topic Re: Neglecting basement's masses in Robot Structural Analysis Forum

Neglecting basement's masses

Tuctas — Wed, 21 Mar 2018 17:20:09 GMT

I am trying to deactivate the masses that correspond to the basement elements (in order not to participate in the superstructure's seismic oscilation) by creating 2 new load cases named 131, 132 that will be converted to mass (which they don't participate to any load combination, they are just created for this purpose), but the results are not satisfactoty. The Total mass in the Dynamic Analysis Results table after this procedure is not the same when deleting completely the basement.

Thank you.

Re: Neglecting basement's masses

Anonymous — Wed, 21 Mar 2018 19:24:38 GMT

I think they are different, because a different form of own fluctuations with the foundation and without it. I would use the file where there is more seimic load.

Re: Neglecting basement's masses

Simau — Wed, 21 Mar 2018 20:59:25 GMT

Hi @Tuctas

You didn't take into account the 2nd level dead load.

Re: Neglecting basement's masses

Refaat — Wed, 21 Mar 2018 21:46:17 GMT

Hi @Tuctas

From the first glance. You had defined the self-weight case of the superstructure a twice (case 1 as whole structure) and (case 132 as part of structure). So all what you need to just redefine (case 1 as part of structure for substructure) as indicated in the attached capture.

Refaat

Re: Neglecting basement's masses

Tuctas — Thu, 22 Mar 2018 06:48:03 GMT

Thanks for the answer,

I didn't want to take into account the basement's roof (actually, its mass won't be activated due to the superstructure's oscilation).

I am just asking about the way of neglecting the whole basement's mass not if it is o.k or not (the later is my choice..:-)). Furthermore, the mass after the procedure that i've done with the creation of the 2 new load cases is larger than the case of deleting completely the basement.

Re: Neglecting basement's masses

Tuctas — Thu, 22 Mar 2018 06:58:40 GMT

Hi Refaat,

Actually, i did want to take into account twice the selfweight of the superstructure, once in the case that it should participate in the load combinations (as a whole structure) and then in the case of being converted in to mass (as a part of a structure, i.e only the superstructure).

Normally, when the basement's selfweight is being taken into account in the load case no 1 that won't be converted in to mass (as for this a have create and assigned to the proper field only the 2 new load cases), it shouldn't be taken into account in the eigen analysis at all.

Maybe (it just came into my mind) i should assign the 2 new load cases as auxiliary?

Re: Neglecting basement's masses

Anonymous — Thu, 22 Mar 2018 08:24:06 GMT

I think that deleting completly the basement is more appropriate...;

Re: Neglecting basement's masses

Tuctas — Thu, 22 Mar 2018 08:57:53 GMT

Yes, but then we should do hand calculations for the basement + foundation...

Re: Neglecting basement's masses

Anonymous — Thu, 22 Mar 2018 09:55:44 GMT

Why do you want to skip the basement participation in the global seismic response?

Re: Neglecting basement's masses

Tuctas — Thu, 22 Mar 2018 10:03:47 GMT

Because if you don't you will have a wrong picture of the superstructure's oscilation...

Re: Neglecting basement's masses

Anonymous — Thu, 22 Mar 2018 10:37:10 GMT

When you compare the frequencies with the basement et without the different is not too big at all. i would just include the monomobe okabe thrust on the basement wall, and considder the global structure in my seismic analysis...no?

Re: Neglecting basement's masses

Tuctas — Thu, 22 Mar 2018 13:29:04 GMT

It is about Dynamic analysis (particulary, eigen analysis). We can discuss it in another post, but here i would like to stay focused on the initial question.

Re: Neglecting basement's masses

Artur.Kosakowski — Thu, 22 Mar 2018 13:49:03 GMT

Hi @Tuctas

The Total mass in the Dynamic Analysis Results table after this procedure is not the same when deleting completely the basement.

I can't see any difference:

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

Re: Neglecting basement's masses

Tuctas — Thu, 22 Mar 2018 14:08:10 GMT

Hi @Anonymous,

The table with the results of the Dynamic Analysis when not deleting the basement is as shown in the attached, that is quite different with the table that you attached which corresponds to the case of deleting completely the basement.

Re: Neglecting basement's masses

Artur.Kosakowski — Thu, 22 Mar 2018 14:16:28 GMT

Hi @Tuctas

Is it the different model or RSA version?

Re: Neglecting basement's masses

Anonymous — Thu, 22 Mar 2018 15:24:11 GMT

Hi @Tuctas

I tried to create a test model and calculate the total mass. and the mass is taken into account for elements even when the dead weight is not specified. I think the essence of the problem here

Re: Neglecting basement's masses

Artur.Kosakowski — Thu, 22 Mar 2018 15:38:23 GMT

HIi @Anonymous

Have you marked the disregard density in the parameters of the modal case?

Re: Neglecting basement's masses

Tuctas — Thu, 22 Mar 2018 15:53:44 GMT

Hi Artur,

I faced a problem with my connection in which i couldn't reply until now.

I got suprised with your answer! I am using ARSA2013, is there such a bug..?

Re: Neglecting basement's masses

Anonymous — Thu, 22 Mar 2018 16:13:19 GMT

Hi @Artur.Kosakowski

No, density was used there, if you remove the density invasion in the setting of the modal analysis, then the mass does not appear. Thanks for the tip !!

Re: Neglecting basement's masses

Simau — Thu, 22 Mar 2018 22:11:15 GMT

Hi @Tuctas

I do not understand the role of this slab;

Why not continius footing under shear walls?