Roof brace axial force during Earthquake?

[drile007]

Hello,

I’m wondering how to correctly estimate/calculate the right axial force in industry hall roof brace (with only tension forces)? Hall main elements are concrete columns (without any shear walls). Roof is made from soft steel sheets which I can’t model as rigid diaphragm. Will modal (combination) analysis give a wrong result?
Other suggestions/thoughts/comments are highly appreciated.

Thank you in advance

 

[KootK]

We're talking about braces is the plane of the roof diaphragm right? Perhaps you can base the design on the code specified diaphragm seismic forces (ASCE7 in US). I'm sure that modal analysis would work but it might be overkill for the problem at hand. What is the lateral system for the building as a whole? One story concrete moment frames? Braced frames?

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.

 

[drile007]

Yes, we're talking about roof braces. The main lateral system are concrete columns without any vertical braces (inverted pendulum). I'd like to connect the column on the roof level to prevent them to swing "for themselves".

 

[KootK]

Yeah, that sounds like a good idea. Standard diaphragm design and lateral design with P-delta effects on a 3D model should get the job done. One might borrow from AISC and apply notional loads to the diaphragm in attempt to account for initial out of plumbness in the columns.

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.

 

[drile007]

But which (how many) mode shapes should I account in Response Spectrum Analysis? Just main mode shape or should I follow 95% modal participation mass?

 

[KootK]

If you do RSA then, yes, follow the ASCE guidelines for modal mass participation. Again though, RSA strikes me as too fancy for what I'm assuming is a fairly regular, one story structure.

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.