Seismic Load Input

[tdunaway]

Would it be accurate to create a seimic load case and input say .04 into the X or Z gravity field as apposed to calculating seismic forces for each column node and inputing by hand?

(assuming your horizontal seismic load is .04W)

 

[JoshPlumSE]

No, I don't think that would be accurate for the following reasons.

1) That would only succeed in capturing the seismic load from the structure self weight (as opposed to any of the applied dead load or the portion of live load that the code requires you to include in your seismic weight).

2) Even if the seismic loading were entirely due to self weight, it still wouldn't represent an accurate distribution of seismic load. Your base shear may be correct, but the portion of the base shear applied at the various floor levels would not be. Therefore, the total overturning moment a the base of the structure would not be correct.

 

[tdunaway]

1. That's a good point. Thanks for the input.

2. Why wouldn't it distribute the seismic correctly? It seems to me, that it would apply the appropriate portion of member self weight at the center of gravity of each member. Not really sure how RISA calculates gravity loads though. Any insite?

 

[JoshPlumSE]

Yes, the program would assign the 0.04g acceleration at the center of gravity of the element. But, that's not what the code usually wants you to do.

The assumed acceleration profile in the code varies the acceleration based on the height of the floor level. Therefore, the mass at the upper portion of the structure gets a disproportionate level of the acceleration.

At least that is the way it is in seismic country. For Seismic Design Category, there may be allowances where you can assume a uniform accleration.

Josh

 

[Stazz]

I second tdunaway's second comment.