topic Seismic Analysis- no activating mass in Robot Structural Analysis Forum

Seismic Analysis- no activating mass

Anonymous — Mon, 19 Feb 2018 16:22:55 GMT

Hi,

I'm working on seismic analysis and I have a problem with activating the mass of the construction when I do "Modal".

I think I'm making a stupid mistake...

Can you please look at the model?

Link:

www.filemail.com/t/d6I6iLXB

Thank you in advance for any help you can provide.

Maja

Re: Seismic Analysis- no activating mass

Rafacascudo — Mon, 19 Feb 2018 20:06:01 GMT

Raise the number of modes to 200 and it will get better.

Probably not enough for seismic analysis , but I cannot help you about it , as I have never done a single one in my life as there are no quakes in Brazil.

Maybe you can find some help from official support here or maybe there is something on this official webinar ,number 7, here Build your Robot Structural Analysis IQ - YouTube

Other than that your top slab (panel 138) is disconnected from the 2 walls (panels 128 and 129).

Just remesh the slab and it will connect to both walls.

Re: Seismic Analysis- no activating mass

Anonymous — Mon, 19 Feb 2018 20:16:41 GMT

Unfortunately, not not enough for seismic analysis... Number of modes is too big...

Tnx for panels 😉 I'll fix it.

Re: Seismic Analysis- no activating mass

Simau — Mon, 19 Feb 2018 21:09:22 GMT

Hi @Anonymous

Your model is correct.

Your building is only 2 stories and you too much walls, so your building is too stiff.

No mass will be activated in the lowest frequencies; only vertical direction of seism will activat some part of slab.

I wonder if it's a real project, and if so, you are lucky to have you're lucky to have so much thick walls in your project.

Nevertheless, there is still some activated mass in the first modes

Re: Seismic Analysis- no activating mass

Muhammed.OPERA — Mon, 19 Feb 2018 22:11:05 GMT

Hi @Anonymous

Like as @Simau have said, this is two story building, it's too stiff too rigid.

But i have something for you >

I didn't recognize the meaning of the load types names but i know that the:

1st representing own weight & the 2nd representing the cover or the wall weight & the 5th representing Live load.

In the Analysis type > Load to mass conversion.

You shouldn't add the 1st case of the own weight as it's taken automatically cause you haven't checked the "Disregard density" in the modal parameters so the own weight now is taken twice [ just delete the 1st case from conversion].

Finally to make you feel comfortable with that : here in Egypt we don't pay attention to seismic analysis only if the building is more than 4 stories

Re: Seismic Analysis- no activating mass

Anonymous — Tue, 20 Feb 2018 16:54:26 GMT

Of course too stiff; even when I delete the bottom raft and go over 200 modes, even with Ritz method.

Arthur will tell us why?

Re: Seismic Analysis- no activating mass

Artur.Kosakowski — Wed, 21 Feb 2018 08:52:34 GMT

Hi @Anonymous

As the structure is low and long and has got rigid walls (300 mm of thickness) with much more flexible floors (150 mm) the vertical shapes of vibrations are found first which causes the need to increase the number of modes to reach significant mass participation in the horizontal directions. You can easily see that switching the Z direction off in the parameters of the modal case. If you decrease the thickness of walls (and define supports under the raft foundation which seem to be missing) the required number of modes will decrease too.

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

Re: Seismic Analysis- no activating mass

Anonymous — Wed, 21 Feb 2018 08:59:22 GMT

Thx @Artur.Kosakowski

The expaination is clear! I appreciate.

Re: Seismic Analysis- no activating mass

Anonymous — Thu, 22 Feb 2018 20:21:03 GMT

Hi @Simau, actually this is a test model... But something very similar will be a real project.

Hi @Muhammed.OPERA, I agree that the own weigt is taken twice and "disregard density" help it. 🙂
Also, that Egypt sesmic analysis is very interesting...in Croatia we pay attention to every type of building.

And for the and, hi @Artur.Kosakowski... Your explanation really helped me to better understand the whole situation. Thank you very much for that.