Spread Footing OT moment SF
Spread Footing OT moment SF
(OP)
Is stated in any code that the 1.5 SF for overturning is required? Using the ASD Basic Load Combos or the Alternative Load Combos.
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Spread Footing OT moment SF
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RE: Spread Footing OT moment SF
RE: Spread Footing OT moment SF
Mike McCann
MMC Engineering
RE: Spread Footing OT moment SF
ASD: 0.6D+W
LRFD: 0.9D+1.6W
RE: Spread Footing OT moment SF
RE: Spread Footing OT moment SF
It seems to me that the factor of sefety of a spread footing against overturning and sliding is built into the allowable soil bearing pressure.
You might ask your geotech if my thesis is correct.
RE: Spread Footing OT moment SF
Different limit states there. The Allowable soil pressure has a safety factor against soil failure under load.
The overturning SF has to do with global stability of the overall structure and doesn't rely directly on the soil bearing capacity.
RE: Spread Footing OT moment SF
I should have added a comment that I agree with your observation that the load combinations( ASD: (0.6D + W) take care of the safety factor of the building to the foundation itself. That same load combination should be used to design the spread footing. Sooner or later that load combination gets to the soil and it dosen't need an additional safety factor against overturning or sliding of the footing.
RE: Spread Footing OT moment SF
Mike McCann
MMC Engineering
RE: Spread Footing OT moment SF
"No safety factors have been applied to these loads, because such factors depend on the design philosophy adopted by the particular material specification" Sec -C2.4.1
While I understand that the commentary may not be considered "part of the code" it does serve a clarifying function.
As far as applying the 1.5 FS for global stability checks I can only find it for retaining walls.
RE: Spread Footing OT moment SF
Mike McCann
MMC Engineering
RE: Spread Footing OT moment SF
There is no 0.6D + 1.6W. The two load combinations are:
0.6D + W (ASD)
0.9D + 1.6W (LRFD)
The commentary section you refer to is stating that the ASD combinations do not have any safety factor applied because these are ASD combinations in 2.4.1. (LRFD is in 2.3). They are simply stating the obvious, that ASD applies its strength safety factors to the resistance side of the equation.
If you read a bit further in the commentary, it says:
"Load combinations 7 and 8 were new to the 1998 edition of ASCE 7. They address the situation in which the effects of lateral or uplift forces counteract the effect of gravity loads. (i.e. overturning) This eliminates an inconsistency in the treatment of counteracting loads in allowable stress design and strength design, and emphasizes the importance of checking stability."
It's pretty clear to me that since the 1.5 overturning "safety factor" disappeared from the code the same time load cases 7 and 8 appeared, that this is the overturning check.
RE: Spread Footing OT moment SF
Nice job of "speaking with data". Dr. W. Edwards Deming would be proud of you.
RE: Spread Footing OT moment SF
JWB
RE: Spread Footing OT moment SF
Sorry for the typo. I was refering to the ASD equation.
I agree the commentary states the obvious. Simply because a factor is now applied to an ASD load combination does not mean a factor of safety is included.
It is my opinion that the load combinations simply determine the most critical load effects to be considered and do not apply a factor of safety for the designer.
If one chooses to take that load effect and not apply a further factor of safety, so be it.
I choose to apply the 1.5 FS. Maybe I am simply being over-conservative, maybe I'm stuck in old code mode or maybe I am just too used to designing to the UFC which includes the stability check with the 1.5 FS when using ASCE 7 load combos.
RE: Spread Footing OT moment SF
Nothing wrong with being conservative. But I have seen and heard that the 0.6D + W was intended to replace the traditional 1.5 OT safety factor.
If you can - post the UFC code provisions. I'd be interested if the 1.5 is really placed on top of the OT load combination.
RE: Spread Footing OT moment SF