IBC Presumptive Load Bearing Values and Safety Factors

[Cpw628]

I am wondering if other engineers use safety factors on the 2021 IBC presumptive load bearing values or overturning checks. Everyone at my office just uses a 1.5 safety factor on all overturning, sliding, and soil bearing checks, but I am wondering if that is too conservative. No one was able to point me to where the 1.5 came from. The only place in the IBC that I could find similar to this is a 1.5 safety factor for sliding and overturning (not bearing) on retaining walls per IBC 1807.2.3. I am not sure why this is explicitly stated for retaining walls, but not shallow foundations.

Is there something else that I am missing? Do most engineers still use the safety factors for mat foundations or spread footings? I've heard that the presumptive values are already pretty conservative, with factors of 2-3 already included, so it would make sense not to use them. On the other hand, it seems very unconservative to not use a safety factor on overturning for shallow foundations.

 

[phamENG]

For building foundations, those safety factors are now baked into the load combinations. Use the search function and you'll find several threads discussing the topic.

Do I apply safety factors to the IBC presumptive values? No. Those are meant to be "allowable" bearing pressures (subject to verification), so safety factor is already applied.

 

[lexpatrie]

It's in the UBC track it from there, but I think it was a 2/3 in the technical sense not (1/1.5).

 

[lexpatrie]

I happened to be in the 1986 SFBC, so here's the relevant provision there, which might help find it in the UBC.

item (d) is pretty interesting, I'm not sure I've seen formal language that specific about weight of earth superimposed over footings or anchors in other codes.