P-Delta analysis question

[chrislaope]

There is an argument in our office regarding whether P-Delta analysis is needed. One of my colleague engineer always applies P-Delta analysis in roof beam design, I told him that his conception is wrong, P-Delta analysis is for column/post design, usually under seismic or wind action, not for beam design. He insist that as long as the roof beam has flexural deformation and there is internal compression in the beam, P-Delta analysis has to be applied.

Is his argument correct? What is your opinion?

Thanks.

 

[MotorCity]

You're both right. It is a global analysis to be applied to an entire frame or model, it is not applied locally to members.

 

[Shu Jiang PEng SE]

Theoretically, any member should be designed per p-delta analysis once it experiences flexural and compression simultaneously. Any lateral displacement in the member can cause extra moment due to the compression force and the extra moment can cause increase of the lateral displacement, it is a iteration process. Refers to Comment C-2 in AISC 360 for cases where the p-delta effect can be neglected.

 

[KootK]

I told him that his conception is wrong, P-Delta analysis is for column/post design, usually under seismic or wind action, not for beam design.

I think that you're predominantly right here.

1) If you're talking P-Big_delta, you'd be absolutely right because the ends of your average roof beam experience negligible relative displacement.

2) If you're talking P-Little delta under significant axial load, you'd be mostly right because a) most roof beams do not experience significant axial load and b) P-Little delta moment magnification is built into the AISC beam-column design procedure.

3) If you're talking P-Little delta in the compression flange under flexure, you'd be mostly right because, again, that destabilizing effect is already accounted for in a) the lateral torsional buckling checks and b) the checks on the bracing used to restrain lateral torsional buckling.

colleague engineer always applies P-Delta analysis in roof beam design

Frankly, this surprises the heck out of me. There a hundreds of textbooks out there that contain example problems of flexural beam design. And I doubt that you'd find a single one where P-Delta on the compression flange is investigated separately from the usual LTB procedure. How does your colleague account for P-Delta in roof beam design anyhow? Or is this just a matter of checking a box within a software package?

P-Delta analysis is for column/post design, usually under seismic or wind action

I'd say that P-Big_Delta is for the design of the parts of the system that limit the Delta movement. That's often braced frames, shear walls, moment frame beams and columns, diaphragms, and collectors.


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.