Roof Diaphragm not analysing 1

[BraSo85]

Hi all,

I've created a really simple model, as per the attached picture below. It has gravity and lateral frames and standard loadings. Nothing groundbreaking! I need the roof diaphragm to model as pseudo rigid however in order to get RAM to analyze the frame at all with this type of roof diaphragm I have to fix every single connection. (XXX - XXX everywhere!) This is incorrect as I need only the major axis to be fixed and torsional fixity. (XOX - XOX.) When I set this type of fixity though I get error messages about local instability at all the joints.

Another oddity which I have noticed, if I set the diaphragm to non rigid it doesn't like the lateral load cases (wind, seismic and notional)...

Does anyone know what is going on and how I can get the model to analyze as I want?!

 

[structSU10]

Are you getting significant weak axis bending that you don't want? I don't see an issue with the fully fixed assumption unless you are getting forces that don't make sense.

When you choose a pseudo-flexible case, you need to have frame numbers assigned so you can assign the proper percentage of the lateral load. If you can, try modeling the diaphragm as semi-rigid. this Link and this Link may help you with that. The semi-rigid actually meshes a diaphragm which should help with stability of the model.