Effective modal Masses

mkumar07 · Post by **mkumar07** » Sat Oct 04, 2008 8:00 am

Hi Silvia,

Is there a command in Opensees to find out Effective modal masses for each mode of vibration?

Look forward to hearing from you.

Regards,

Kumar,M.

silvia · Post by **silvia** » Sat Oct 04, 2008 10:46 am

why take that fun away from you!!!
it's all in post-processing, isn't it?!

mkumar07 · Post by **mkumar07** » Sat Oct 04, 2008 11:38 pm

Dear Silvia,

Thanks for your reply.

I needed it becoz, I am performing NTHA on same structure with Opensees and another software. I am getting same top displacement and different base shear with two software. If I calculate effective modal masses manually and multiply with relative mode acceleration and sum them i m getting the result matching Opensees (following Chopra Example). Whereas from other software i m getting different base shear and different effective modal masses.

Thats why I wanted to know if I can get it directly from the OpenSees to validate my calculations and find the reason for difference in two software.

If there is some way you could help.

silvia · Post by **silvia** » Mon Oct 06, 2008 5:15 am

it is hard to have exactly the same models.

kishor · Post by **kishor** » Mon Oct 06, 2008 2:46 pm

Dear Silvia,
Can you please give me tips on how to calculate effective modal mass as well as response of each mode in time history analysis in Post processing?

silvia · Post by **silvia** » Mon Oct 06, 2008 8:18 pm

i hope someone here can help, i haven't done it nor looked at that dynamics in a long time.

kishor · Post by **kishor** » Wed Oct 08, 2008 10:46 am

Thank You!
I came with a similar question.
Can we do Time history analysis considering only first five modes in OpenSees?

silvia · Post by **silvia** » Wed Oct 08, 2008 3:03 pm

if you model those only.

bayram_aygun · Post by **bayram_aygun** » Thu Oct 16, 2008 10:22 am

Hi,

I'm attaching two links which will have explanatory notes on how I tackle the mass participation factor issue in OpenSees. Most of the codes are NOT written by me and I have to admit that the codes might have to be modified according to your individual needs.

www[dot]owlnet.rice.edu/~ba2/Modal_Analysis_OpenSees_Forum.pdf

www[dot]owlnet.rice.edu/~ba2/Modal_Analysis_OpenSees_Forum.doc

The point I'm posting these is not that I claim these codes to be a universal answer to your individual problems but I want to share the information I have on the mass_participation issue. That's all it is.

Please don't expect everything to be very straightforward and treat the notes as a "Buffet" and try to "eat" as much as you can.

Hope this helps a couple of people out there,

I will try to do my best to answer any questions which might arise...

Thanks,