Hi,
Is there a way to model drag struts in RAM Structural System that would break up the diaphragm forces? I am not concerned so much about design of the strut but more so the distribution of the forces into the lateral system (perimeter precast concrete shear walls).
I am analyzing an existing building - where the diaphragm will be going from a flexible condition to a semi-rigid (untopped metal roof deck). With flexible, the program allows you to control the % distribution into the lateral system. For a semi-rigid diaphragm, it will distribute the forces depending on the stiffness of the system/diaphragm. Does the program take into account the drag strut essentially breaking up the roof diaphragm? Do you have any recommendations how to properly model/run it to take the drag struts into consideration?
Thanks,
Michael Bui
You'll find a few other posts on the topic on Communities, Here's a reusable search: https://communities.bentley.com/search?q=ram%20frame%20drag&group=326
The best you can do is make the diaphragm semi-rigid and adjust the deck stiffness region by region. You can then work with frame story shear output or diaphragm section forces to get the desired types of forces.
Thank you for your quick reply! I will look into breaking up my diaphragm by region.
I found this post below however the 2nd link in the answer doesn't work. Do you know what the link was referring to?
https://communities.bentley.com/products/ram-staad/f/ram-staad-forum/108757/diaphragm-collector-chord/330542#330542
Also, I have the diaphragm modelled as semi-rigid currently but I am getting huge deflections along one of the drag struts (modelled as lateral beams with pinned fixities) in the dead load case. Do you know why this would occur?
If the diaphragm is semi-rigid and 1-way, then the gravity loads are pre-collected during the framing tables process and applied directly to the beams (not to the elements of the diaphragm). As such we should expect to see the lateral beam deflection based on those tributary loads. You say they are huge, but that looks like 2" to me, so that may or may not be huge all depending on the span.
The diaphragm shells are connected to the beam and they "go along for the ride" unless there is some significant out-of-plane stiffness to contribute and reduce the overall deflections.