To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: Re: Is anyone interested? You Bet!
From: Rafael Sabelli <sabelli(--nospam--at)dasse.com>
Date: Tue, 8 Jun 1999 10:45:13 -0700
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
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.