topic Diaphragm action floors and centre of rigidity in Robot Structural Analysis Forum

Diaphragm action floors and centre of rigidity

Anonymous — Sat, 28 Oct 2017 16:31:16 GMT

Hi there,

I am currently modelling a steel frame supporting cast insitu concrete slabs and I have following issues:

1) Columns stretching at 2 storeys are not assigned to a storey automatically. When I assign them manually I am getting another warning message saying that columns exceeding storey dimensions. I am not running an earthquake analysis so I assume that this is not an issue. Could you please confirm?

2) I want to accelerate the solution procedure so I don't want to assign shell elements to the concrete slabs. My model seems to be working fine if diaphragm action is ignored. This is not the case if diaphragm is switched on and I am getting a warning message for instability type 3. I don't understand this issue because the instable node is a node at the midspan of a beam element connected to the horizontal brace although all beam elements on all sides of the node are fully connected. Also the green lines are not connected to correct centre of rigidity. Could you please help me with this issue?

3) Could you please confirm that the diagonal braces located underneath the slab are not carrying any vertical load? Is there a special element type I should use for this purpose?

Thanks in advance for your help.

Regards,

Dimitris

Re: Diaphragm action floors and centre of rigidity

Artur.Kosakowski — Mon, 30 Oct 2017 10:38:37 GMT

Hi Dimitris,

1) If there is no seismic analysis the stories are not necessary to be defined so it is no issue at all.

2) The instability of type 3 is the large difference in stiffness of elements of a model. When you define diaphragm you introduce 'artificial' RLink elements of the stiffness which is much higher than of other elements of the model.

You can display the details of the rigid links generation in this way:

3) You need to provide the list of bars you don't want to transfer cladding load to in the dialog shown below:

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

Re: Diaphragm action floors and centre of rigidity

Anonymous — Mon, 30 Oct 2017 11:21:00 GMT

Thanks a lot Artur! I now understand how the program works!