Seismic load mass conversion

[mgaafar]

Hi, I was designing a multi-storey structure in RSA, and I was using the equivelant lateral force method for ASCE 7-10.

Now I know that RSA takes the selfweight by default in the seismic calculations and any other added loads in the "Load to mass Conversion" tab.

in my case, I need RSA to ignore the selfweight and use only specified loads in the "Load to mass Conversion" tab.

How can I do that?

Thanks

[dimogrec]

Maybe defining a material with no weight should work...

[StefanoPasquini6790 · Accepted solution]

Hi, quickly and easy.

Create a load case called "self weight_seismic" and one other called "self weight_non seismic". In the first one assign self weight load type selecting, from load table, part of the structure and assigning the list of the bars, panels ecc that compose the seismic mass. Otherwise selct all the non seismic element and assign it to self weight_non seismic load condition.
When you run the modal analysis you have to ignore density and, in the load to mass conversion table, select the self weight_seismic and add to conversion.

This procedure can be used also for dead loads and variable loads.

In the first time you made this I suggest you to check it manually, define first the effective seismic mass than check the robot value in the table. I make it everyday and it's more difficult to explain that to do it.

Hope I help you, greetings.

[mgaafar]

Thanks, So when you say that I have to ignore density, does this mean that I have to create a material with Zero density or there is another way to ignore density. I know you can check the "ignore density" in the modal analysis but there is no such option in the equivelant lateral force window option.

Thanks

[dimogrec]

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@mgaafar wrote:

.... I know you can check the "ignore density" in the modal analysis but there is no such option in the equivelant lateral force window option.

Thanks

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That's the reason I suggested you to set a zeo value on density (weight) of your material. ( You don't have to define new material just edit the excisting one from Tools->job preferences->material->modification.. .)  I don't know if there's an other way for equivalent lateral force method

If "ignore density" tab was available you could you some other tricks as Stefano proposed.

[Anonymous]

Good mornign ,

I am trying to run a modal analysis in Costa Rica Central america

I have an acceleration spectrum loaded but when run analysys forces and displacement become all zero

The  Question is :

To run the analsys do I have to create an additional node at gravitational structure center , assign mass to the the node ?

Is there any mass source node in Robot ?

Thank you very much for your kind answer

[Rafal.Gaweda]

File please.

[Anonymous]

Herein you will find , file and spectrum

[Rafal.Gaweda]

 If your spectrum is correct (max acceleration = 0,5m/s2), and if mass is correct (almost 1,45 mass of self weight (1.0 sw mass from modal caase defintion + 0,45 of case 18 load to mass coversion) + 0,45 load to mass of case 2) so the results are also correct but to see them you have to change scale coeff and units

[Anonymous]

If I can recall, the SW can be ignored using ignore density option in the Modal Analysis. Then Use Convert Loads to masses. Don't use load to masses if you did not uncheck this option or your seismic mass will double