Load Combinations

SteelPE · Mar 27, 2014

I have a case where I have a large heavily loaded cantilever that is doing some weird things to my structure. I am currently trying to design my foundation and I am having a hard time with the load combinations found in IBC 2009. I have a case where the "anchor" is resisting uplift load from the cantilever. I have other loads on the column (dead, live and snow). Now when it comes time to check the IBC load combinations I was all set to use 0.6D but that is only a combination for Wind and Seismic not for D+L+S. I am probably going to end up using 0.6D but I am just wondering what others would do in this instance?

JoshPlumSE · Mar 27, 2014

I agree with your engineering judgement in this case. I would also tend to use 0.6DL.

I believe there are some notes saying that you can use 0.9DL for some of the other load combinations whenever dead load has a stabilizing effect. Then if you apply a 1.5 safety factor (i.e. you use the method from the old code) then you arrive at a de-facto 0.6 DL.

AELLC · Mar 27, 2014

= 0.9*1/1.5 = 0.6

Lion06 · Mar 27, 2014

I take the same approach as you, SteelPE. I have even gone so far as to use 0.9DL + 1.4DL for DL for certain cases. For example, when I have a dead load that is causing overturning. I will use 0.9DL for the axial portion (stabilizing effect) and 1.4DL for the overturning portion (destabilizing effect).

Even if it's not an explicit load combination, I want to meet the intent of the combinations. My example may be a bit over the top, but it was a critical condition with an unreinforced concrete member.

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