Modal Mass Participation Ratio 1

[AniMhj]

Dear everyone,
I am modelling a five storied building with one basement. The basement walls are modelled at basement. The building also has vault rooms in the Ground Floor and the First Floor. So, I have to model the shear walls in Ground Floor and First Floor. I am getting low modal mass participation ratio. In 1st mode, the model mass participation ratio is 3.82% along X direction and 36.13% along Y direction. In 2nd mode, it is 36.04% along X direction and 3.36% along Y direction. The sum of mass participation up to three modes are 39.88% along X direction and 40.11% along Y direction. What should be done to increase mass participation?

 

[OldDawgNewTricks]

More modes.

 

[muruep00]

Add more number modes in the modal analysis (I would recommend to reach minimum horizontal of total 90%).

 

[NedGan76]

What software do you use?

Consider adding rigid floor diaphragms, if possible, and concentrating the masses (and mass moments of inertia) in the respective mass centers.

It is also quite possible that your building does not have sufficient torsional stiffness or large eccentricities, so torsional or mixed (translational-torsional) modes play a big role here. Of course you can increase the number of modes until you get 90% mass participation, but if the torsional stiffness is not enough is not good for the building in general.

You'd better post some scheme and output here to discuss.

Cheers!
Ned Ganchovski

A better software for your calculation notes, free and open source:

https://github.com/Proektsoftbg/Calcpad

 

[AniMhj]

Thanks everyone for your answers.
I am using ETABS software for the analysis.
Please see the attached screenshot of the modal mass participation table.

 

[JoshPlumSE]

My thoughts:
1) For a 5 story building I would generally expect the first mode in each direction to have 50% mass participation or more. That's not a rule. Just an expectation. When it doesn't happen, then you generally need to increase the number of modes until you get above 90% for each of the two horizontal directions. I would expect you to be able to get there after capturing at least another 10 modes or so. Which will probably include one or two torsional modes and one or two 2nd order modes for each direction.

2) Another problem that can occur when you have a lot of the structures mass at floors that are not going to be dynamically active.... Like a basement. In cases like this, I might define a different load pattern for the definition of mass than I use for the design of the structure. In this load pattern, I would NOT include the mass of the any elements that are at floors that can't move. Meaning they've got horizonal boundary conditions that restrict that mass from being active.

3) I'll state #2 a little differently: I only want to model the ACTIVE mass of the structure, so that the mass participation will be more realistic.

4) Another solution can be to switch from an eigen-solution for modal behavior and instead use Ritz Vectors. They tend to do a better job with mass participation. They're often very close to the natural modes of the structure, but they're biased in a way that prevents a lot of spurious modes with low mass participation ratios from interfering with your dynamic solution.

 

[rscassar]

I would interrogate the modes which have very little mass participation like modes 5 and 6. If you are performing this analysis as part of Earthquake design you are interested in the UX, UY and RZ. It appears you might have "Include Vertical Mass" selected under mass source. This will generate modes for all the floor panels "bouncing" up and down independently from the rest of the structure.

 

[bobbyboucher]

make sure to manual mesh your basement walls... etabs does strange stuff with walls

 

[chris3eb]

Your first mode is the mode that corresponds to the largest period (ie most flexible). For a building of uniform stiffness, the first mode (or first 3 modes which are usually X, Y and torsion) should account for a large portion of the mass. But if you have a relatively flexible superstructure on top of a very heavy, stiff substructure, then the fist modes will excite the superstructure, but not the substructure, as Josh mentioned - this isn't necessarily a problem. Rather than defining mass separately, you can determine the mass associated with the superstructure and with the substructure (by bringing up the "mass summary by story" table). Then you can ratio the mass participation factors to determine what percent of the superstructure's mass is activated by the first modes. So in you case, if your superstructure accounts for 65% of the total building mass, the first mode would be exciting 55% (36% / 0.65) of the superstructure's mass.