Structures
Design
Age
Building materials
Measuring force on
structures
Acceleration
Resonance
Acceleration
• A measurement made on structures
relative to gravitational force
• 1 g = 32 ft/sec squared or 9.8
meters/second squared
• Building codes are at about 40-60
percent of that or written as .4 to .6
Acceleration
• Added strength is needed to
maintain a structure’s integrity when
subjected to lateral accelerations
Structures are built to maintain their integrity due
to gravity
Accelerographs are placed on man-made structures to
measure performance during an earthquake
• Acceleration
readings vary with
earthquakes
• What type of fault
would produce the
highest
accelerations?
Acceleration
-Horizontal
accelerations reached
1.19 and 1.02 g at the
base and 1.8 g on the
roadway.
-The Los Angeles River
sediments underlie this
bridge.
-What happened?
Freeway Collapse
Freeway collapse
Simi Valley freeway collapse due to high
accelerations.
Northridge Earthquake
• Increase with building height
Acceleration
San Jose High School, 1906
earthquake: stiff building
material and increase
acceleration with height
• Decreases with
distance from
epicenter
Acceleration
Period and Resonance
• Period is the amount of time it
takes one wavelength to pass a
point
• Seismic waves with a long
wavelength have a larger period
(2-4 seconds)
• Seismic waves with a short
wavelength have a shorter period
(1/2-20 cycles/sec)
Wavelength
Period and Resonance
• Buildings also have a period
• The period (long or short) is
determined by the number of stories
• Resonance occurs when the seismic
waves pass through the earth material
producing a particular wavelength and
this wavelength matches the buildings
period (wavelength).
• Remember: frequency is inverse of
period
• Resonance causes the motion of
the bldg to increase
• 0.1 second for a one-story building
• 1-2 seconds for a 10-20 story building
Period and Resonance
Common Building Failures
• Resonance:
when the period
of the seismic
wave matches
the period of a
structure
• 30 seconds of
shaking put the
structure into
resonance
Wood Shear Wall
Construction
Structural Failure associated
with the Loma Prieta
Earthquake
Bay Bridge failure
• First pier into bay
mud off Yerba Buena
Island (bedrock)
• Connection failed
due to low
frequency seismic
waves (mud) and
high frequency
seismic waves
(bedrock)
Cypress structure, Oakland
Reinforced
concrete
failure
1950 structure:
lacked seismic
design
Earth
material:
bay mud
Seismic
waves
amplified
Liquefactio
n
Marina District
Earth
material:
unengineer
ed fill
Liquefactio
n
Seismic
waves
amplified
Soft story: a floor of a multiple
story building that lacks the
structural strength or symmetry of
the other floors
Downtown Santa Cruz
Earth
material:
unconsolidate
d sediments
deposited by
the San
Lorenzo River
Seismic
waves
amplified
Liquefactio
n
Unreinforced masonry (URM)
failed
Structural Failure
Man-made structures:
– Structural design and age
– Building materials
– Fire
– Infrastructure failure: gas lines, water
lines, electrical wires or transformers, cell
phone towers
Soft Story: one floor
has less support than
the adjacent floors
Soft Story Collapse
• Parking garage is a
soft story
• Scenes like this
were familiar near
the epicenter
• Where have you
seen this type of
structure?
• Soft story: inadequate lateral
bracing
Structural failure,
Northridge Eq
Kobe, Collapse of 5th Story
• Another example
of soft story
collapse
• 5th floor restaurant
• Open structure
• Stories above and
below have more
support
Stiff building material
Pakistan, 2005:
Mw 7.6
8:50 AM, local
time
80,000 fatalities
200,000 injuries
Unreinforced Masonry
Wall Failure
Traditional structures failed-
unreinforced
brick
L’Aquila, Mw 6.3
April 4, 2009
• Seismic waves travel horizontally and
vertically
• Failure occurs at the connections
• Increase in acceleration with height
Irregularly shaped buildings
Irregularly shaped
structures
Irregularly shaped buildings
• T-shaped structure
• Communication
center in Mexico City
• The city lost
international
communication after
the 1985 earthquake
Resonance
• Resonance:
when the period
of the seismic
wave matches
the period of a
structure
• 30 seconds of
shaking put the
structure into
resonance
Mexico City,
1985
Earth material
• Loosely consolidated sediments and water
saturated mud or sand amplify seismic shaking
• Liquefaction often occurs
• Failure at connections where earth material
varies
Bay Bridge
Cypress
Structure
Moss
Landing
House falls off foundation
Foundation
Sill plate
House attaches to the
foundation through the sill
plate
HOG: house over garage
Open, weakly supported garage
fails with heavier and sturdier
structure above
Cripple wall failure
• The wall between
the sill plate and
the house
Mexico
Silent earthquakes
• Yellow: GPS data
– Slow slip or silent
earthquakes
– Early- 2002, mid-2006
• Red/Green: seismic
stations
– Circled area,
earthquakes
Silent earthquakes: indicative of
earthquakes
Mexico City Earthquake
• 50 x 170 kilometers of displacement
along the subduction zone
• M 8.1
• Mexico city is 400 kilometers away
• City was built on the sediments of
Lake Texcoco
Mexican subduction
• Cocos tectonic plate is
subducting under the
North American Plate
• Two plates lock
• Stress builds and
energy is stored
• Stress exceeds
frictional force
• Release of energy in
terms of an earthquake
Mexico City
• Drained Lake Texcoco
• Clay sedimentary layers
• Low frequency surface
waves amplified
• 1-2 second frequencies
• Matched the periods of
buildings 6-16 stories
Mexico City: Common
Building Failures
• Top floors fail-resonance
• T-shaped structures
• Flexible structures
between stiff structures
Mexico City: Building
Failures
• Hammering
Soft story
collapse
Chile, Mw 8.8, 2010
Uplifted terrace with
lighthouse
Intertidal fauna
exposed
3-6 feet of uplift along the
coast
Conception:L-shaped
structure failure
Failure of URM and soft
story
Balcony
beams and weak
internal wall caused
buckling of building.
Conception: failure of
concrete walls
Liquefaction induced
failures
seismic construction
• Huaca Pucllana
• Lima, Peru
• 200-700 CE
• Bricks built in a
trapezoid pattern
with spacing
• Accommodates
seismic shaking
Which structural designs tend
to fail during ground shaking?
• Soft story
• Structures constructed from stiff building
materials
• One weak point initiates other failures
• Irregularly shaped structures
• Structures that move into resonance
• Earth material fails
• Hogs
• House off foundation
• Cripple wall failure
Constructing model
buildings and subjecting to
shaking
• Building must be:
– At least 30 cm high
– At least 3 stories
– No central post or uprights
• Materials are limited
• Complete construction in limited time