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Regarding Seismic Mass

 
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rpyadagere
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PostPosted: Fri Oct 06, 2017 1:45 pm    Post subject: Regarding Seismic Mass Reply with quote

Dear All,

While arriving at the seismic mass we consider 25% to 50% of live load depending upon its intensity. Once the seismic mass is defined we use this mass for analysis. My question is regarding load combination of 0.9DL+/-1.5EQ. Why we consider live load portion for seismic mass for this load combination? I believe this load combination applies for seismic during structure is not occupied. Seismic mass here is only structure self weight. Kindly share your views.
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kartikeya.structure
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PostPosted: Sat Oct 07, 2017 3:31 pm    Post subject: Regarding Seismic Mass Reply with quote

Hey ,

You have interpreted it right. One does not need to include live load while calculating seismic weight for the 0.DLl+-1.5EQ loadcase.
People have been doing it just to avoid making the task complexer.
But it can affect the economicsof the structure by upto 15%.
So as a sound structural analyst and engineer, one should do analysis with two seismic definitions or two seperate analysis.

Hope it answers your query.
For more clarity on the load combinations try exploring IRS or IRC codes.

Cheers
Kartikeya Gupta
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paresh_kothari
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PostPosted: Sat Oct 07, 2017 5:11 pm    Post subject: Reply with quote

Dear rpyadagere,
Nice observation,
You are right. For the load case where Live load is not present in combination, so we should not consider the live load in seismic masses.

I never did the above exercise.

Thanks,

Regards,
Paresh Kothari
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sakumar79
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Joined: 18 Apr 2008
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PostPosted: Sun Oct 08, 2017 5:48 am    Post subject: Reply with quote

Dear Sirs,
   I agree that logically for a combination with only DL and EL, the inclusion of Live Load component in seismic mass appears to be incorrect. But I am not aware of any provision in our code which allows two separate seismic masses to be calculated. Is there any such provision in any international code? Or perhaps even in any book?

   Seismic Resistant Design is by itself a probabilistic solution where the actual effect of earthquake is not known. Considering this, I feel it is better to be on the safer side and consider the effect of LL in seismic mass even for load combinations excluding LL unless there is clear codal provision or a good reference for the same. Otherwise, in the case of peer reviews, there are likely to be disputes.

   This is my humble opinion.

Yours sincerely
Arunkumar
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kartikeya.structure
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PostPosted: Mon Oct 09, 2017 6:36 am    Post subject: Reply with quote

Dear Arun,

It is always preferable to be precautious. In structural engineering, we have to agree that we face exhausting technical challenges but there is even bigger economics involved. Seismic coefficient (Luxury coefficient as I call it)to be taken will always be as per economic stature of the project and there is no exact resolution for this God made force.

We cant let rudimentary practice to invalidate the meticuluously run practice.

There is no provision of taking live load in seimic weight calculation for such cases in IRC and IRS codes. As these days, we have ample of road works and rail works going on. We can always give reference to atleast to these Indian codes to present our case.

Codes always give guideleines parallel to commonly faced cases. It is always going to be the job of  analysis and design structural engineer to construe it in a  right  wayand pragmatic way.


Regards
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