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[SEAOC] Wind loading vs. Seismic loading

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Comparison of wind to seismic loading must be done judiciously.  The 
probabilities of the events are not equal, neither is the expected 
performance.

The ASCE 7-95 commentary provides guidance on the probability of wind speeds 
and the calculated wind loads.  The Blue Book and the NEHRP Commentary 
provide good resources for probabilities on seismic events, calculated 
loads, and expected performance, although residential construction is not 
the primary focus of these documents.  In general, seismic loads are 
determined with a 10% chance of exceedance in 50 years, however ASCE 7 wind 
loads have a 64% chance of exceedance in 50 years.

At first glance this appears to be an extraordinary disparity, but one must 
then look at the expected performance.  The performance in seismic events is 
aimed at life safety, stability, and collapse prevention, and assumes non 
linear response for forces larger than those calculated using the code.  The 
intended performance for wind is elastic, and linear under design loads. 
 Remember also the special detailing and limitations for seismic loads are 
required to insure proper non-linear performance.

If an engineer is going to make any kind of assurances as to the performance 
of rigid cladding such as plaster or masonry, it would be prudent to perform 
extensive analyses including site specific response and non-linear dynamic 
analysis.  This is generally a tall order for residential construction.

HOS

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