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RE: RE: Load Combinations for ASD 9th

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No doubt there are several anecdotal incidences where all possible
conditions will occur simultaneously, but is it prudent to design for this
small probability? Before you answer this question unilaterally, you should
consult with the owner who is paying for your decision. I think we need to
consider that there are situations where structures will fail. I believe
engineers in tornado country have accepted this fact.

But this thread does provoke a thought. I wonder how many structures failed
because they were being re-roofed during the Northridge earthquake (or any
other earthquake for that matter) and the structure failed specifically
because of the fact that the roof live load was not considered. If this
number is known, I wonder how it compares with the buildings that suffered
structural damage due to the fact that there was an incomplete load path
from roof to foundation even though the package of design calculations was
volumnous and apparently considered a very rigorous analysis at the time of
submittal to the building department.

Sorry to get back on this soap box again and I know this has nothing to do
with floating bridges, but no matter how many load conditions we consider
nor no matter how much we bump up the design loads the structure still is a
poor design if there is not a well defined and detailed load path from roof
to foundation.

So, don't we have to ask ourselves which is more important: calculations or



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

||-----Original Message-----
||From: Rhkratzse(--nospam--at) [mailto:Rhkratzse(--nospam--at)]
||Sent: Monday, August 21, 2000 10:02 PM
||To: Bill(--nospam--at); seaint(--nospam--at)
||Subject: Re: RE: Load Combinations for ASD 9th
||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) writes:
||<< Even if you can visualize an earthquake occurring during a
||re-roofing job,
||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.
||Bill Allen, S.E. (CA #2607) >>

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