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Re: P-Delta

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Things get interesting with another software company joining in the discussion.

> Because the forces which are obtained in step 5 form a couple and hence
> contribute no net force, the net additional horizontal force or vertical
> force on the structure is zero. The only effect that PDelta analysis
> should produce - when compared with a situation where PDelta effects are
> not evaluated - is additional moments on the structure which should be
> reflected in the support reactions. The software should not produce any
> extra horizontal or vertical reaction forces.
>
> Kris, STAAD Support Group

It may be a good idea if the Knowledge Base at STAAD is updated to reflect this statement. I quote:

STAAD.Pro Knowledge Base
Issue #: SP-1505 Date Posted: 10/26/2001
Description: Why is it that I cannot get a STATICS check report if I do a PDELTA analysis?
Version: 2001 Build No: ALL

Solution: A static equilibrium check usually means verifying the following :
APPLIED LOADS = Negative of the SUPPORT REACTIONS

For a PDELTA analysis, the support reaction side of the equation contains the secondary effects, which does not show up on the APPLIED LOAD side of the equation. So, an equilibrium check cannot be obtained due to this disparity.


Another point:

Looking at Kris' algorithm, STAAD apparently uses Pseudo Force method and not the more general method using geometric stiffness matrix.

Wouldn't that imply that other non-linearities and dynamic effects cannot be modeled simultaneously with P-Delta analysis?

If so, how does one do a response spectrum analysis for a slender steel frame where P-delta effects will be significant?

Rudra Nevatia
Structural Engineers' eBook

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