.6D + W for footing bearing pressure?
.6D + W for footing bearing pressure?
(OP)
When designing a footing with moment, do you consider the .6D + W combination from IBC when comparing to the allowable soil bearing pressure as provided in a geotech report? I understand the need to use that combo (.6D + W) when considering sliding & overturning of the particular footing, but am unclear about it's use when it comes to the soil pressure itself. My understanding was that soil pressures are compared to true service loads (1.0 factor on everything).






RE: .6D + W for footing bearing pressure?
Service load does not mean 1.0 for everything. It just means that it is based on the 're-occurrence interval' associated with service level loads. Therefore I would say the 0.6D + 1.0W should apply.
EIT
www.HowToEngineer.com
RE: .6D + W for footing bearing pressure?
For all other load combinations we try to avoid any net uplift on a footing edge.
RE: .6D + W for footing bearing pressure?
RE: .6D + W for footing bearing pressure?
If you are just making sure the soil pressure is not exceeded, I would use the old 0.9D, as the pressure due to dead load will be more, and may be controlling.
Mike McCann
MMC Engineering
RE: .6D + W for footing bearing pressure?
Separate but related - 1807.2 in the IBC allows retaining walls to be designed without the load combinations of 1605 (use 1.0 for all nominal loads except E) but requires a SF of 1.5 for overturning & sliding. What do you guys use for checking bearing pressure at a retaining wall? If you are using the 1.0 loads, do you apply a SF of 1.5 against allowable bearing? If not, why would it not be similar condition for my footing - use the extra safety (either with 1.5 sf or .6 DL) for OT & sliding check but not for bearing check?
RE: .6D + W for footing bearing pressure?
Mike McCann
MMC Engineering
RE: .6D + W for footing bearing pressure?
EIT
www.HowToEngineer.com
RE: .6D + W for footing bearing pressure?
The exception to this would be to pour a small footing the diameter of the shaft at the bottom after the auger is pulled out, and then either install the pole, or pour it to the top and make a moment connection just above the surface of the soil.
Mike McCann
MMC Engineering
RE: .6D + W for footing bearing pressure?
The crux of the question is whether the DL should be marginalized for bearing pressure checks (regardless of if that is done with a 1.5 SF or by using the .6 DL). I agree that it should be done for overturning & sliding, just not sure it should apply to bearing pressure. With the retaining wall design, you are not required to use the .6DL and nothing indicates using a SF of 1.5 for bearing pressure, so I question whether the .6DL really applies to the bearing pressure check for a footing that has a similar moment loading.
RE: .6D + W for footing bearing pressure?
If we are talking about a building isolated pad footing then I would use the 0.6DL + WL and allow for uplift as JAE noted. Even for a retaining wall condition I would do this. I suppose you could use 0.9DL as M^2 suggests as well depending on how 'accurate' you feel the dead load is. In your case of the netting support, it seems like the dead load would be pretty well defined.
However, I think the main question is now - why are you using a rectangular footing for this application. I would think it would be more economical to use a shallow drilled pier. This is probably why you were led to these questions in the first place - cause the footing seems large in order to get your bearing to 'work'. Or maybe not...
EIT
www.HowToEngineer.com
RE: .6D + W for footing bearing pressure?
RE: .6D + W for footing bearing pressure?
If not, limiting the bearing could be achieved with a footing with a larger footprint, up to a point of course. Are you at that point?
Mike McCann
MMC Engineering
RE: .6D + W for footing bearing pressure?
RE: .6D + W for footing bearing pressure?
RE: .6D + W for footing bearing pressure?
RE: .6D + W for footing bearing pressure?
RE: .6D + W for footing bearing pressure?
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RE: .6D + W for footing bearing pressure?
I'd only worry about shear failure of the soil from wind so find out what the allowable pressure is when not considering settlement.
RE: .6D + W for footing bearing pressure?
ASCE 7-95 does not have the 0.6D+W (or 0.6D+0.7E) load combination. For the traditional 1.5 factor of safety against overturning and sliding, they say a couple paragraphs after the load combinations that the overturning moment or sliding force must not be more than two thirds of the stabilizing effects form the dead load. 1/1.5 = 2/3, so this is the traditional way of doing things we’re all familiar with.
ASCE 7-98 does have the 0.6D+W (and 0.6D+0.7E) load combination. Also, the paragraph about overturning and sliding was removed. The Commentary states that these two load combinations were added to remove the inconsistency between ASD and LRFD, and to increase the emphasis on stability checks.
I think an unintended consequence is that the bearing pressure calculated has now been increased by about 1.5. This does not make sense to me. Overturning and sliding are phenomena that occur or don’t. You don’t have “a little” overturning. So you provide a factor of safety of 1.5 to make sure it doesn’t happen.
The allowable soil bearing pressure already had a factor of safety applied to it. There is no need to apply an additional 1.5 to it, like there is for overturning or sliding. A strict reading of the code tells you to compare 0.6D+W to the allowable bearing pressure, but I do not think that was the intent.
It would be nice if the powers that be would clarify this.
RE: .6D + W for footing bearing pressure?
I don't understand what the debate is. It's written in plain English: use the load combination when designing foundation elements.
(ASCE 7-10 has changed the load combination to 0.6D + 0.6W because wind has been changed to a strength case. But it is the same combination as the 0.6D + 1.0W we are talking about)
RE: .6D + W for footing bearing pressure?
Except that retaining walls are given an exception (in IBC 1807.2.3) to revert to the old 1.0 factors and the 1.5 SF. It would appear this also means that bearing pressure calcs for a retaining wall only require the 1.0 factors.
Why then would other footings that have moment conditions require .6D to be used for the bearing check? The inconsistency creates the debate.
RE: .6D + W for footing bearing pressure?
RE: .6D + W for footing bearing pressure?
Here's how I see it.
This is in regards to checking the bearing pressure:
If you are designing a building footing, where you have large trib areas and dead load can vary (aka almost all buildings). Then yes you use the 0.6DL + W and as previously noted some uplift on the footing is probably acceptable.
If you are analyzing a retaining wall condition the concrete and soil could be taken as 1.0 for dead load. However if you have other dead loads applied, say to the stem wall or on the heel then I would analyze both conditions - one with 1.0DL' and one with 0.6DL' where DL' is the weight applied to the stem or heel (not the soil/concrete). Compare the results to the allowable soil bearing values. Once again some uplift is probably acceptable for the 0.6 case.
Side note:
When you go to do the concrete design then I would start to use the load factors and even more debate in my mind lies here (should you factor the applied bearing pressure found with ASD loads, should you apply LRFD and reanalyze to find new applied bearing pressure, what happens if your eccentricity is now too high to perform the analysis, etc.)
EIT
www.HowToEngineer.com
RE: .6D + W for footing bearing pressure?
I don't think anybody is denying that. When you follow the building code, you have to check the bearing pressure using these 0.6D+W load combinations.
The problem is, bearing pressure has not traditionally been checked this way. The change to the code requiring this was made to address stability concerns (overturning and sliding). This appears to be an unintended side effect, and designers are resisting.