Modal Mass Participation

[Olivaw]

I am designing a 10-story building with 2 basement levels. Im using ETABS.

I don't know if there is something wrong but I have low mass participation ratios on the firts couple of modes. See tables below.

Mode 2 has 24% mass participation, but mode 20 has 18% and mode 21 has 12%.

Mode 1 has 24% mass participation, but mode 15 has 23%.

It seems weird that those higher modes have mass participation close to the fist and second modes. I was expecting that the first modes would concentrate at least 40 to 50% of the mass.

Any idea what I may be missing here?

 

[JoshPlumSE]

I find that the best thing to do is to look at the mode shapes themselves. Maybe animate them. You'll really see what portions of the structure are being excited by which modes.

Then, obviously, I try to understand whether this is a real characteristic of the structure, whether there's a mistake in how I've modeled the stiffness or mass of the structure. Maybe even if there is an issue with local modes that could be solved by changing how I define the mass of the structure.

 

[bones206]

Ditto everything Josh said. If I had to guess, you probably have a lot of mass modelled in the sublevels that is not actively participating due to fixed boundary conditions. You could make another version of the model with above-grade levels only and get better results for your superstructure modes.