Footing Uplift 5

[slickdeals]

Folks,
When you calculate the uplift load on a footing, the code requires the use 0.6D + W. Assume this gives a net uplift of 30 kips.

When you calculate your resistance to uplift in terms of the footing weight and the weight of a truncated soil pyramid (based on a 30 degree angle), do you use a 0.6 factor on the uplift resistance and compare it to the above calculated uplift of 30 kips?

It seems like double dipping (very conservative) to use a 0.6 factor on the resistance side also. Thoughts?

 

[dcarr82775]

The 0.6 applies to DL and soil load resisting uplift. There is no double dip so to speak.

Example:

- Uplift due to Wind = 50 kips
- assume resisting load of Dead+soil = 100 kips

100*.06 = 60 kips > 50 kips so ok.

 

[Lion06]

Agree with dcarr, the bottom of the footing is subject to rhe combination 0.6D + W and you can't have a net uplift. I agree it's conservative, but it's at least partially to account for the F.S. of 1.5 that is no longer required.

 

[msquared48]

Used to be .9D + W with a FS of 1.5. The relative degree of conservatism depends on the relationship between the allowed dead lload and the wind uplift.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[frv]

I think Mike has it right..

I've seen some engineers use 0.6DL+1WL and further use a 1.5 factor of safety.. That would be double dipping.

 

[BAretired]

Yes, but they are double dipping on the right side...conservatism.

BA

 

[slickdeals]

Actually my question was more regarding using the 0.6D factor on the weight of the footing and soil.

I was thinking along the lines of calculating the uplift on the superstructure above the footing based on 0.6 D and comparing it to the resistance based on 1.0 * (weight of footing and soil) and providing a F.o.S of 1.1 or so.

But I think the consensus is that I may be violating the letter (or spirit?) of the code.

 

[ToadJones]

I see what you are saying Slick, and I have done it in the past, but usually when doing so I still had a pretty big F.O.S.
I never saw a reason to multiply the actually footing weight x 0.6.
The soil maybe, as it could be excavated. My biggest concern would be the dead loads of and on the superstructure.

 

[SteelPE]

I think this was covered before:

http://www.eng-tips.com/viewthread.cfm?qid=183966&page=1

 

[dcarr82775]

The new is the same as the old : (1/0.6)*0.9 = 1.5

They just re-wrote 0.9D with a 1.5 factor of safety into the standard load combination equation form.

There has always been a 1.5 factor against overturning conservative as it may be it has always been there and you need to follow the code.

 

[SteelPE]

Here is the article from Structure:

http://www.structuremag.org/OldArchives/2004/september/Codes and Standards.pdf

 

[msquared48]

... Or just run the numbers and prove it to yourself.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rscassar]

I would apply the 0.6 factor to all dead loads which have a stabilizing action on the structure.

 

[ron9876]

The code says 0.6D. It doesn't say 0.6 x the dead load that you would like to use. If you calculate the gross uplift and the total dead load you need a safety factor of 1/0.6=1.67.

I think it is an overkill. In hurricane areas with one story light framed structures the size of interior footings is not reasonable. When hurricane Andrew came thru I don't remember anyone reporting footings failing in uplift and I don't think many of the older buildings considered uplift on footings at all.

I think it is really crazy when you are designing something like a big shearwall pilecap. The design of the concrete and reinforcing steel is done with load factors so they have their safety factors. The piles have a safety factor from the geotechnical side. Then you add the 1/0.6 safety factor for the pile loads and you have safety factor x safety factor. There should be some type of allowance for this condition.

 

[abusementpark]

To kinda pile on to ron9876's post, I think it becomes even more overly conservative when the geotechnical engineer wants you to take the buoyant weight of the footing and soil in consideration for uplift resistance. So then to truly meet to the code, you have to multiply the bouyant weight of the footing and soil by the 0.6 factor. In hurricane country, you can end up with some unreasonably sized footings.

Another monkey in the wrench, is whether or not you consider adhesion on the sides of the footing for uplift resistance. In my area, some geotechnical firms give you an allowable adhesion (usually 500 psf) to consider acting on the sides of the footing for uplift resistance. This can help tremendously and then using the 0.6* buoyant weight of footing and soil becomes less significant as you can usually get a significant contribution from the adhesion.

However, as I said above, for some reason, only a few geotech firms allow you to consider that, which has always been peculiar to me and could be a whole separate thread. I've often found it too conservative to use the 0.6 factor, the buoyant weight, and no consideration of adhesion, which unfortunately would be the "by the book" way one would have to design a footing for uplift in certain instances.

 

[structuresguy]

personally, I always have a problem using the weight of the soil to help hold down the building, adhesion or no. The reason is there is no guarantee what condition the soils will be in when they are most needed. In fact, in hurricane country anyway, it is most likely that the soils will be in their worst condition when you need them. I mean that hurricanes typically happen during the wet season. So there is highly probability that the soils could be saturated during a hurricane. The saturated soils will tend to "flow" off the top of the footing as it lifts up, due to the saturated condition, more easily then in dry condition. This could also happen to saturated clays, particularly those subject to liquifaction.

So I generally don't use the weight of the soils, at least when I think it is reasonable to assume saturated soils during peak uplift event. I prefer to rely on something "concrete", so to speak. Something I can be 100% sure that it will do what I need it to do. Besides, concrete in the ground is pretty cheap insurance.

 

[ron9876]

Boy you must get some really big footings. For me when I have a 8'x8'x4' footing to hold down an interior column in a retail space I can't see doing anything to add to that.

 

[slickdeals]

@ron9876
I am in the same boat as you are. I have a 7.5'x7.5'x3' footing 3' below grade for an interior column in a retail building as well.

 

[msquared48]

Interesting. You must be in a hurricane region with high uplift.

I am in a region where we frequently have to design failure panels for structures in flood areas to allow flood waters to pass through without failing the structure.

As a point of conversation, has nothing similar been either tried or proposed in hurricane regions to limit the uplift seen by the footings?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[structuresguy]

@ron9876: Yes, occasionally I get some good size footings. But since they are only 16" down usually, the weight of the soil above them is not that significant anyway. Not like they were 4 feet down, with piers for the columns. So adding an extra 8-12 inches of concrete gets me the weight of the soil above the footing. But I also don't do a lot of large 1-story warehouse buildings, where I would have large column bays and little dead load to resist uplift.

Just as a sanity check, are you calculating net uplift using MWFRS or C&C wind pressures? We always use MWFRS for uplift. But I have heard some people using C&C, which would result in much larger uplifts.