In ASD load combinations, 0.6D is used in the current codes instead of 0.9D in the older one (i.e. UBC 97). It appears to me that factors of safety for foundation stability(sliding and overturning) can be checked against unity, not 1.5. In practice I use 1.5 but what about for the exam, should I use 1.0 or 1.5? Thanks.
Foundation Overturning
- Thread starter bcy
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I don't seem find an explicit statement of FS>=1.5 for founcations in IBC, except for retaining walls. Would you be able to point me to the IBC section regarding this? Thanks.
Casey
Well-known member
I recently designed a shallow foundation with a overturning FS of 6....
I originally had it at 4, but the client was still a bit worried and asked me to make it a bit bigger, so I did.
Actually, they did complain a bit about the quantity of concrete required (27 cy) and I said I could reduce and they said "no, it's ok..."
The foundation only supports a 100ft high light pole, but the wind gave it a huge overturning moment (300k-ft I think), plus the foundation was about 10 feet away from a 100ft high MSE wall with really expensive operations equipment below (they would lose a couple million each day if it was out of order).
Overkill? Yeah, probably, but I can sleep at night not worrying about my company being sued into oblivion...
Again, I don't care for the minimalist approach and prefer some over design...
I originally had it at 4, but the client was still a bit worried and asked me to make it a bit bigger, so I did.
Actually, they did complain a bit about the quantity of concrete required (27 cy) and I said I could reduce and they said "no, it's ok..."
The foundation only supports a 100ft high light pole, but the wind gave it a huge overturning moment (300k-ft I think), plus the foundation was about 10 feet away from a 100ft high MSE wall with really expensive operations equipment below (they would lose a couple million each day if it was out of order).
Overkill? Yeah, probably, but I can sleep at night not worrying about my company being sued into oblivion...
Again, I don't care for the minimalist approach and prefer some over design...
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Make sure to take out the 0.14SDS for ASD seismic load combinations. THis will often reduce your dead load by 15%, which in turn, will create a larger net moment at the bottom of the footing.
outatime2002
Well-known member
Wow, your client must have alot of disposable cash. I work in the petrochemical industry and our clients are huge oil companies with tons of money. Even these clients are looking for ways to save material cost by forcing us to use seismic importance factors of 1.0 rather than 1.25, etc. We should be using an importance factor of 1.25 since we are designing for structures in occupancy category III (they support sufficient quantities of toxic or explosive substances). Everyone, rich or poor, is looking to save a buck.
I would've held my ground on a F.S. of 4. If you're using a F.S. with the 0.6D+1.0W (or (0.9-0.14Sds)D+rhoQe) load combination, 1.5 is plenty good enough.
