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EIGEN VALUE OR RITZ VECTOR?

EIGEN VALUE OR RITZ VECTOR?

(OP)
In an analysis of a 30 storied RCC building with moment frame and shear wall, I found that modal participation mass ratio is 54%, though I have used the number of modes 48.
What could be the reason?

What are the checks in dynamic analysis before go for design?

Can anyone send me a file of complete dynamic analysis.

thanks
shams

RE: EIGEN VALUE OR RITZ VECTOR?

I suspect there a bunch of elements that are not connected to the structure that are causing it to flop about. Look at your dead load deflection animations. Usually this will show you the rogue elements.

By code, a modal analysis should typically capture at least 90% of the building's modal mass.

RE: EIGEN VALUE OR RITZ VECTOR?

...and though you didn't ask it in your actual question, from the title of your post ritz vectors are generally better for 3-dimensional analysis as they capture coupled modes.  

 

RE: EIGEN VALUE OR RITZ VECTOR?

In an analysis of a 30 storied RCC building with moment frame and shear wall, I found that modal participation mass ratio is 54%, though I have used the number of modes 48.
What could be the reason?

---Because you have a lot of local modes in your ETABS model. This situation might be normal if you have highly non-regular structures. Increasing the number of modal shapes might be the only solution.

and though you didn't ask it in your actual question, from the title of your post ritz vectors are generally better for 3-dimensional analysis as they capture coupled modes.

---be careful about Ritz vectors, which are likely to cause significant calculation errors when high Ritz modes are used. In one case, when I increase the number of Ritz vectors from 200 to 300. Base shear tripled, although in both of two cases, modal participation mass ratio are beyound 90%
 

RE: EIGEN VALUE OR RITZ VECTOR?

EIGEN VALUE OR RITZ VECTOR

- RITZ Vector is more quick than EIGEN VALUE but at a large numbers of modes as larger than 250 modes may be become unuseful. So, you must lead to incrase manual meshing for elemens which may usally lead to acheive 90% of the building's modal mass participation.

- If there are free movement elements in the model, the model gives you low values of modal participation. So, the modal must be checked again. (see ETABS animation may help you).
- Don't define walls as membrane which leads to negative time periods and low mass participation percentages.

Best Regards,
ATOOMUS

RE: EIGEN VALUE OR RITZ VECTOR?

All answers are good.
besides, you have to use much more than 48 modes. you can use 3 modes for each storey, so that in total 90 modes

RE: EIGEN VALUE OR RITZ VECTOR?

One reason may be that you are using spring supports under a slab foundation. I had this recently in one structural model. You model the thing with non yielding supports and participation skyrockets.

RE: EIGEN VALUE OR RITZ VECTOR?

Every seismic code should be having a cut-off frequency (separating out-of-phase modes from rigid modes), where the modes with frequencies higher than that should not be considered for your Resonse Spectrum. I wonder haven't that limit been crossed when increasing no. of modes to 300 or so?

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