Factor of Safety for Foundation Overturning 2

[Deadblow]

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!

 

[jechols]

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.

 

[Lomarandil]

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.

 

[Deadblow]

Ok, that makes sense. Thank you!

 

[Guastavino]

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

 

[appot]

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.

 

[TLHS]

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

 

[TLHS]

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

 

[mike20793]

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.

 

[Lomarandil]

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.

 

[WillisV]

See my response from...geez...8 years ago: thread507-202887

 

[m1208]

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