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P-Delta for Wind Design?

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??

RE: P-Delta for Wind Design?

My take on it:
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?

Agree with JAE. P-Delta should be evaluated for all loads, including gravity, at the least by way of thinking about it and exercising engineering judgement. I'm sure it just gets so much airtime in seismic because your drifts could be to the tune of h/40 whereas with wind it's probably more like h/400. I've run into the same thing when trying to figure out proper modelling parameters for shear walls under wind. 5 Billion papers for cracked section properties at plastic hinges and next to none for wind.

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?

(OP)
Thank you for your input. The project that is raising this issue is a lightweight greenhouse structure governed by AISI. It has a large footprint, but the frame spans and building height aren't big (15 ft eaves, 20x30 ft column spacing) However, being comprised of lightguage steel frames, it's very flexible so when P-Delta effects are considered with wind, it has a sizeable impact on design. For that reason alone, it seems logical that they should be considered (in my opinion), but I want to also be clear on the code requirement surrounding it.

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 would generally tend to agree that P-Delta should be considered. Given it's a greenhouse perhaps the implications of not doing so aren't high. They're rarely occupied and even less so during high winds, though that's already baked into your importance factor or wind speed.

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?

At this point, if the building code doesn't require it, the material code will force you into it in many cases. For structural steel, you just about have to if you want to avoid figuring all the "K" values. It's similar for concrete.

RE: P-Delta for Wind Design?

Quote (OP)

However, being comprised of lightguage steel frames, it's very flexible so when P-Delta effects are considered with wind, it has a sizeable impact on 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?

(OP)
MrHershey, I agree 100% on your sketchy lightweight structure comment. The fast-and-loose design approach seems to be rampant in this corner of the industry.

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?

Yes. Also frequently get the 'we've done it this way for thirty years and never had a problem' argument. Which is nice, I'm certainly happy with the absence of structural issues. It doesn't mean much though, the design wind event is pretty unlikely to actually have occurred in the last 30 years.

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