P-Delta for Wind Design?
P-Delta for Wind Design?
(OP)
Reading ASCE7-10, P-Delta/second-order effects are mentioned several times in the seismic chapters but nothing is said about it in the wind chapters. A similar thing occurs in the California Building Code (I assume the IBC is worded the same).
Why is this? P-Delta effects are required to be considered for wind design as well as seismic, correct??
Why is this? P-Delta effects are required to be considered for wind design as well as seismic, correct??






RE: P-Delta for Wind Design?
ASCE 7 deals with the development of loads for buildings and other structures.
For seismic, your seismic demand is based upon a non-elastic response where second order effects would alter and affect the seismic loads as the seismic resisting system changes in stiffness through the event.
In wind, where you remain in the elastic range, second order effects don't affect the wind load.
In IBC 2012, section 1604.4, it suggests that the structural analysis takes into account "general stability". That to me implies second order effects if they are such that they would affect the final design.
That is up to the engineer's judgement. A very large warehouse type structure with a long spanning diaphragm, or a very tall building, will certainly develop second order effects from building drift that should always be taken into consideration.
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RE: P-Delta for Wind Design?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: P-Delta for Wind Design?
AISI S100-2007 also says in Appendix 2 that for systems made of moment frames, braced frames, shear walls, or combinations thereof, second-order effects shall be considered. Interestingly, this is absent of any wind/seismic wording. As I read IBC Section 1604.4 combined with AISI Appendix 2, P-Delta effects must be considered for systems matching this description.
I've had others tell me they don't think P-Delta analysis is necessary, so I want to make sure I'm interpreting the requirements correctly.
RE: P-Delta for Wind Design?
I've reviewed some pretty sketchy cold-formed lightweight structures, actually recently reviewed a greenhouse with some fairly dubious load paths. So I'd be a little wary of any 'this is how we've always done it' arguments. (Not to say anything about you or your company, that you're questioning things is great.)
RE: P-Delta for Wind Design?
RE: P-Delta for Wind Design?
I had a greenhouse design adventure last year and my experience was the same. At first blush, it's tempting to declare that, since the gravity load is so small, P-delta ought not matter. Two additional phenomenon crop up though:
1) As the frames drifts relative to the compression reaction coming up from the foundation, that reaction produces kind of an upside down p-delta effect that amplifies the moment in the frame joint on that side of the structure.
2) The compression in the compression side columns reduces their flexural stiffness and gradually erodes their contribution to frame stiffness.
These things are inconsequential in "normal" buildings but tend to have an exaggerated impact on these noodly structures.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: P-Delta for Wind Design?
It's always a tough argument telling owners and contractors that just because these are going up all over and/or they've done it many times before doesn't mean it's code compliant.
RE: P-Delta for Wind Design?