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Foundation Design and LLR

Foundation Design and LLR

Foundation Design and LLR

(OP)
I don’t really work with many projects where I can take LLR. I have a project where it is possible and at my column base I have reaction of

Dead=75
Live = 35 (includes LLR)
Snow = 15

So, to design the foundation in accordance with equate 16-10 of IBC2009 I would use 1.0D + 0.75 L + 0.75 S = 113 kips.

Just want to make sure this is correct.

RE: Foundation Design and LLR

(OP)
I guess what I am worried about is the fact that it appears as if you are double dipping with the LLR and the load combinations.

RE: Foundation Design and LLR

I see the LLR (Load Load Reduced, not Live Load Roof) as saying: If you're having a party, you wont have the live load on a huge area all at the same time.
I see the load combination as saying: if there's a huge blizzard, you won't be having a party at the same time.

I wouldn't hesitate to utilize both reductions.

RE: Foundation Design and LLR

I'd use both as well. The 0.75L is 0.75 x whatever your calculated live load is, which is allowed to be reduced per the code provisions. The Code is probabilistic with respect to risk, and doesn't require designing for the full impact of two unlikely events simultaneously. It's roughly the same reason that there's no load combination that includes wind and seismic forces.

RE: Foundation Design and LLR

Yes - sorry - I failed to grasp the question - using a reduced live load AND using the 0.75 in that load combination with snow is OK. You still have the D+(Red LL) combination as well.

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RE: Foundation Design and LLR

Yes, you can use 0.75 with a reduced live load.

Your combinations to check are:

2) D + L = 75 + 35 = 110

4) D + 0.75L + 0.75S = 75 + 0.75(35) + 0.75(15) = 112.5

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