P-Delta and Seismic Overstrength

[JoshPlumSE]

When running an analysis which includes the Omega overstrength factors from ASCE-7's seismic requirements is it still required to run a P-Delta analysis?

I specific looking at steel structures subjected to seismic loading. But, the question I have could also apply to concrete moment frames.

I cannot find any specific exclusion that would prevent PDelta from being required. (Though I have to admit that I haven't looked all that hard).

But, it would seem to make sense. After all, those overstrength load combinations are there mostly to prevent localized failure in connections so that you can achieve higher ductility.

 

[ToadJones]

If you're talking Steel Braced Frames, I'm guessing P-delta wont have a big effect anyway.

 

[JoshPlumSE]

It's the moment frames that I'm primarily concerned about.

 

[JAE]

JoshPlum,

Here's what I think you are asking:

Option 1
Determine V
Distribute V vertically through stories - each story having an Fx applied to it.
Perform a PDelta analysis (if required under 12.8.7)
Take the resulting "effects" and multiply them by [Ω] (when required by the code for the element being designed).

OR

Option 2
Determine V
Distribute V vertically through stories - each story having an Fx applied to it.
Apply an [Ω] factor to the loads that are applied to the structure.
Perform a PDelta analysis (if required under 12.8.7)
Take the resulting "effects" which are already multiplied by [Ω] and use for design (when required by the code for the element being designed).

I think Option 1 is correct but the code doesn't specifically say either way.

The question is really - should the overstrength factor on the lateral forces be used in the PDelta analysis....or is the [Ω] factor just an added safety factor on the original PDelta results.

If you look in section 12.8.7, the definition of Vx in equation (12.8-16) just says "seismic shear force". That doesn't clarify whether [Ω] is in there or not.

But again - I'd say Option 1.

 

[haynewp]

I treat omega on the design side and do not input into the analysis nor disregard p-delta because of omega.

 

[JAE]

haynewp - I agree - I think what you are saying is what I implied with Option 1 above.

 

[haynewp]

Yes, option 1 is what I do. ASCE 41 similarly designates 'force controlled' elements that are on the design side as well.