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Re: Is anyone interested? You Bet!

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Dear Frank McClure:

Please elaborate on your objections to the treatment of overturning in FEMA 
273.  I have used FEMA 273, but not the linear static procedure, to which, 
I believe, your objections are limited.

As I read it, FEMA 273 defines a "pseudo lateral load" for the purposes of 
determining structural deformations and drifts when a linear static 
analysis is employed (pages 3-7 to 3-8); this force is explicitly stated to 
be potentially larger than the actual strength of the building.  Actual 
building performance and element behavior is expected to be nonlinear, so 
forces corresponding to the computed deformations are likely to be 
significantly lower than those calculated assuming elastic behavior.

The sidebar, "Overturning and Alternative Methods" (page 2-38), states 

If dead loads are used to resist overturning, then the overturning is 
treated as a force-controlled behavior and the overturning demands are 
reduced to an estimate of the real overturning demands which can be 
transmitted to the element, considering the overall limiting strength of 
the structure.

This would seem to require a limit-state analysis, with the foundation (or 
other element resisting overturning) remaining elastic and the soil 
remaining within an acceptable pressure range; such an analysis imposes 
equilibrium between the structure above and the element below.  While this 
requires more than a linear analysis procedure, it does not imply lack of 
equilibrium.  I agree that the alternative method in the sidebar 
calculation (Q = 0.9 QD + QE/ ROT) is less than rigorous, but I won't be 
the one to say that linear analysis procedures should no longer be allowed. 
 I will say that one can have more confidence in the answers derived by 
more rigorous analyses.

Treating a foundation as deformation-controlled is less clear, and I 
hesitate to include it in this discussion.  Nevertheless, as an interesting 
sidebar of my own, I draw your attention to a thread on the required 
strength of moment connections.  Charlie Carter wrote "another (limit to 
required strength) is if there is a system limitation, such as foundation 

I suggest that this approach, which I have also seen applied to braced 
frames, be used very cautiously.  Although statics tells us that the 
footing cannot weigh more than its mass times the acceleration of gravity, 
once we have allowed uplift to occur the foundation enters the realm of 
dynamics, and its upward acceleration contributes to the force it imposes 
on adjoining elements.  I have seen this very neatly demonstrated using 
time-history analysis; a static analysis is not the best-suited for 
addressing this behavior.

I welcome your thoughts and comments.


Rafael Sabelli