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off their foundations were also observed. Fuller (111 noted that the greatest deficiency of wood-frame construction was its lack of resistance to torslonal racking caused by the second story being stlffer than the flrst (nonsymmetry). The floor diaphragm rotated In plan. lifted the rear wali from Its foundatlon. and left only interior walls to resist further eerth-quake motion. Arnold [12] observed that 65% to 80% of the buildings he surveyad either were not symmetrical horizontally or did not have a uniform story plan vertically.

Two- and three-story apartment buildings. Iow-rise shopping centers. and schools also experienced dam-age due to lack of adequate bracing (101. Large openings for parking or Windows in the flrst story again resulted in nonsymmetry of the lateral resistlng elements. Industrial buildings of tiltup concrete or masonry walls with a Berkeley roof system of pre-framed plywood (13) separated at the plywood-to-ledger beam connection. This resulted in loss of lateral support at the top of the wali. Although no statistics have been reported. it appears loss of lateral support was a frequent problem.

A review of photographs taken during damage surveys showed that the frame dwellings in the Alaska earth-quake had simple rectangular configurations. continu-ous floors, and smali window and door openings. These features provided a desirable symmetrlc box-like lateral resistive system that performed well. On the other hand, the newer San Fernando resi-dences wlth split-level floor configurations. multiple roof levels. and large percentages of window and door openings were not inherent earthquake resistive Systems. Their nonsymmetry madę them vulnerable to torsional as well as lateral motion. A number of commercial and industrial buildings with plywood roof diaphragms were observed In the San Fernando earthquake; few were observed In the Alaska earth-quake.

COMPONENT AND BUILDING RESPONSE

Floor and roof diaphragms and vertical sheer (racking) walls constltute the lateral load-resisting elements of timber structures. The combination of these components is generally classified as a box system in building codę nomenclature.

Floor and roof systems act as horizontal beams spanning shear walls. Whether they are rigid or flexible depends on their relatlve rigidity compared to that of the vertical shear walls. The diaphragm is rigid if its distortion is smali compared to that of the vertical shear walls; this is the usual assumptlon In lumped mass dynamie analysis. The diaphragm is flexible if its distortion is comparatively large; this is the case for a timber diaphragm with concrete. masonry. or timber shear walls.

McNatt and Galllgan (141 gave an overview of the types of diaphragm and framing materials available. Carney (151 presented a bibliography on wood diaphragms for literaturę prior to 1975; In generał, he considered only statically loaded diaphragms. Peterson [161 presented a bibliography of literaturę through 1982 that included static and dynamically loaded wali and floor diaphragms.

Several studies (8. 17. 181 present lateral diaphragm deflection due to static loading; total deflection includes bending and shear deflection. nail slip. and chord splice slip. Foschi [191 has presented a gener-alized structural analysis of diaphragms that incorpo-rates piąte action and nonlinear connection behavior. Bower (201 considered diaphragms of inregular shape.

Studies [21, 221 related to vertical floor vibration in conjunction with determination of allowable live load deflection have been conducted. Fundamental frequencies of horizontal floors of 12 to 17 Hz and damping ratios (as a percent of crltical damping) of 7% to 11% were found [22-251; empirical rełation-ships for frequency and damping to the stiffness for tongue-and-groove roof diaphragms were also presented Rudder [26) found out-of-plane vibrations of building floors and walls to be 20 to 40 Hz when they were subjected to trafflc-induced vibration.

Shear (racking) wali strength has been discussed for statically loaded walls. little is known for dynamically loaded (In-plane) walls. Medearis [271 presented wali natural frequency as a function of roof load. sheer wali weight, and stiffness. Young and Medearis [281 investigated the damping and energy absorption characteristics of plywood shear walls. A force-deflectlon mathematical model for diaphragms subjected to dynamie loads has been presented [291. Two parameters, initlal stiffness. and ultimate strength were used to descrlbe the model. Adham and