CSMIP 94-02

CSMIP 94-02

"Degradation of Plywood Roof Diaphragms under Multiple Earthquake Loading"

by J. Bouwkamp, R. Hamburger and J. Gillengerten

February 1994, 32 pp.

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This report summarizes the findings of a study examining the recorded response of three buildings with concrete walls and plywood roof diaphragms to repeated strong motion events. Observed stiffness characteristics of the diaphragms are compared for each successive event and with that predicted by design formula and available data from static tests. Recorded response of the diaphragms indicates an initial dynamic stiffness substantially in excess of that predicted by static tests and design formulae. Damping for these diaphragms is determined to generally be low, on the order of 5% or less. Degradation of dynamic stiffness of highly stressed diaphragms is apparent. However, the observed degraded stiffness of these diaphragms is still in excess of that predicted by conventional design formulae. It was determined that the design of roof diaphragms for a response modification factor (Rw) of 6 is appropriate, regardless of the building Rw. The building period increases after repeated earthquakes due in part to the degradation of the plywood diaphragm. Following a strong earthquake (a design level earthquake), the roofing should be removed to inspect the nailing and renail as appropriate in order to restore the stiffness of the diaphragm.