Load combinations for foundation sizing

[Tstruct]

According to ACI/UBC/IBC, what load combinations should be used to check foundation soil pressure against bearing capacity for the following cases:
1. Considering only gravity loads
2. Considering both earth quake and gravity loads

In my area engineers use D+L for the first case and D+L+0.714EQ for the second case with 33% exceedance allowed (from allowable bearing capacity).
I am unable to find references for the above. Please point me to the reference.

Thanks

 

[lexpatrie]

The 33% stress increase is really old and unless the alternate load cases are still around, this was effectively removed circa 2000 IBC. The current load cases have a 3/4 on paired or more transient loads (L, Lr, S, vertical EQ) that would potentially apply.

 

[Eng16080]

To answer your questions, according to IBC 2018, I would use:

1. Equation 16-9: D + H + F + L
2. Equation 16-14: D + H + F + 0.75(0.7E) + 0.75L + 0.75L

H is the soil pressure load which, per Exception 3 of IBC Section 1605.3.1, will have a load factor of 1.0 if it adds to the overall load effect, or 0.6 if it resists it and the load is permanent
F is a fluid load (like standing water), probably zero in your case

Concerning the 33% increase you mention, I would read Section 1605.3.1.1, which generally prohibits this increase.

It seems that the method you mention above would result in the seismic portion of the load being 36% more while also allowing a 33% stress increase. If the only loads were seismic, then I'd say this method is perhaps more conservative. But since you also have dead and live loads which are not increased in your load combination but which nonetheless benefit from the 33% stress increase, I would say that this method would be non-conservative. I'd recommend not using this method which I believe is a relic from the past.