Modeling Flexible Diaphragm

[structeng2]

Hi Everyone,

I have a few questions about flexible diaphragms and the hand calculations vs. modeling results (using SAP2000).

By hand calculation, a flexible diaphragm would equally distribute forces to lateral systems based solely on tributary width regardless of stiffness. I have built a model of a building that we are retrofitting. It has concrete shear walls on the exterior and we are adding internal steel bracing to try and break up the span of the diaphragm. I know that the shear walls are much much stiffer than the braces and would expect more load to go to the walls if the diaphragm is anything but fully flexible.

The existing diaphragm is a poured gypsum concrete. We are adding horizontal bracing at the roof level and assuming the gypsum fails. I guess I have 2 questions:

1.

I've reduced the f11 and f22 properties to 1% to make the poured gypsum (shell elements) as flexible as possible but still be there to distribute the vertical loads to steel framing. However, I am still not getting the tributary distribution of forces I would expect from a flexible diaphragm. Is the diaphragm still acting as semi-rigid?​

2.

Is any type of diaphragm actually fully flexible in real life? Won't the diaphragm always act as a semi-rigid diaphragm, thus distributing forces according to stiffness?​

Thank you for your help!

-Peter

 

[JoshPlumSE]

To answer your 2nd question.... There is no such thing as a perfectly flexible or perfectly rigid diaphragm. Reality is always somewhere in between.

As long as you use shell elements to model your diaphragm it will be some form of semi-rigid. The more flexible the material the closer to idealized flexible you should get. The more rigid the material the closer to idealized rigid diaphragm behavior you should get.

 

[structeng2]

Thanks, Joshplum.

This is what I figured.

If I remove the roof shell elements and apply my vertical loads directly to the steel framing, would that be closer to a flexible diaphragm?

 

[structSU10]

If you are adding in horizontal bracing in the plane of the roof just model that and ignore the rest of the diaphragm. you need to understand the forces in the braces anyway.