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Re: Factor of Safety against Uplift

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Nels,

What about the Olive View Hospital stair towers???

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Nels Roselund wrote:

. > Structures in seismic events do not fail due to overturning instability
. > unless a structural element fails.  Foundations do not get rooted out of 
. > the ground.  That is because earthquake loads are dynamic -- they are 
. > cyclic.

. > Stability under earthquake shaking is not like stability under soil 
. > pressure or other constant loading. Earthquake loads are cyclic.  A 
. > building with a period of, say, 0.2 seconds and any significant amount of 
. > overturning resistant mass will not have time to progress toward 
. > instability due to earthquake loads before the loads reverse. The 
. > structure's period of response that we use for design is much shorter 
. > than the period of the overturning response. While the maximum design 
. > loads may develop due to a cyclic response in a small fraction of a 
. > second, it will take a much longer period of time for any significant 
. > overturning displacement to develop in the system.

. > The overturning resistance mass in the foundation of a structure need be 
. > no more than enough to provide the structural system with a polar moment 
. > of inertial that has a long period compared with the response period of 
. > the structure.  Our seismic codes may still be conservative on this issue
. > because we don't compare the structural system's overturning response
. > period with its flexural response period -- for simplicity we arbitrarily
. > require static stability based on unfactored service loads.

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