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Re: ASD Load Combinations

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The reason we need L and E both in this case is for ordinary floor
loads, which may be there during an earth-quake. (May be a factor of
0.75L or something would be acceptable for this). It would be difficult
for all of us if the code differentiated between different live loads
and put a proper factor for each type under various combinations.  I
vote for keeping all live loads together, even if it means designing for
improbable events in some structures.

Bill Allen wrote:
Even if you can visualize an earthquake occurring during a re-roofing
job, I
have to ask:

What is the probability of THAT happening?

Isn't the design level earthquake something like 2% exceedance in 50

A re-roof occurs every 10-20 years?

Are we talking about a 500-1,000 year event for a structure that won't
50 years anyway?

Yes, it CAN happen. But, so can an engine falling off an airplane
hitting a
roof. Are we going to design for THAT?

O.K., so my statistical analysis isn't accurate, but you get the idea.

IMO, combining wind OR earthquake load with roof live load (even 50%
live load) is pretty ridiculous.


Bill Allen, S.E. (CA #2607)
Consulting Structural Engineers
Laguna Niguel, CA
V (949) 365-5696
F (949) 249-2297

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