Seismic load combination

Question

Seismic load combination

SHBH (Structural)

(OP)

3 Apr 07 07:52

Hi,
I was checking cl 12.4.2.3 for seismic load combinations, in which 0.2Sds is used to account for the seismic load in vertical direction. For rest of the horizontal seismic loads , one can use 100% in X dir and 30% in Z dir (seismic design cat D).
What if 100% is in the vertical direction, should it then be 100%Vertical + 30% hor-X + 30%hor-Z
I have seen this combination to be used in some other design offices, but where is the ref in code.

JAE (Structural) · Answer 1 · 2007-04-03 09:23:36

100% Vert + 30%X + 30%Z?? No.

The primary seismic is always either X or Z. The vertical is just an added seismic effect that accompanies the X or Z.

mrMikee (Structural) · Answer 2 · 2007-04-03 10:45:26

I've seen something like this before and I think it could possibly have come from ASCE 4 for "Seismic Analysis of Safety-Related Nuclear Structures" although I can't find it in the spec now. It might have been in other contract specs.

Regards,
-Mike

SHBH (Structural) · Answer 3 · 2007-04-04 08:44:55

Thanks JAE & Mikee,
From this discussion now i can remember that the 100% vertical seismic effect was really mentioned in the nuclear DBR.
so the eccence is vertical has to be 20% if i am using ASCE7 load comb.

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Seismic load combination