P-delta

[Drej]

I have a relatively squat structure which is basically a series of connected boxes supported on box section legs. The legs are about 1m high and on these sit steel boxes of dims approx 2m by 1m by 1m. This is a nuclear glovebox structure and has to be qualified seismically to code. We're running a response spectrum analysis with orthogonal SRS and am interested to know if anyone has any opinions on the inclusion of p-delta effects for this structure using this approach. Cheers.


------------
See faq569-1083 for details on how to make best use of Eng-Tips.com

 

[johnbridge231]

You can compare the first order to the second order moments. If their ratio is more than 20 then no P-delta analysis is required.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[JoshPlumSE]

Not sure that JohnBridge stated that correctly.

Stated a different way: If the theta factor is less than 0.10 per ASCE-7 section 12.8.6 (whereas JohnBridge suggested 0.20) then the effect of P-Delta can be ignored for seismic. If it's greater than you can multiply your results by 1 / (1-theta).

When looking through ASCE-4 (Seismic Analysis of Safety-Related Nuclear Structures), I don't see any direct requirement to include P-Delta. Though, I am looking at the 1998 version. It is possible that I have missed something or that newer versions have added it. Therefore, you might want to look for your self.