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Factor of Safety for Foundation Overturning

Factor of Safety for Foundation Overturning

Factor of Safety for Foundation Overturning

(OP)
Hello All,

I am designing the foundations for a pre-engineered metal building and would like to know what safety factor you all are comfortable to use for overturning checks. I am using IBC 2009 and as far as I can tell, code does not mandate a specific S.F. for the overturning of foundations. I see that in 1807.2.3, the minimum S.F. for retaining wall design is 1.5. I would think 1.5 should be my S.F., but some of these foundations must be very large to meet a S.F. of 1.5.

Thanks for any thoughts!

RE: Factor of Safety for Foundation Overturning

I think you only need to satisfy the load combinations. 0.6D + W or 0.6D + 0.7E. Using the load combinations gives you a factor of safety of 1.67 using service level loads which is D + W or D + 0.7E.

RE: Factor of Safety for Foundation Overturning

Exactly, some load combinations have taken to embedding the 1.5 F.S. against overturning into the combination -- often without a lot of explanation that they're doing it.

RE: Factor of Safety for Foundation Overturning

(OP)
Ok, that makes sense. Thank you!

RE: Factor of Safety for Foundation Overturning

My take is that the load combinations of 0.6D+W, etc. take care of it! Similar for the 0.9

RE: Factor of Safety for Foundation Overturning

What about a case when you only have 60% of your dead load to resist the wind? I do not consider 0.6D + W to be an extreme case where I can take FS=1.0, that is, I use an "additional" factor of safety on my 0.6D + W load case.

RE: Factor of Safety for Foundation Overturning

That isn't a factor of safety of one, though? You have an inherent factor of safety in your dead load reduction.

RE: Factor of Safety for Foundation Overturning

I'm not bothering to double check the way the limit states factors work in the IBC at the moment, but as an extra point of clarification I'm assuming it's a 1.5 factor of safety against overturning on top of an assumption that reducing the dead load to 90% makes it a conservative value. This would be generally in line with the way it works in Canada.

You're basically checking:

(0.9 * Dead) / Wind >= 1.5

RE: Factor of Safety for Foundation Overturning

0.9/0.6 = 1.5. As mentioned above, the load combination of 0.6DL+WL has the factor of safety built in, they just do a very poor job of explaining that in the code. If you're using unfactored loads, then yes, you want the factor of safety of 1.5.

RE: Factor of Safety for Foundation Overturning

Swiver -- depends how dead your dead load actually is. I've also seen the 0.9 value as a conservative (or realistic) proportion of DL to use when it is beneficial to your system.

RE: Factor of Safety for Foundation Overturning

I have used the 1.5 FS for overturning since late 1970's. I only use 1.0 FS for foundation sliding.

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