Good Evening. I'm currently designing an industrial warehouse. I modelled the main structure entirely with frames and assigned live and dead loads to the model. Afterwards, I tried to execute a modal analysis and several errors regarding the mode shapes appeared. The first mode seems allright but the next ones are just the braces vibrating on their own like on the figures below:
Is there any way to stop this from happening? The modal analysis is just wrong...
The modal analysis is correct but includes the modes that you would like to skip for spectral analysis. The solution could be to remove selfweight from the bracings leaving it assigned to columns and roof girders only and then convert the mass of the bracings into nodal masses at their end nodes. You may run the auxiliary model with bracings only that have fixed supports at their ends to find out the values of reactions for their self weight and then use these values for nodal masses for the complete model.
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you can also perform the modal analysis with the matrix set as "concentrated with no rotation" in the parameter dialog box right? This usually avoids local modes like this if your bracing are in one piece (no division at the bar's crossing).
Thank you for your assistance. I will try this option, but let me tell you it is a huge amount of work. As you could see the size of the model is quite high. It would be great for future versions or a service pack, if there was an option where the program automatically skips these modes, specially when performing seismic analysis.
Such option already exists for seismic and spectral load cases (you can skip the modes which e.g. Contribute less than specified % of participating masses) but the issue is that the number of such local modes in your case is very large due to the fact that you have nodes at intersections of bracings.
So there is no other way to remove local modes? It has to be done manually? It is really not efficient and poorly developed, specially when you design trusses and basically any structure with X bracings...
Yes... I know of this function. But since I only get 2 significant modes out of 100, it does not work for my purpose. I'm still working on the other method you proposed. I'll let you know how it goes.
Let us now about the results of your investigation..
When you avoid generating intermediate nodes at the intersection of the X braces, generally the results are better (in such case and if in practice you implement the intersection, in the design you should assign as buckling lenght the half of the real brace's lenght).