Seismic Load Combinations
Seismic Load Combinations
(OP)
In reference to this problem:
http://shop.iccsafe.org/media/wysiwyg/material/901...
For the given values of : ρ = 1.3, SDS = 1.10, the seismic load combinations are:
1.2D + 1.3QE + (0.2)(1.1)D + 0.5L = 1.42D + 1.3QE + 0.5L (Comb. 5)
when the signs of QE and D are the same, and
1.2D + 1.3QE − (0.2)(1.1)D + 0.5L = 0.98D + 1.3QE + 0.5L (Comb. 6)
when the signs of QE and D are opposite.
0.9D + 1.3QE + (0.2)(1.1)D = 1.12D + 1.3QE (Comb. 7)
when the signs of QE and D are the same, and
0.9D + 1.3QE − (0.2)(1.1)D = 0.68D + 1.3QE (Comb. 8)
when the signs of QE and D are opposite.
Among these combinations, Comb. 5 & 8 governs (according to my reference).
My question is: Between combinations 6 and 8, why does combination 8 governs, when both have 1.3 QE, and Comb. 6 having 0.98D compared to 0.68D of Comb. 8?
I just got stuck here! Thank you so much!
http://shop.iccsafe.org/media/wysiwyg/material/901...
For the given values of : ρ = 1.3, SDS = 1.10, the seismic load combinations are:
1.2D + 1.3QE + (0.2)(1.1)D + 0.5L = 1.42D + 1.3QE + 0.5L (Comb. 5)
when the signs of QE and D are the same, and
1.2D + 1.3QE − (0.2)(1.1)D + 0.5L = 0.98D + 1.3QE + 0.5L (Comb. 6)
when the signs of QE and D are opposite.
0.9D + 1.3QE + (0.2)(1.1)D = 1.12D + 1.3QE (Comb. 7)
when the signs of QE and D are the same, and
0.9D + 1.3QE − (0.2)(1.1)D = 0.68D + 1.3QE (Comb. 8)
when the signs of QE and D are opposite.
Among these combinations, Comb. 5 & 8 governs (according to my reference).
My question is: Between combinations 6 and 8, why does combination 8 governs, when both have 1.3 QE, and Comb. 6 having 0.98D compared to 0.68D of Comb. 8?
I just got stuck here! Thank you so much!






RE: Seismic Load Combinations
RE: Seismic Load Combinations
Just thinking lateral for a second. If you push on top left corner you'll be pushing downward on right support and pulling upwards on left support. When you then add gravity loads (dead/live) in, the gravity loads will pushing down on both supports. So your gravity load is in the same direction as your lateral for the right support, but opposite for your left. That's why the two answers that are correct are the one with the most gravity load (1.42D+0.5L) and the least gravity load (0.68D). They'll give you the worst case downwards and uplift reactions, respectively.
Either combination 5 or 8 (as you've numbered them, numbering is different in the example you provided) will always control for lateral earthquake cases. 6 and 7 (again as you've numbered them) never will.
RE: Seismic Load Combinations
Kamal Safa, PhD
Ecole Polytechnique
RE: Seismic Load Combinations
To edit my response, I was referring to your load combination 8. I generally wouldn't do the load combinations 6 and 7 that you have shown. I normally only add the 0.2SdsDL to the 1.2DL case and subtract it in the 0.9DL case.