foundation uplift safety factor

huangyhg

foundation uplift safety factor
i have always been instructed to us a 2 to 1 saftey factor when designing concrete footing/foundations to resist uplift. is this in any code (florida)?
asce7 requires a multiplication factor of 0.6 for dead loads resisting asd wind uplift. maybe that is where the 2 factor comes in.
who instructed you to do that?  it might just be company policy.  the fbc does not require a safety factor of 2 explicitly though you may use what you like if it meets the code minimum.
i have used 1.5 against uplift my entire career. it is not very hard to accomplish. using a safety factor of 2:1 sounds a bit conservative.
i heard all kinds of arguments through the years; but i feel 1.5 is a good safety factor against uplift and sliding.
regards,
lutfi
overturning failure can be defined different ways.  one of the standards i use requires the design moment be multiplied by 1.5, then the foundation checked against ultimate bearing.  this will usually work out to a factor of safety around 1.8 based on rigid-body-overturn.
in some cases, it is desired to avoid any theoretical uplift, and that will imply a safety factor compared to rigid-body-overturn of perhaps 2.0 to 4.0, and varies with the shape of the foundation.
i have always used 1.5 safety factor, also.  
i've been taught 1.5 is the standard but where does this come from?
i do feel combining 0.6*dead and the 1.5 safety factor is a little conservative although it is the approach i typically take.
the 1.5 safety factor for overturning has been specified by building codes for decades.  leafing through the codes on my shelf, there are many examples...
1946 ubc sec. 2307: "the overturning moment from the wind pressure shall in no case exceed two-thirds of the dead load resisting moment."
2003 ibc sec. 1806.1: "retaining walls shall be designed for a safety factor of 1.5 against lateral sliding and overturning."
i could go on, but you get the point...
you should use 1.5-2. 1 is not a safety factor. you can find almost all the information you need to design foundations on the following army coe document (found free on the internet):
tm 5-818-1 : soils and geology procedures for foundation design of buildings and other structures (except hydraulic structures)
in the past i always used a factor of safety of 1.5 along with the dl likely to be in place (not overestimating the amount of dl).  i interpret the ibc to require a factor of safety of 1.66, since their load case is 0.6dl to resist uplift.  again, this is using the dl likely to be in place, not overestimating the amount.  the only justification i can see for using both the 0.6 factor and 1.5 would be if you are using a very conservative dl and expect the 0.6 factor to reduce it what is likely in place.