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Instabilities using plates as semi rigid diaphragm STAAD

Instabilities using plates as semi rigid diaphragm STAAD

Instabilities using plates as semi rigid diaphragm STAAD

(OP)
I am modeling a gabled CMU building with bar joists and a series of ridge beams and columns and a steel deck diaphragm.

I attempted to model the steel roof diaphragm according to the tips given in this thread here:

http://www.eng-tips.com/viewthread.cfm?qid=317163

Reading through the above linked thread the it seems that the plates should be modeled as plane stress only and unconnected to the joists.

However, when I have the plates as plane stress elements I generate over 1000 warnings and notes for instabilities. In addition the diaphragm deflects wildly. I cannot figure out what I have done wrong, however I have not previously modeled a diaphragm in a building.

Also I am not sure where exactly in the input file the command line should go. I currently have it located after members have been defined but before loads are called up.

Thanks for any help you can provide.

RE: Instabilities using plates as semi rigid diaphragm STAAD

Just as an FYI I've never found a successful method for modeling flexible or semi-rigid diaphragms in staad before.

That said, I'd suggest not modeling the base of the columns as released in torsion and try adding a torsional only support for your roof plates. Check the instabilities and find out if they're FX,FY,FZ or MX,MY,MZ and see what you need to do to support those instabilities. Check the deflected shape to try to see what's flying off and why.

If staad doesn't throw an error about your command then it's probably in an acceptable place. I've never personally used plane stress elements myself.

When I typically model a diaphragm in staad I make sure it's rigid and then use master-slave nodes to simulate the diaphragm.

Maine EIT, Civil/Structural.

RE: Instabilities using plates as semi rigid diaphragm STAAD

If you're using plane stress plates then don't apply any transverse loads to those plates.... not even self weight.

RE: Instabilities using plates as semi rigid diaphragm STAAD

(OP)
I don't believe there are any loads applied to the diaphragm.

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