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Re: RE: Load Combinations for ASD 9th

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Bill, I agree, but once in a while something like that happens, to wit: 

What are the chances of a strong steady wind aligning perfectly with a narrow 
inlet at low tide ... and sinking a floating bridge?  Well, it happened to 
the Hood Canal Floating Bridge in Washington (state) twentysome years ago.  
If any of those factors had not occurred "just right" the bridge would have 
survived.  (The low tide was required to slack the lateral anchor cables to 
allow sufficient movement to allow the waves, etc., etc.)  Very small 
probability and I imagine the designers considered it practically impossible, 
if they considered that combination at all. 

Ralph Hueston Kratz, S.E. 
Richmond CA USA


In a message dated 8/18/00 8:17:39 AM, Bill(--nospam--at)AllenDesigns.com writes:

<< 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 years?


A re-roof occurs every 10-20 years?


Are we talking about a 500-1,000 year event for a structure that won't last

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% roof

live load) is pretty ridiculous.


Regards,


Bill Allen, S.E. (CA #2607) >>


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