7.3.2.2

Heating, ventilating and air conditioning (HVAC) systems may be helpful in the control
of fire and occupant survival if linked to fire detection equipment or informed manual
control. For this purpose, control of ventilation fans needs to include, for example, the
ability to shut them down quickly in a fire-affected zone, and to pressurise adjacent
compartments and escape routes appropriately.

7.4 Escape, its management and the emergency services

7.4.1 Escape routes and emergency services access

7.4.1.1

For tall/large buildings, a high level of physical protection of escape routes appears
desirable, for example by requiring robust shaft construction and stairwell protection by
ventilation, pressurisation or smoke lobbies. Further examination is needed of the
processes by which smoke may spread during major emergencies and of methods of
providing improved protection of occupants over extended periods in the building and
during evacuation.

7.4.1.2

Shafts containing escape routes need to have sufficient structural robustness and
integrity so that there is only a small risk of them becoming impassable by occupants
during an extreme event. Routes should be separated or separately protected even if
placed together in the same shafts. They also need to provide a secure environment so
that occupants, who may be on the route for an extended period, are safe. For this
purpose, shaft pressurisation and blast-resistant doors to lobbies might be considered.
Diversity of numbers and location of escape routes and exits is desirable to provide
occupants with more options for escape. This would reduce the likelihood that all routes
become impassable in an extreme event. Placing entrances to stairs on opposite sides of
a central core to give alternative exits from each floor may be a way of increasing
diversity. Alternatively, the use of more than one core might be considered.

7.4.1.3

The physical size of escape routes, e.g. stair widths, should also be such that they have
sufficient capacity to allow simultaneous evacuation from a number of adjacent floors and
possibly the whole building. The dimensions of staircases need to be sufficient so that
congestion and delays in evacuating affected floors are avoided in most circumstances.
Escape routes also need to be usable by occupants with a wide range of physical
capabilities. It may be that properly designed and protected lifts can be used for
evacuation generally. The development of design requirements and operating protocols
for the use of lifts for evacuation is needed. In ‘fire’ emergencies, escape by lift needs to
be restricted to floors not affected by the fire as the risk of waiting for a lift on the ‘fire
floor’ is too high. Lift control systems should be designed so that signals from the fire
alarm system prevent lifts from stopping at ‘fire floors’. Provisions should be made for
escape from ‘stopped’ lifts.

7.4.1.4

Escape route provision should allow all occupants to evacuate the building without
becoming distressed by congestion or delay. Support systems, e.g. ventilation/air
conditioning of shafts, electricity supply and communications systems need to have

Recommendations for consideration: Passive and active fire protection

•

Provide robust, resilient and durable passive fire protection.

•

Treat active fire protection, e.g. sprinklers, as an addition to, and not a substitute for,
passive fire protection, and do not consider it for extreme events.

•

Ensure compartments are gas tight and seals are sound on building completion by
inspection, testing and certification.

•

Provide protection to compartments and mitigate the spread of smoke.

•

Design building to survive complete ‘burn out’ of contents.

•

Require independent approval, as a part of licensing and periodic audit of life-safety
systems, of modifications to passive and active fire protection.

IStructE Safety in tall buildings and other buildings with large occupancy

built-in redundancy. Effective live communications between the building management
and occupants, easy-to-follow signs, and reliable lighting of routes, e.g. using battery
back-up power, are important provisions.

7.4.2 Management of escape

7.4.2.1

The primary aim in management of emergencies in tall/large buildings is to maintain a
safe environment where occupants are located in the building, especially in normal
circulation areas and escape routes. A second aim is to manage the occupant population
and ensure the optimal means of getting people safely to a place of safety, away from
danger and out of the building if necessary.

7.4.2.2

Maintenance of a safe environment depends firstly upon the continuing structural
stability and integrity of the building. Continuing structural stability is required for at
least sufficient time for occupants to receive warning and evacuate to a place of safety
within the building, or if the overall stability of the structure is in doubt, to a safe
distance from it. Continuing stability is also important to enable emergency services
personnel to retreat clear of the building.

7.4.2.3

Secondly, keeping the environment safe usually also depends upon prevention of the
spread of smoke and other airborne hazardous substances. Prevention of their spread can
be achieved by compartmentation, assisted by HVAC systems that control air circulation
and ventilation, pressurise compartments, e.g. stairs, and contain or purge contaminated
air as appropriate.

7.4.2.4

Meeting the second aim depends mainly on:

• Implementation of an emergency response strategy appropriate to the emergency

scenario.

• Provision of adequate means of detecting, locating and assessing the hazards and

providing appropriate information and requests/instructions to occupants. Sensors are
not available or are not reliable for many possible contaminants. As a result, strategies
that do not rely on feedback have to be used.

• Provision and protection of safe areas in the building and of emergency means of

escape that have adequate capacity to enable occupants to reach a place of safety,
when necessary, without being exposed to hazardous conditions.

7.4.2.5

The development of better management, training and information systems is needed to
enable effective management of major emergencies as well as those emergencies that
can be considered as more conventional. Building managers need to have a wide range
of extreme event scenarios in mind.

7.4.3 Interaction with emergency services

7.4.3.1

New emergency response strategies and protocols need to be developed for the
management of occupants applicable to the different scenarios that may arise. The
nature of the extreme event and its location will have an important bearing on the risk
to occupants and how their safety is best protected. For the wide range of potential
hazard scenarios, it is necessary to consider how building management will be able to
obtain sufficient reliable information during an incident to enable them to decide on an
appropriate plan of action and how they will communicate with occupants and
emergency services.

7.4.3.2

A key member of the building management team needs to be made responsible for the
preparation of emergency response strategies. Appropriate structures for devolution of
responsibility are required. Training of the building management team in the handling
of emergencies is crucial. They need to be familiar before a major emergency occurs
with the hazard scenarios that may arise so that they can identify them and decide
quickly on an optimum response in any particular case. Knowledge, experience and
training are perhaps the best safeguards against human error in the handling of
emergencies. This consideration is also relevant to building occupants: they need to be
familiar through training with what could happen and how they could escape.

IStructE Safety in tall buildings and other buildings with large occupancy

7.4.3.3

Strategies need to be defined for each of the main types of major emergency that may
arise. Necessarily they involve control of HVAC and other services, management of
occupants and the emergency services. There is justification for preparations for major
emergencies in tall/large buildings including exercises involving all emergency services
and local hospitals as are carried out for other hazard scenarios, such as train crashes.

7.4.3.4

For any emergency, rapid identification of the nature and location of the emergency is
required so that the optimum management response is selected. Communication systems
that work in a major emergency are vital to the survival and safety of occupants. A
further critical feature is the location and protection of the building management
communication and control system. As far as is practical it should be protected from
becoming inoperable in extreme events.

7.4.3.5

A performance approach based on time to escape is usually employed in modern fire
safety design of large, complicated and heavily populated buildings, although these
times are normally based on phased evacuation, and may not be valid for simultaneous
evacuation. This is a more rational approach for such buildings compared to the
traditional prescriptions given in codes that use distance as the escape criteria. Code
requirements for maximum travel distances in tall buildings vary significantly around
the world. A performance approach is likely to be more suitable for major emergencies,
including those that are not fire-related. It can be used for existing as well as new
buildings.

7.4.3.6

In many aspects of life safety in major emergencies, time is of the essence. Time to
detection of incident, to action by building management to control the incident, to
movement of occupants to a place of safety, and to intervention by emergency services
can all be critical factors in the survival of occupants. Further work is needed to improve
predictive modelling of incident development, of movement and evacuation of people in
relation to areas where the air is contaminated by the event, and of protective actions by
management and emergency services

(9)

. These tools may then be used to identify harmful

areas in the building and suggest design solutions and/or management strategies.

7.4.3.7

In relation to building maintenance and management more generally, periodic third-
party audit and certification is recommended during building use to make sure that life
safety installations are maintained as intended. In addition, the management of the
operation of the buildings’ systems, including emergency response strategies and plans,
should be subject to independent audit and certification to make sure they remain alert

IStructE Safety in tall buildings and other buildings with large occupancy

and capable of responding effectively to potential risks to safety. Consideration should
be given to adopting a licensing system for tall/large buildings akin to the systems used
for sports stadia in the United Kingdom.

7.5 Other issues

7.5.1 Security and safety of cladding, including glazing

7.5.1.1

It is possible through appropriate selection of materials and design of glazing systems
to reduce injuries to people caused by fragments of glass when explosion or fire occurs
within or outside of the building. Levels of protection in existing buildings can be
improved by the use of anti-shatter film. For new buildings, a higher level of protection
can be obtained, for example by using laminated glass with an interlayer together with
suitably designed window frames and fixings. A combination of laminated and
toughened glass can be used in particularly vulnerable locations.

7.5.2 Security and safety of building services

7.5.2.1

The probability of occurrence of extreme events in which building services are used to
create a hazard to occupants can be reduced and the effects on occupants mitigated by
a broadly-based strategy. The strategy needs to involve both the design of the building
and its management. Measures for risk reduction that may be considered include:

• Minimise the risk of hazardous substances being brought into the building, see Section

7.5.3.

• Make the air and water distribution systems in the building secure and with vulnerable

points (e.g. air intakes, air handling units, air ducts and plenums, motor controls and
mechanical equipment rooms) difficult to access by unauthorised people, and
monitored with vision systems or detectors.

• Reduce the vulnerability of air distribution systems by designing for rapid shut-down

and incorporating redundancy in routes and provisions for isolating damaged or
contaminated zones. Arrange systems so that contamination released in a zone can be

Recommendations for consideration: Escape, its management and the emergency
services

•

Provide protection to escape routes from ingress of smoke.

•

Protect vulnerable parts of building services systems and incorporate redundancy.

•

Provide separate stand-by power for vital building services and for lighting of escape
routes.

•

Provide robust adequately-sized escape routes and diverse locations for them and provide
protection for final exit routes.

•

In addition to phased evacuation for emergencies, plan for timely simultaneous
evacuation of a large proportion of floors in major emergencies, including use of lifts as
well as staircases.

•

Be prepared for extreme event emergencies through development and trial use of
emergency response strategies that guide decisions on evacuation, communication with
occupants and the emergency services

•

As part of preparedness, make sure that: plans of the building are deposited in a remote
accessible location; engineering advice can be obtained quickly during an extreme event;
communication systems with floors, stairwells and lifts are in place and functioning;
training for the management team, emergency services and occupants is given; and
evacuation procedures are practised at regular intervals.

•

Require independent approval, as a part of licensing and periodic audit of life-safety
systems, of modifications to escape routes.

IStructE Safety in tall buildings and other buildings with large occupancy

purged quickly or contained as appropriate, and more generally so that they can
respond flexibly to a range of scenarios. Provide redundancy in the siting of plant.

• Make entrances to the building, e.g. lobbies, mail rooms and utilities entries, separate

air distribution zones with separate air supply and extract.

• Locate building air inlets so as to minimise risk of externally released substances

being drawn into them.

• Filter or treat inlet air. These measures are desirable and may be appropriate in some

cases. They are not sufficient alone to reduce risks to occupants from airborne
contaminants. In addition, a flexible capability for pressurisation or depressurisation
of compartment volumes relative to those nearby and to the outdoors, and good
airtightness are desirable to enable control of contaminated air. Such measures involve
large consumption of energy. Provision for natural ventilation may be more suitable
since a higher rate of air change is possible compared to that achieved by mechanical
systems.

• Seal air distribution systems, including preventing bypass around filters. Make

compartment boundaries good physical barriers by sealing unnecessary penetrations.

• Include redundancy and isolation capability in water supply systems. Dedicated risers

for fire fighting purposes may not be the only water supply redundancy that is
justified. Ability to isolate parts of services supply can be especially beneficial in
preventing growth of an emergency, e.g. ability to stop oil being pumped through
pipes adjacent to a fire.

• Control and vet food and catering services.

7.5.2.2

Surveillance and security measures used in the management of the building can also
contribute to reducing risks to occupants from malicious acts against building services
systems, see Section 7.5.3. Access to vulnerable points around and within the building
can be made as difficult as possible and, in addition, fences, grilles and locks can
contribute to security. Access attempts can be deterred by the use of CCTV and other
surveillance methods that make apprehension and identification likely. Detectors may
also be used within services systems to detect some types of harmful substances.

7.5.2.3

In addition to strategies of deterrence and protection described above, methodologies for
clean-up and recommissioning after incidents need to be in place.

IStructE Safety in tall buildings and other buildings with large occupancy

7.5.3 Security against unauthorised entry

7.5.3.1

The approach needs to focus on measures of deterrence and defence involving both the
design and management of the building since detection of many hazardous substances
is not practical. Measures that may be justified to reduce the risks of unauthorised entry
include:

• Provide no more entry points than are needed to enable efficient use of the building

and evacuation in major emergencies.

• Tightly manage and control entries, including goods delivery and basement parking.

Sophisticated security equipment and vetting of security and contract staff may be
needed.

• Install surveillance and monitoring systems, both outside and within the building, to

give immediate warning of any suspicious activity and to deter access attempts and
make apprehension and identification likely.

7.5.3.2

Effective security measures against unauthorised entry to tall/large buildings (and
also against approaching into close proximity carrying destructive substances on the
person or in road vehicles) can do much to reduce risks to occupants from malicious
acts. Whilst architectural and engineering design can be made so that possible points
of entry are limited in number and able to be controlled effectively, means of escape
for occupants should not be prejudiced. Security systems can act as a barrier and
deterrent to potential intruders. Constant surveillance may assist by providing early
detection and record for subsequent police investigation. Provision of effective
security is more difficult for tall/large buildings with multi-occupancy and/or multi-
functions. A security policy is needed for each building implemented by a
responsible team.

7.5.3.3

Preventing road vehicles from coming into close proximity of a tall/large building is an
important mitigation measure for protecting occupants against malicious acts involving
explosives. Physical barriers such as ditches, bollards, large planters and fountains can
be designed and placed to keep unauthorised vehicles at a distance from the building.
On the other hand, the design needs to allow access by emergency vehicles.

7.5.3.4

For security, performance monitoring and post-event analysis, the use of a ‘black
box’ – analogous to those used in aircraft – could be considered. Technology
associated with ‘intelligent’ buildings could be used to record useful data about the
‘health’ and status of the structure, the building systems, and occupant activity in and
around the building.

7.5.4 Implementation of design and construction

7.5.4.1

The best intentions to provide for the safety of occupants can be undermined during the
processes of design, construction, maintenance, repair and building management by:

• Errors in design.

• Defective construction (below-standard components and installation) not in

accordance with the design and specifications.

• Shortfalls in the maintenance and repair of the building fabric and its systems.

• Shortfalls in the management of the building that allow management system failures

to remain uncorrected and preparedness plans to lapse.

7.5.4.2

The large potential consequences in tall/large buildings caused by extreme events make
it necessary for higher standards of risk control to be adopted in these processes.

7.5.4.3

To control the risks, independent third-party inspection and certification of the safety-
critical aspects of design, construction and maintenance is needed to give adequate
assurance of safety

(15)

. In particular, stricter and tighter on-site construction control is

necessary, especially for safety-critical parts.

7.5.4.4

For tall/large buildings especially, independent third-party inspection and certification
of fitness for use of products and installations should be required. The costs of the

IStructE Safety in tall buildings and other buildings with large occupancy

increase in quality and certainty of system effectiveness would probably be no more
than the costs of reworking faulty installations. Inspection should focus on the quality
and soundness of those parts of the structure and building systems that are critical to life
safety in extreme events, e.g. structural system, cladding, fire protection systems,
services supply facilities, and alarm and security systems.

Recommendations for consideration: Other issues

Safety of cladding, including glazing

•

Use laminated and/or toughened glass with fixings designed taking account of potential
explosion loading.

Security and safety of building services

•

Use a broadly-based strategy involving design and building management to reduce the
risks.

Security against unauthorised entry

•

Reduce the probability of occurrence of extreme man-made events with potential to
cause progressive collapse, where practicable. For this purpose, use incident prevention
or limitation measures, e.g. provide barriers to protect the base of the building from
vehicle impact or explosion, and provide security against unauthorised entry.

•

Use both design and management provisions to deter and protect against extreme man-
made events taking place in or near the building.

Inspection of design and construction

•

Reduce the risk of the building performance being compromised during the design and
construction processes by appropriate use of independent third-party inspection, testing
and certification of safety-related structure and systems.

IStructE Safety in tall buildings and other buildings with large occupancy

8 Development and research needs

8.1

Development and research work on the following topics, many of which are interrelated,
are needed to assist consideration of the initial recommendations for enhancing the safety
of occupants in extreme events. In some cases, original study and testing is not needed.
Rather development work is needed to bring together existing knowledge and
understanding in order to develop practical guidance.

8.2

Vulnerability to progressive collapse

(1)

Robust structures for tall/large buildings – the provision of ductility, energy
absorption capacity and redundancy, and the design and protection of structural
elements fundamental to safety.

(2)

Provision of robustness and protection for stairwells and lift shafts.

(3)

Analytical tools to support performance-based engineering design of buildings
for extreme events, and in particular for combinations of events.

(4)

Guidance on design of robust structures based on parametric studies of ductility
in different construction systems, building types and details.

8.3

Passive and active fire protection

(5)

The durability in a tall/large building environment of passive fire protection and
its resilience to extreme events and to distortion of the base material.

(6)

The behaviour of whole building structures in real fires using fire modelling.

(7)

Compartmentation:

(a)

Ability of compartments to prevent the spread of smoke or contaminated
air.

(b)

Diversity and robustness of escape routes.

(8)

Standards of fire load and fire size for use in building design.

8.4

Escape, its management and emergency services

(9)

Escape route flow and number, location and occupancy capacities of stairs in
emergency situations where many occupants may wish to evacuate over a short
period of time.

(10) Protection of escape routes from smoke or contaminated air for extended periods.

(11) Decision support and information/communication systems for implementation of

response strategies and management of emergencies, including the escape of
occupants and the protection of key personnel.

(12) Guidance on operational planning, including major emergency planning and

management, based on emergency response strategies and protocols for the wide
range of extreme event scenarios that can be foreseen as significant risks.

(13) Enhancement of the linkages between building management and the emergency

services within emergency response strategies.

(14) Making communication between building management and occupants in a major

emergency more effective.

(15) Occupant evacuation models for engineering design relating to fire and other

extreme event scenarios. Modelling of incident development and occupant
movement in order to inform response strategies, including testing of models.

(16) Detection systems for providing building management with real-time information

on the conditions within and around the building and the status of building
services and security systems.

(17) Use of lifts for evacuation and other use in emergencies.

IStructE Safety in tall buildings and other buildings with large occupancy

8.5

Other issues

8.5.1

Security and safety of cladding, including glazing

(18) Cladding and glazing systems with minimum propensity to cause injuries

following impact, fire or explosion.

8.5.2

Security and safety of building services

(19) Robust and protected building services systems, their performance and control.

(20) The location and protection of plant rooms, water and oil storage.

(21) The means of protecting against dispersion of airborne contaminants in and

around tall/large buildings in major emergencies.

(22) The siting and number of air inlets for tall/large buildings.

8.5.3

General

(23) Risk management processes.

(24) Strategies for risk avoidance, reduction and acceptance.

IStructE Safety in tall buildings and other buildings with large occupancy

9 Concluding remarks

9.1

Current world wide social and political conditions suggest that it is now necessary explicitly to
take account of risks arising from a wider range of extreme events than has been traditionally
considered in the design, operation and management of tall/large buildings. Consideration by the
Working Group of recent extreme events causing danger to occupants and damage to tall/large
buildings has identified a number of multi-disciplinary and interrelated safety issues.

9.2

The safety of occupants in new and existing tall/large buildings can be enhanced in many extreme
event scenarios by reductions in vulnerability to disproportionate damage and more effective
protection through design, construction and building management measures. The Working Group
believes the key to minimising risks to occupants in extreme man-made events is to use overall
strategies involving design, construction, maintenance, operation and management of the
building. The initial recommendations made in this Report indicate the main directions for
reducing risks to occupants.

9.3

The Working Group recognises that implementation of the recommendations in these directions
will depend on the ‘profile’ of the building and the extreme man-made events considered in any
particular case. Development and research are required to provide the necessary tools and
standards. In this way the safety of occupants in new and existing tall/large buildings and the
safety of the buildings themselves can be enhanced in the future.

9.4

The Working Group benefited from drawing on a wide range of expertise across disciplines and
from world-wide locations. In itself this collaboration has proved fruitful and may serve as a
model for future investigations/reports into other building/construction issues.

9.5

The salutary reminders of the scale of loss of life and human tragedy at the World Trade Center
have been at the forefront in discussions of the implications. The Working Group acknowledges
that 11 September 2001 will remain a defining moment in the history of building performance in
the face of a malicious attack on civilised life.

IStructE Safety in tall buildings and other buildings with large occupancy

10 References

(1)

Hart, F. et al.: Multi-storey buildings in steel, 2nd edition. London, Collins, 1985

(2)

Federal Emergency Management Agency. World Trade Center Building Performance Study:
Data Collection, Preliminary Observations, and Recommendations. FEMA 403, May 2002

(3)

11th September 2001, Supplement to High-rise Buildings, Munich Reinsurance Company, 2001

(4)

Standing Committee on Structural Safety. Structural Safety 1996-99. Twelfth Report, London,
SETO, 1999

(5)

American Society for Testing and Materials. Standard Test Methods for Fire Tests of Building
Construction and Materials. ASTM E119-00. West Conshohocken, Pa, ASTM, 2000

(6)

Steel Construction Industry Forum. Investigation of Broadgate Phase 8 Fire. Ascot, Steel
Construction Institute, 1991

(7)

Bailey, C. G., Lennon, T. and Moore, D. B.: ‘The behaviour of full-scale steel-framed buildings
subjected to compartment fires’. The Structural Engineer, volume 77, no 8, pp 15-21, 20th April
1999

(8)

Thomas, I. R. et al.: Fire Tests of the 140 William Street Office Building.
BHPR/ENG/R/92/043/SG2C. Melbourne, Australia, BHP Research, 1992

(9)

Chartered Institute of Building Services Engineers. Fire Engineering, CIBSE Guide E. London,
CIBSE, 1998

(10)

National Fire Protection Association. Recommended Practice for Smoke Control Systems. NFPA
92A. Quincy, Mass., 1996

(11)

Katzman, G. M. et al.: ‘Risk Assessment of Citicorp Center Original Design’. In: Safety, Risk and
Reliability – Trends in Engineering, International Conference, Malta, 2001, p303-308. Zurich,
IABSE, 2001

(12)

Do Valle, G.: ‘Failure of a building in Rio de Janeiro’. In: Safety, Risk and Reliability – Trends in
Engineering, International Conference, Malta, 2001, p699-704. Zurich, IABSE, 2001

(13)

Nugent, W. J. and Schmidt, M. K.: ‘Failures of Modern High-Rise Building Facades and
Components’. In: Safety, Risk and Reliability – Trends in Engineering, International Conference,
Malta, 2001, p711-716. Zurich, IABSE, 2001

(14)

‘Pirelli floor beams sag after Milan plane crash explosion’. New Civil Engineer, 25 April 2002,
pp 6-7

(15)

Standing Committee on Structural Safety. Structural Safety 2000-01. Thirteenth Report. London,
SETO, 2001

IStructE Safety in tall buildings and other buildings with large occupancy

Appendix A: Recent extreme event damage to tall/large buildings

A1 Damage caused by explosions

Ronan Point, London 1968

A1.1

In the United Kingdom, the progressive collapse of part of the 22-storey Ronan Point flats
following a gas explosion on the eighteenth floor is well known

(A1)

. There were four fatalities.

Subsequently the phenomenon of progressive collapse was demonstrated in the laboratories
of the Building Research Establishment in the United Kingdom.

A1.2

Following the Ronan Point collapse, the UK Building Regulations were revised to include a
requirement for buildings of 5 or more storeys to be designed with the aim that damage caused
by an extreme event is not disproportionate to that event, see Appendix B.

World Trade Center, New York 1993

A1.3

A large car bomb was detonated against the south wall of the 110-storey north tower (WTC1)
of the World Trade Center in an underground garage two levels below ground

(A2)

. There were

only six fatalities but over 1000 people were injured. Electrical and water supplies were cut
and sprinklers and standpipes were made inoperable. The most severe structural damage
occurred in the basement levels, creating extensive bomb craters on some of the levels. A
shock wave propagated throughout the basement structure, causing the slabs at parking levels
to shear free from their supporting columns and other restraint locations. In certain positions,
the steel columns that were once braced at the parking levels had unbraced lengths as large as
21m after the explosion.

A1.4

The structural integrity of the tower was not threatened due to the ductility of the framed tube
of structural steel and the provisions made in the design of the tower. It was designed to resist
a 240km/h wind storm, the loss of perimeter columns by sabotage, and the impact of a fully-
loaded Boeing 707 aircraft at any height. Although lateral horizontal pressures during the
explosion were severe, the tower did not collapse because the magnitude was insufficient to
cause the columns to fail in shear or in combined axial load and bending.

A1.5

Buildings adjacent to the WTC1 tower were designed to less onerous requirements and
suffered extensive damage that threatened their structural integrity.

Murrah Federal Building, Oklahoma City 1995

A1.6

A large vehicle bomb was detonated approximately 5m from the north face of the Murrah
Building

(A3)

. The explosion and resulting collapse caused 168 fatalities and substantial damage

to the Murrah Building and to other buildings in the vicinity of the blast. The nine-storey
Murrah Building of reinforced concrete slab and column construction was damaged severely
at the north face where three of the four external columns and an internal column were
destroyed causing a 3rd floor spandrel to give way. As a result, eight of the ten bays along the
northern half of the building collapsed progressively, together with two bays on the south side.
Surveys of the damaged building found that progressive collapse extended the damage
beyond that caused directly by the blast.

St Mary Axe and Bishopsgate, London 1992/3

A1.7

Two separate incidents of detonation of relatively large bombs occurred in London

(A2)

. Only

one building suffered complete collapse, a 14th century church, but many suffered
considerable damage to cladding and internal fixtures and fittings. Only four buildings
immediately adjacent to the explosions suffered severe local structural damage.

A1.8The European Bank for Reconstruction and Development, approximately 150m from the

bomb in St Mary Axe, suffered extensive glass damage. The building was shielded from the
blast by an adjacent building and so did not suffer structural damage.

A1.9

The glass damage to the European Bank building illustrated the influence of glass type, size,

IStructE Safety in tall buildings and other buildings with large occupancy

strength, and orientation. Nearly all windows on the upwind faces were shattered. Large
annealed glass windows were blown in and glass shards were projected well into adjacent
offices. Where blinds were drawn on windows, the projectile hazard was reduced noticeably.
It was also evident that blast effects of the explosion on the interior were of low intensity. The
only windows to survive on the upwind face of the building were double-glazed in toughened,
10mm-thick glass. These windows were found to be crazed. The 33mm-thick laminated glass
windows at street level survived without crazing.

Manchester City Centre 1996

A1.10

A large bomb was detonated in the central shopping district of Manchester, England causing
extensive damage

(A2)

. There were no fatalities but many injuries were caused by flying glass.

Structural assessments found that the damage caused by the explosion was mainly to glazing
and cladding panels. Although glazing damage was extensive, it appeared to be randomly
distributed. Ground-floor windows relatively close to the blast remained intact, whilst
windows much further away and at high elevation were shattered. The worst case of structural
damage occurred near to the heart of the explosion where the structural frame of a 200 tonne
pedestrian bridge was twisted and lifted off its bearings. A retail store immediately adjacent
to the site of the explosion was subsequently demolished.

London Docklands 1996

A1.11

There were two fatalities and office buildings and nearby homes were damaged extensively
when a home-made vehicle bomb was detonated in London Docklands

(A2)

. There was little

structural damage to nearby buildings. However glazing and cladding damage was extensive.
No glazing within 50m of the blast survived.

A2 Damage caused by fire

Seven World Trade Center 2001

A2.1

The 47-storey building known as Seven World Trade Center (WTC7) was set on fire by debris
from the WTC towers (WTC1 and WTC2) when they collapsed on 11 September 2001.
WTC7 collapsed totally about seven hours later. The collapse appears to have been due
primarily to the effects of fire, and not to impact damage from the collapsing WTC towers

(2)

The collapse may have been associated with the burning of a large quantity of diesel fuel
stored in tanks on the 5th, 7th and 8th floors, and with nearby steel trusses used to bridge the
building structure over electricity substations. No other case of a fire-protected steel-framed
building collapsing totally in fire is believed to have occurred in spite of there having been
several cases world wide of large uncontrolled fires in tall buildings, even where the fire has
burnt out all combustible materials inside. The mechanisms causing the total collapse of
WTC7 have not yet been confirmed. Loss of structural integrity in one of the load transfer
systems caused by fire has been suggested as the ‘trigger’ event.

Andraus Building, Sao Paulo 1972

A2.2

The fire developed on four floors of the 31-storey department store and office building. It then
spread externally up the side of the building involving another 24 floors. Wind and
combustible interior finishes and contents contributed to the fire spread. The building was
constructed of reinforced concrete. Its façade had extensive floor to ceiling glazed areas, with
a spandrel of only 350mm in height and projecting 305mm from the face of the building. After
the fire broke through the windows, three to four floors above the department store floors were
exposed to a flame front. The front increased in height as more floors became involved. At its
peak the mass of flame over the external façade was 40m wide and 100m high and projecting
at least 15m over the street. There were 16 fatalities.

Joelma Building, Sao Paulo 1974

A2.3

Fire started on the 12th floor near to a window of this 25-storey office building of reinforced
concrete construction. The in situ concrete floor slabs projected 900mm on the north wall and
600mm on the south wall. The exterior facade was made of hollow tiles rendered with cement

IStructE Safety in tall buildings and other buildings with large occupancy

plaster on both sides and aluminium-framed windows. The fire spread externally up 13
storeys on two of the facades to the top of the building, readily igniting combustible finishes
inside the windows of the floors above, enabling the vertical spread of the fire to continue.
There were 179 fatalities.

Las Vegas Hilton Hotel 1981

A2.4

This 30-storey hotel of reinforced concrete construction had windows between floors
separated vertically by a prefabricated spandrel of masonry, plaster and plasterboard on steel
studs. The fire started on the 8th floor of the east tower lift lobby involving curtains, carpeting
on the walls, ceiling and floor, and furniture. An exterior plate glass window shattered
allowing a flame front to extend upwards outside the building. The fire spread from the 8th
floor up 22 storeys to the top of the building in about 20 minutes.

A2.5

The vertical fire spread was facilitated mainly by two mechanisms. Flames outside the upper
windows radiated heat through the windows and ignited curtains and timber benches with
polyurethane foam padding which then ignited carpeting on room surfaces. The second
mechanism involved the flames contacting the plate glass windows. It is believed the
triangular shape of the spandrels and recessed plate glass caused additional turbulence which
rolled the flames onto the windows causing their early failure.

A2.6

There were 9 fatalities. The doors to the hotel rooms where four fatalities occurred were open
or had been opened by the fire. There were no fatalities in rooms where the doors had been
kept closed.

First Interstate Bank Building, Los Angeles 1988

A2.7

This 62-storey building had sprinkler protection only in the basement, garage and
underground pedestrian tunnel. The building had a structural steel frame with sprayed fire
protection and steel floor pans and lightweight concrete decking. The exterior curtain walls
were glass and aluminium with a 100mm gap between the curtain wall and the floor slab, fire
stopped with 15mm gypsum board and fibreglass caulking.

A2.8The fire started on the 12th floor and extended to the floors above primarily via the outer walls

of the building. Flames also penetrated behind the spandrel panels around the ends of the floor
slab where there was sufficient deformation of the aluminium mullions to weaken the fire
stopping allowing the flames to pass through, even before the windows and mullions had
failed. Flames were estimated to be lapping 10m up the face of the building. The curtain walls
including windows, spandrel panels and mullions were almost completely destroyed by the
fire. However, the building structure as a whole did not collapse. There was one fatality.

One Meridian Plaza, Philadelphia 1991

A2.9

The construction of this 38-storey bank building used structural steel with concrete floors
on metal decking and protected with spray-on fire protection. The exterior of the building
was covered by granite curtain wall panels with glass windows attached to perimeter floor
girders and spandrels. Only the below-ground services floors were fitted with sprinklers at
the time of construction. Subsequently sprinklers had been installed on the 30th, 31st, 34th,
and 35th floors and to parts of the 11th to 15th floors. Fire broke out on the 22nd floor,
penetrated through the windows and heat exposure from the fire plumes ignited materials
on the seven floors above. The fire was stopped as it approached the 30th floor which had
sprinklers. Although the fire burned for 19 hours, the structure did not collapse. Three
firemen lost their lives.

President Tower, Bangkok 1997

A2.10

This 37-storey retail, commercial office and hotel development was under construction.
Interior fit-out was not fully completed and the sprinkler system was not yet operational. An
explosion and fire on level seven caused the destruction of the aluminium framed curtain
walling. The effectiveness of fire stopping at the floor edges was compromised by floor to

IStructE Safety in tall buildings and other buildings with large occupancy

floor cabling. Window and spandrel glass shattered. The fire spread up three floors to level
ten. There were three fatalities.

A3 References

(A1)

Ministry of Housing & Local Government. Report of the inquiry into the collapse of flats at
Ronan Point, Canning Town, London. HMSO, 1968

(A2)

Yandzio, E., and Gough, M.: Protection of buildings against explosions. SCI Publication 244,
Steel Construction Institute, 1999

(A3)

Federal Emergency Management Agency. The Oklahoma City Bombing: Improving building
performance through multi-hazard mitigation. FEMA 277. Reston, Pa., ASCE, 1996

IStructE Safety in tall buildings and other buildings with large occupancy

Appendix B: Regulations and codes of practice

B1 General

B1.1

The regulations and directives governing the construction of tall/large buildings generally
cover a similar, but not always the same, scope in each country. National and/or local
regulations require application for permission, often in the form of a licence, to construct
buildings. Other regulations govern the form and detail of the building itself. The latter are
usually intended primarily to ensure personal safety and, as a requirement of lower
importance, to protect the building against damage and defects. There do not appear to be
regulations in any country requiring a licence to operate and use a tall/large building once
built, although there are controls on some aspects of buildings such as emergency exits and
fire escapes, e.g. in hotels. In comparison, licences to operate some other types of facility
where large numbers of people are accommodated, e.g. sports grounds, are required in some
countries. These licence systems are generally for the control of safety-related aspects of the
facility and its operation.

B1.2

Regulations governing protection against natural hazards, such as wind and earthquake, are
usually related to requirements for structural stability of the building. The severity of the
natural hazard that must be resisted is usually specified, sometimes via associated standards
and codes. These requirements usually also serve to protect people in the vicinity from falling
parts of the building, especially parts of the façade. In some cases regulations give specific
requirements for the structure to be resistant to progressive collapse in the event of an
accident. Generally, man-made hazards to the structure are known as accidents, e.g. impact
and explosions. Malicious acts are specifically excluded or are not specifically referred to.
Guidance on the magnitude of accidents to take into account in design is sometimes given in
codes of practice.

B1.3

Regulations generally recognise fire as a major risk to buildings and require provisions for fire
protection that cover fire resistance, compartmentation, sprinklers and escape routes. The
requirements may be more onerous for tall buildings than others. The differences reflect the
higher risk in tall/large buildings of spread of fire and smoke and the greater limitations in
such buildings on escape and on the ability of emergency services to rescue people at height
and to fight fires within the building.

B1.4

Regulatory requirements for operational security usually include the safety of lifts, stairs,
guard rails and parapets, emergency lighting and non-slip floor coverings.

B1.5

In England and Wales, approved documents together with codes provide guidance on meeting
the performance requirements of the Building Regulations

(B1)

. They relate to performance on

completion of construction. Similar requirements apply in other parts of the United Kingdom.
National standards and codes in the United Kingdom are increasingly influenced by
developing European codes that are expected to supersede the national standards in due
course.

B1.6

In the United States, there is no national Building Code and most of the states have their own
code. Each community determines its own building code requirements

(B2)

. There are, however,

model building codes:

• Uniform Building Code by the International Conference of Building Officials.

• National Basic Building Code by the Building Officials and Code Administrators.

• Standard Building Code by the Southern Building Code Congress.

• Codes relating to fire by the National Fire Protection Association.

B1.7

An International Building Code by the International Codes Council (applicable in United
States only) also exists. It is essentially a conventional prescriptive code obtained by merging
the three United States model codes. An alternative, the International Codes Council
Performance Code, has recently become available.

B1.8None of these codes is mandatory but many states adopt one of them, at least in part. Others,

IStructE Safety in tall buildings and other buildings with large occupancy

such as New York and Florida feel their specific needs are best met by a locally-developed
code. Some cities, such as New York, Chicago and Los Angeles, also have unique codes that
do not entirely conform to the local state codes. Many codes in the United States, e.g. New
York City, provide exceptions/exemptions for government agencies and public utilities. The
New York City Building Code did not require detailing for seismic events when the WTC
towers were designed and built. Requirements for fire (covering compartmentation, fire
resistance and escape routes) are quite detailed.

B1.9

The National Fire Protection Association in the United States has produced a ‘Life Safety
Code’ that is specific to fire

(B3)

. An NFPA task group has developed guidance on performance-

based design.

B1.10

Many codes for design are largely prescriptive. The relationship to life safety is not always
clear. However, performance-based guidance is becoming more widely established.

B1.11

This very brief review of some of the regulations that relate to the safety issues in tall/large
buildings indicates that requirements are not consistent around the world. The differences are
due largely to independent development of regulations in each country and local experience
and conditions. Even where regulations are the same, important differences in the detailed code
rules for implementation exist that may substantially influence the levels of safety achieved.

B1.12

Development work is needed in many areas of performance-based design covering the main
safety issues. Performance-based fire safety engineering design is perhaps the area where
development is already well advanced and can be speeded up. The approach has already been
used in the design of tall/large buildings and other facilities with unique design features, e.g.
airport buildings, railway stations and tunnels. Generally time, e.g. time to escape, is likely to
be the performance parameter of greatest relevance in many aspects of building design,
operation and management for extreme events.

B1.13

The discussion below briefly describes some of the main requirements and provisions of
Regulations and codes of practice in the United Kingdom, United States, Australia, Hong
Kong and some other countries. The discussion is not intended to be exhaustive but rather to
illustrate the large number and different scope of regulations and code requirements that exist
around the world relating to safety issues in tall/large buildings.

B2 Vulnerability to progressive collapse

B2.1

Some regulations and codes of practice explicitly recognise the design principle for buildings
that damage should not be disproportionate to the cause. Currently, regulations and codes of
practice for buildings in the United Kingdom, United States and elsewhere have different
requirements for design against progressive collapse. In the United Kingdom, there is a
regulatory requirement, originally introduced following the progressive collapse of Ronan
Point in 1968, to provide (in buildings over 5 storeys tall) structural resistance with the aim of
limiting damage caused by an accident so that it is not disproportionate to the cause. Building
Regulations Approved Document A and British Standard codes of practice give advice on
meeting the requirement. During 2001 the UK Department of Transport, Local Government
and the Regions (DTLR) consulted on proposals to amend the Regulations to bring all
buildings within the compass of the requirement. The associated British Standard codes of
practice provide guidance on designing the form and detail of structures for ductility and
robustness. Structural elements fundamental to the survival of the structure are recognised.
Effective vertical and horizontal tying forms the main thrust of the approved design rules.

B2.2

The Eurocode EN1990: Basis of design

(B4)

adopts, as a fundamental requirement, the principle

that a structure shall be designed in such a way that it will not be damaged by events like fire,
explosion, impact or consequences of human errors, to an extent disproportionate to the cause.
It gives strategies for avoiding or limiting damage along the lines of the recommendations in
Section 7.2. Essentially, avoid or reduce hazards, select a redundant structural form with a low
sensitivity to the hazards considered, and design and connect the structure together with strong
ductile elements and connections so that it can absorb energy and survive removal of parts in
an extreme event.

IStructE Safety in tall buildings and other buildings with large occupancy

B2.3

In the United States, whilst there is no explicit provision aimed at prevention of progressive
collapse, current design guidance of cast in situ reinforced concrete structures and structural
steel frames (with properly designed and constructed connections) generally produces
structures with substantial ductility. For zones of high seismicity, the model codes in the
United States have detailing provisions that are intended to increase structural ductility and
toughness, thereby reducing the risk of progressive collapse during earthquakes. Following
the explosion at the Murrah building in 1995, see Section A1.6, the potential of failure of key
elements to trigger progressive collapse has been recognised

(B5)

B2.4

Australian requirements are given first as a functional statement of capability of the building
to withstand combinations of loads and other actions to which a building may reasonably be
subjected

(B6)

. Associated performance requirements include resistance at an acceptable level of

safety to the most adverse combinations of loads that might result in potential for progressive
collapse.

B2.5

The Hong Kong Building Authority uses locally-developed codes of practice for the structural
use of steel and concrete. The approach to structural robustness, accidental damage and dispro-
portionate collapse essentially follows the principles and methods adopted in the United
Kingdom, although there is little specific reference to robustness in the Hong Kong Building
(Construction) Regulations and Hong Kong codes of practice for structural design. The code:
Structural Use of Steel 1978 issued by the Building Authority gives no guidance on the issue,
either in principle or prescriptive. The code: The Structural Use of Concrete 1987 does however
state the principle – ‘The structure should be designed to support loads caused by normal
function, but there should be a reasonable probability that it will not collapse catastrophically
under the effect of misuse or accident. No structure can be expected to resist excessive loads or
forces that could arise due to an extreme cause, but it should not be damaged to an extent dispro-
portionate to the original cause.’ From time to time Practice Notes for Authorised Persons and
Registered Structural Engineers (PNAPs) are issued by the Building Authority. PNAP 140 gives
a list of standards that are considered to satisfy the technical requirements of the Building
Regulations. This list includes British Standards BS 8110 and BS 5950. It is through these two
particular codes that the conventional provisions for tying, localisation of damage, and key
elements, as used in design in the United Kingdom, are applied.

B2.6

Overall therefore, regulatory and code requirements across the world differ in the extent to
which they recognise vulnerability to progressive collapse. There appear to be none that deal
explicitly with the issues of weakening from impact or explosion combined with further
weakening from a major fire.

B3 Passive and active fire resistance

B3.1

There are regulatory requirements in the United Kingdom for inhibiting the spread of fire
within a building through the use of linings that resist the spread of flame, and through fire-
resisting construction that sub-divides the building into fire compartments. Overall, these
requirements seek to prevent the premature failure of the building structure in a fire. There are
also requirements to restrict fire spread over external walls and roofs and from one building
to another.

B3.2

Sprinklers are recommended in all buildings (except those for residential use) where they
exceed 30m in height to the highest floor. Under the Building Regulations, the sprinklers need
to be designed to a higher specification of ‘life safety standard’. The higher specification
includes additional measures that reduce the likelihood of sprinkler failure. The regulations
relating to fire work together as a package. Compartmentation is required to contain the spread
of a fire, sprinklers to stop the fire developing sufficiently to breach the compartmentation,
and protected shafts to enable people to escape safely when, by necessity, they have to escape
passed the fire.

B3.3

In the United States, many states and cities have fire codes that give building requirements.
Building code requirements for structural fire protection are based on laboratory tests, the
ASTM E119 standard fire test on building components

(5)

. This standard test provides

comparisons between component behaviour under controlled conditions. Similarly to the

IStructE Safety in tall buildings and other buildings with large occupancy

BS 476 standard test used in the United Kingdom, the test is not intended to predict actual
behaviour of the component in a building during a real fire. A handbook of fire protection
engineering has been published by the Society for Fire Protection Engineering

(B7)

B3.4

Australian building codes require fire-resisting construction according to building size,
building over 5 storeys being the highest category. Load-bearing elements are required to
maintain integrity and insulation for specified times. Lightweight non-combustible materials,
specified for protecting structure from heat, are required to meet prescribed mechanical tests.
Requirements for compartmentation are specified in terms of floor area and volume. Fire
stopping of services penetrations is required.

B3.5

In Hong Kong, the Building Code

(B8)

for fire resisting construction has been derived mainly

from earlier British counterparts. A barrier is required at openings in floors to prevent the
spread of fire and smoke. Curtain walls extending beyond one storey must be of non-
combustible materials and have fire stops in any void between the wall and the building
perimeter.

B3.6

Pressurisation methods required for the control of smoke from fire and prevention of its
spread through a tall/large building differ across the world. In the United Kingdom, positive
pressurisation of stair wells and negative pressures on fire floors are required. In Hong Kong,
the fire floor does not have to be depressurised, whilst in Australia additionally the floors
above and below the fire floor have to be positively pressurised.

B3.7

Overall, the requirements for the fire protection of building structures and smoke control vary
significantly around the world. In many countries, e.g. United Kingdom, United States,
Australia, Hong Kong, Sweden and Singapore, the requirements for fire protection are
obtained from tabulated data of the performance of structural elements in standard laboratory
tests. There are anomalies in the ratings that are derived. Other methods are available for
deriving requirements, e.g. the Eurocode method

(B9)

. These methods are based on ‘real’ fire

scenarios and provide more realistic gas-temperature/time curves that can then be used to
input into structural fire analyses to give predictions of the behaviour of the load-bearing
system as it is heated by the fire. Proposals being considered by the ISO/TC92 Committee for
a framework for long-term standardisation of fire safety in support of performance-based fire
engineering design may provide an effective international forum.

B4 Escape, its management and the emergency services

B4.1

The Building Regulations of the United Kingdom have requirements in Regulation B1 for
means of escape in case of fire

(B10)

. Provisions for early warning of fire and for means of escape

to a place of safety outside the building are required. The requirements for escape routes
depend on the use, size and height of the building. They cover number and capacity of routes,
distance of travel, protection, lighting, signing and facilities to limit ingress of smoke or to
restrict the fire and remove smoke. There are also requirements for fire precautions that
require a fire certificate for a tall/large building

(B11)

. The precautions required, in addition to

means of escape, include the provision of fire alarms and fire fighting equipment. As a whole,
the requirements for fire safety are designed to ensure the provision of adequate general fire
safety, means of escape and related fire precautions.

B4.2

Phased evacuation is recognised in several countries, e.g. United Kingdom

(B10)

, United

States

(B3)

and Australia

(B6)

, as an appropriate way of evacuating tall buildings. The Australian

building code provisions for escape require at least two exits for tall buildings and they must
be fire-isolated and exit within a certain distance to an open space. There are limits to
distances in the building from exits. The size of the exits is related to the number of people
accommodated in the building. Barriers must be provided to prevent vehicles blocking exits.

B4.3

In Hong Kong, the Building Code is also prescriptive but well developed on the basis of the
long history of tall buildings there. Prescriptive measures include stair pressurisation of fire
fighting lift and stair shafts, and provision of refuge floors. Means of escape are defined using
total evacuation as the escape strategy. Escape stairs must lead directly to a street and exit
doors must be easily operated from within. The width of staircases depends on the number of
occupants. Refuge floors are required every 20 storeys, except for residential buildings where

IStructE Safety in tall buildings and other buildings with large occupancy

the requirement is relaxed to 40 storeys. In Germany, concrete shafts are required for escape
stairs.

B4.4

The use of lifts for evacuation in emergencies in airport control towers is allowed in the
American code NFPA 101

(B3)

and, in the United Kingdom, Part 5 of BS 5588

(B12)

allows their

use in buildings.

B4.5

Code requirements for fire detection systems vary significantly around the world. For
example, in Australia, both smoke detectors and sprinklers are required in tall office buildings
whilst, in Hong Kong, only sprinklers are required for the detection and suppression of fire.

B4.6

Various standards exist for informative warning systems, including BS 5839: Part 8

(B13)

AS 2200

(B14)

, and NFPA 72

(B15)

. In many countries, only relatively simple alarm systems are

required, e.g. a bell.

B4.7

The provision of access and facilities for emergency fire services are required in the United
Kingdom. Designated fire fighting shafts (lift and stairs) are required that have additional fire
protection measures to protect ‘emergency services’ personnel and to facilitate their fire
fighting work, i.e. the shafts may be pressurised or ventilated. Similar requirements apply in
Hong Kong. Other countries, e.g. Australia, do not have this requirement.

B4.8Overall current regulations and codes are focussed on emergencies and means of escape in

case of fire. Further research is needed not only on systems for escape and emergency services
access in case of fire, but also on life safety in non-fire types of extreme event where different
evacuation and rescue strategies may be needed.

Other issues

B5.1 Security and safety of cladding, including glazing

B5.1.1

In the United Kingdom, cladding, including glazing, is considered in the Building Regulations
to be ‘structure’. The regulatory requirements for safety of the structure and resistance against
disproportionate collapse therefore apply. Approved documents give guidance on design of
cladding and fixings to meet the requirements. Enhanced glazing is only required at locations
where occupants may accidentally impact against it.

B5.2 Security and safety of building services

B5.2.1

There are no regulations in the United Kingdom specifically covering the security and safety
of services in buildings. However there are regulations and standards controlling the supply
of electricity and clean potable water.

B5.3 Security against unauthorised entry

B5.3.1

The introduction of regulatory requirements for entrance security of buildings is being
considered in the United Kingdom.

B6 References

(B1)

The Building Regulations 2000. London, TSO, 2000

(B2)

Pachecano, R. R., and Goldsmith, J.: ‘One Size Does Not Fit All’. Facilities Design and
Management, April 2002, pp26-28

(B3)

National Fire Protection Association. Code for safety to life from fire in buildings and
structures. NFPA101A. Quincy, Mass., NFPA, 2000

(B4)

prEN1990, Basis of design, CEN, July 2001

(B5)

Hinman, E. E., and Hammond, D. J.: Lessons from the Oklahoma City bombing: defensive
design techniques. New York, ASCE, 1997

(B6)

Australian Building Codes Board. Building Code of Australia, 1996 – Canberra. ABCB, 1996

(B7)

Society for Fire Protection Engineers. Handbook of Fire Protection Engineering, 3rd edition.
Quincy, Mass., NFPA, 2002

(B8)

Building Department, Hong Kong. Hong Kong Building Code: Code of practice for the

IStructE Safety in tall buildings and other buildings with large occupancy

provision of means of escape in case of fire, 1996; Minimum fire services installations and
equipment, and inspection, testing and maintenance of installations and equipment. Hong
Kong, Fire Services Department, 1998

(B9)

Draft prEN 1991-1-2, 2000. Actions on structures exposed to fire, CEN 2000

(B10)

The Building Regulations 2000: Approved Document B: fire safety. London, TSO, 2000

(B11)

Fire Precautions Act 1971, London, HMSO, 1971

(B12)

BS 5588 Fire precautions in the design, construction and use of buildings, Series, BSI,
London

(B13)

BS 5839: Part 8: 1995. Code of practice for the design, installation and servicing of voice
alarm systems, Fire detection and alarm systems for buildings, BSI, London

(B14)

Standards Australia. Emergency warning and intercommunication systems in buildings.
Australian Standard 2220, Sydney, 1989

(B15)

National Fire Protection Association. National Fire Alarm Code Handbook. Quincy, Mass.,
NFPA, 1999

IStructE Safety in tall buildings and other buildings with large occupancy

Appendix C: Use of risk management processes

Virtually all human activity involves risk. Owners and occupiers should therefore appreciate that
absolute safety in tall/large buildings is not achievable. Design, operation and management can
only seek to keep risks to occupants and the building itself at an acceptably low level.

A practical overall aim of design of a tall/large building against extreme events with a low
probability of occurrence is to make provisions, both in the building and in its operation and
management, such that the damage caused is not disproportionate to the event. The ‘damage’ of
primary concern relates to the safety of people. The physical damage to the building itself is also
of concern, particularly since damage to the building usually places people at risk. Minimising
the damage to the building fabric and its services systems can minimise the ‘damage’ to people
in many, but not all, cases.

Codes and standards have evolved to enable provision of safe buildings. They provide reasonable
protection for the occupants of a building in ‘normal’ hazard events, e.g. ‘conventional’ fire
scenarios. As a result, modern tall/large buildings designed using current good practice to resist
normal loading conditions and recognised extreme events such as extreme winds, earthquakes,
and road vehicle impacts, have performed well. This success can be attributed to the provision of
generally robust structures and systems, and of protective measures within and around buildings
to protect the buildings and their occupants from such events. Tragic incidents with loss of life
often stimulate a re-evaluation of codes and standards and lead to changes in practice which
improve levels of safety.

Safety and the protection of occupants provided by design and by building management for
normal circumstances may be strengthened and made more effective in extreme events by
specifically identifying possible hazard scenarios, assessing the risks and improving robustness
and/or protective measures and emergency response plans accordingly. A rational structured
consideration of the hazards and risks of extreme events that may occur during the life of a
tall/large building can assist designers and building management to enhance protection and
advise building owners and operators.

Explicit processes for identifying potential hazard scenarios and for managing risks due to extreme
events have not yet been generally adopted world wide in current regulations and codes relating
to building design and management. There is, however, a trend in this direction. Use of explicit
risk management processes in structural engineering has been advocated elsewhere

(C1, C2, C3)

. Their

use has been encouraged in some other industries, e.g. offshore oil and railways, following reports
on incidents of extreme event damage. The reports on, for example, Flixborough oil refinery
(1974), Seveso chemical plant (1976), Piper Alpha off-shore oil platform (1988), and King’s
Cross Underground station (1987) strengthened the trend away from prescriptive design methods
towards probabilistic analyses and performance-based design.

In the United Kingdom, the use of risk-based scenarios as the basis of design of structures is
becoming established practice. Some relevant standards have been produced, e.g. BS 7974

(C4)

This fire engineering standard recommends an initial qualitative design review by several experts
to decide what are the realistic scenarios and the fire safety objectives. The draft European
standard for structural design against accidental impact and explosions

(C5)

uses the concept that

some damage is acceptable and gives design guidance on measures for reducing the probability
of the event and the consequences. In other industries in the United Kingdom and elsewhere, e.g.
offshore oil, railway and nuclear power, explicit risk management processes are required by
regulations and supported by codes.

Well-developed techniques of hazard identification and risk assessment exist to inform risk
management processes. Their use can aid judgments by designers and building managers on the
risks of man-made hazard scenarios for which it is appropriate to make provisions or enhanced
provisions.

C8Such processes usually begin during the early stages of feasibility and development of the clients’

requirements and brief. They can enable more consistent implementation of the principle in
design that damage should not be disproportionate to the cause. Application of these processes to

IStructE Safety in tall buildings and other buildings with large occupancy

tall/large buildings necessarily embraces consideration of the building as a whole, its design and
construction, its protective systems, operation, management, and links to the emergency services.
The process typically includes identification of potential events/threats/hazards, assessment of the
risks judged against acceptability criteria, and choices and decisions about how the risks will be
managed. A range of techniques is available to assist, see for example, reference (C6). Although
there is no certain way of identifying all potential hazards and the judgment of what is acceptable
is subjective, the process of thinking through different scenarios can be helpful in identifying
those measures – whether simple or complex – that have the greatest potential within the
constraints of the project to improve life safety.

Specific consideration of risk in extreme event scenarios can play an important role in
determining what ‘enhancements’ should be considered, for example, relating to provisions for
fire:

• The use of phased and simultaneous evacuation.

• Use of lifts for evacuation.

• Target time for building evacuation.

• Evacuation management regimes.

• Selection and training of fire marshalls.

• Increasing robustness of escape stairs.

• Robustness of fire protection.

C10

More explicit risk management processes along the above lines could become a wider part of the
routine of the creation of tall/large buildings with potential benefit for occupant safety.
Development work is needed to transfer and develop the relevant risk management processes
used in related industries for tall/large buildings.

C11 References

(C1)

Standing Committee on Structural Safety. Thirteenth Report, 2000-01. London, SETO, 2001

(C2)

Schneider, J.: Introduction to safety and reliability of structures. IABSE Structural
Engineering Document 5. Zurich, International Association for Bridge and Structural
Engineering, 1997

(C3)

prEN1990, Basis of design. CEN, July 2001

(C4)

BS7974. Application of fire safety engineering principles to the design of buildings, Code of
practice. London, BSI, 2001

(C5)

Draft prEN 1991-1-7. Accidental actions due to impact and explosions. CEN, March 2002

(C6)

Managing safety risk. Guidance Note RT/LS/G/001. Railtrack plc, June 2000

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