Well...I think the 1.5 has recently been replaced by load combinations that include a hefty safety factor for overturning - the IBC for instance has removed the 1.5 and included in it place the following two load combinations:
I kind of agree with JAE. And I should have stated for WIND effects. Ron can you point where it talks about in the IBC? I can not find it, not even in the ASCE. There are requirements for seismic but not wind.
StruKturdg,
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.
JAE,
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.
I disagree that the ASD Basic Load Combination 0.6D + 1.6W includes any factor of safety. These combinations appear in ASCE 7-05 (and earlier) and the commentary states in black and white;
"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.
Remember that the code is an engineering minimum, not a strict guideline here. If professionally you feel that the design is more to the intent of the code by raising the bar, then do it.
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:
[blue]"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.[/blue] (i.e. overturning)[blue] This eliminates an inconsistency in the treatment of counteracting loads in allowable stress design and strength design, and emphasizes the importance of checking stability."[/blue]
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.
Something to be aware of is that the default factor of safety for overturning on many foundation software packages is 1.5. I had this issue with Enercalc yesterday while checking an existing footing. The controlling issue was overturning. Once I reset the factor of safety to 1.0 the footing worked for overturning.
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.
that ASD applies its strength safety factors to the resistance side of the equation.
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.
Here is the relavent paragraph from the UFC. I cannot find similar verbiage in the IBC, except for retaining walls specifically. Hmmmm
1-12. STABILITY. The building foundation must be capable of safely transferring all vertical
and horizontal forces, due to specified design load combinations, to the supporting soil or rock.
The mechanism used for the transmission of horizontal forces may be friction between the
bottom of the footing and ground, friction between the floor slab and ground, and /or lateral
resistance of soil against vertical surfaces of grade beams, basement walls, footings, piles, or
pile caps. Net upward forces on footings and piles, which must be resisted to prevent
overturning and/or flotation, will be considered in the foundation design. Dead load should
include the benefits of weight of the overlying fill in resisting sliding, overturning, and flotation.
Structures will be designed to resist overturning effects caused by seismic forces in
accordance with TI 809-04. For load combinations other than earthquake when load
combinations applicable to allowable stress design (ASCE 7, paragraph 2.4.1) are used for the
foundation design, the building will have a minimum safety factor of 1.5 against sliding,
overturning, and flotation.
