I need help with my confusion about modal mass participation ratios. 3

[AzzA16]

Dear everyone,

I am modeling a five-story building and I am confused because I keep getting similar modal mass participation ratios along the x and y directions in the first mode. 26% along the x direction and 44% along the y direction. While these numbers are not exactly the same, the difference is usually quite large. Is this normal, or is there something I need to address?

 

[Krishna7568]

I think it is kinda normal, but I would suggest to reduce your number of modes.

 

[CSIESPL INDIA]

Having different modal mass participation ratios along x and y directions can be normal depending on various factors related to your building’s design and modeling approach. It’s essential to ensure that your model is accurate and reflects real-world conditions while also considering both directional responses during any dynamic analysis.

https://csiespl.com/

 

[Shu Jiang PEng SE]

Those modes with small mass participation factors are local vibrations. The local vibrations do not affect dynamic analysis and design f the structure as a whole. You can not model those members in your FEA to facilitate dynamic analysis.

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Shu Jiang, SE (Nevada). PE(Michigan, South Dakota,Missouri,Oregon), PEng (Ontario)

 

[AzzA16]

Thank you for all the help, guys!!. but maybe out of the subject a little bit, how do I know if my model is accurate and reflect real world condition

 

[KootK]

Thank you for all the help, guys!!. but maybe out of the subject a little bit, how do I know if my model is accurate and reflect real world condition

Show the details of your building and its lateral system to a trusted, senior engineer and see if your results matches their expectation.

 

[JoshPlumSE]

X and Y are your two horizontal directions, right?

When you have a mode that has similar mass participation in BOTH direction, it just means that this mode is probably something of a Torsional mode rather than being a pure flexural model in the X or Y direction. Pure flexural modes, of course, would have mass participation that is almost entirely in one direction.

It's not necessarily anything to worry about.

FWIW, I always find it helpful to view the mode shapes to get a better sense of what's happening with that mode.