Regarding the modal participation mass ratio 3

[Sakitech76]

Hi everybody, I am new member in this forum and I have one query regarding the modal mass participation, the periods are showing more than the 1.2 sec after analysis in the etabs and also mass participation is less than 90%,kindly go through an attached snap and suggest the solutions to overcome from the modal mass participation issue..

 

[Agent666]

Something is fundamentally broken in your model. Your periods are all greater than 7 million seconds. Check your deflected shapes, etc, you'll find numerous issues I bet. This is not a realistic result that should set off alarm bells when you're reviewing the analysis results.

I'd also expect with those results you model isn't even solving. Review the analysis logs for ill conditioning errors and other similar issues and work through solving these. Using the standard solver can report locations where errors are occuring. Making use of the check model function may also highlight modelling issues.

As far as getting greater than 90% mass participation, you need to include more modes in your modal analysis. It's that simple.

 

[STrctPono]

Agree with Agent 100%. The first thing you should be checking is deflected shape. Any issues may be immediately apparent. Improper boundary conditions is a classic culprit. With your results and those periods I would expect your model is completely unstable and throwing up error messages.

Start small. Before you run your response spectrum analysis on the model, run a simple elastic analysis with a test static load and CHECK YOUR DEFLECTED SHAPE! Make sure the model is behaving as expected. Then you can tackle a dynamic analysis.

 

[JoshPlumSE]

You must have some instabilities or near instabilities that are causing these problems. You have to review you model.

I suppose it's possible that your mass is way off (entered in kips instead of pounds or something), but usually when I see periods like that it is an issue with the stiffness of the model. It could be localized (like having mass applied to an extremely thin plate or cable), or you could have portions of your structure that just aren't connected together.

 

[r13]

Check your mass density and member rigidity specifications. Either one is in error, or both.