by J. Bouwkamp, R. Hamburger and
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.