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Re: P-Delta

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The program is only reporting the reactions at the base of the lateral-force-resisting elements.  The P-Delta effect of the leaner columns does increase these reactions.  To satisfy equilibrium, there are equal and opposite reactions at the base of the leaner columns.  These leaner column reactions are not reported in the frame analysis program, so it can be confusing when the reported base shear is larger than the applied loads.

Jason Emoto


>>> "Michael L. Hemstad" <hemstad.ml(--nospam--at)tkda.com> 04/06/04 08:21AM >>>
John Peterson wrote:

"When looking at individual frames in the small model, with only the
wind loads applied in one direction (also the innate mass load, which
would be the "P" of P-delta), the horizontal reactions at the base are
greater than the applied story forces for each direction.  I modeled one
of the end frames using Visual Analysis 4.0 and did not get the same
results as I did with RAM despite using the P-delta analysis.  The
reactions added up to the applied loads in VA.  I contacted RAM to see
if I was interpreting their program incorrectly and the representative
stated that since I used the P-delta calculation, the building is no
longer in static equilibrium and will have base horizontal reactions
greater than the applied lateral loads."


John,
If you're doing a static (not dynamic) analysis, the frame as a whole
still has to be in static equilibrium.  The only explanation that makes
any sense (aside from simple roundoff error) is the idea put forth by
Craig Leech, that the software is applying a "small disturbing load" (or
words to that effect) to generate a lateral displacement, then
forgetting to take it out.

Leaner columns can't defy statics--the reactions of the whole building
still have to add up to the lateral forces applied to it.  If the RAM
rep actually said that, he should be made to take his licensing exam
again.

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota

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