Modal Analysis - Excessive Releases

[Anonymous]

Hello,

I am currently running a modal analysis in Robot to determine the mode shapes for a steel floor framing structure (see attached screen capture). When running the analysis I received a error message that there are "Excessive releases at the bar". When I hoover over this message all of the beams are highlighted. I want all connections to be "pin-pin" connections (not transfer any moment). To model this, I placed pin supports where the beams will be connected to a concrete wall and released the moment in all directions for each beam (see attached screen capture). I am also receiving several instability warnings for each member regarding the rotation. Is there a better way to model the pin-pin connection so that I won't receive these errors and warnings? The results of the modal analysis seem quite a bit off so I'm not sure whether this is something that  I can ignore or not. This is my first modal analysis in Robot so I really appreciate the advice.

Thank you in advance,

Lauren

[Anonymous]

As an update, I am was somehow able to manage getting rid of the warning but that hasn't changed my results much. These results aren't what I expected. I am looking for suggestions on ways to accurately model the ends of the beams as "pinned". It seems when I fix all the ends, the results are more reasonable, or even consider them as have some elastic rotational resistance. Has anyone experienced problems with beam connections in modal analysis before?

Thank you in advance,

Lauren

[EduardoMeruvia · Accepted solution]

Hi Lauren.Schuster, 

From what I can gathered seems like you have your all your bar members with pin-pin release. Also looking at the plan view image you attached I can see that  you also have PIN supports on your bars. This is probably why you are getting a instability warning when running your model, as you can not have a Pin released node supported on a pin supports, this creates a mechanism and thus your structure is unstable. 

To achieve a "pinned frame" the way that you should model should be as figure shown below, by only releasing the ends that have no pin supports. 

Also not sure if you have this situation in your model but you also should have two members joint together with their joining node being realsed from both members. this will also crease a mechanism as these members are free to rotate in and out of plane. 

Hope this helps 

Ed