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RE: effective length factor for sway moment frames? K for frames
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- Subject: RE: effective length factor for sway moment frames? K for frames
- From: "Martin W. Johnson" <MWJ(--nospam--at)eqe.com>
- Date: Mon, 2 Apr 2001 08:59:08 -0700
I confess that this is a topic that has troubled me for a long time. I do not think it proper to assume K=1 just because I turn on the P-delta feature on software, for reasons I shall describe below. And since the normal AISC equations are intended to account for P-delta effects, it seems overly conservative to include the additional stresses/deformations in the forces when I do a code check. So the P-delta is an interesting exercise that is basically used to check drift limits. My problem with the P-delta analysis in computer programs is that they do not include the erection tolerances for out-of-plumb conditions and permitted camber or sweep that is permitted in fabrication - and these constitute a large part of the P-delta effects found in the AISC equations. So the computer is only able to calculate the P-delta deformations of a theoretically perfect structure, that is erected perfectly plumb and with totally straight members. In theory you could add nodes at mid-points of member spans and shift coordinates slightly to include these effects, and then in theory the P-delta analysis might come out just right, but who the heck is going to do that? regards, Martin
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