Bending of footings under uplift

[sadienerd]

I have seen many post on here regarding uplift calcs on footings. I don't want to get the .6*(footing weight) debate started again. I am curious how everyone checks there footing reinforcement for bending under the .6D+W combination. It seems to me that if there is net uplift on your footing (i.e. PEMB) the load transfer is such the anchor bolts engage the pier, the pier engages the footing, and the footing engages the soil (all of this weight is included in uplift resistance). All of this would end in negative bending of the footing. I typically provide two mats in the footing in that case so my concretes D value is equivalent in bending in either direction when there is a substantial uplift but I have never seen an example of a flexural or shear design check of a footing under net uplift. Does anyone have an example or guidance for a young engineer on how the scenario of uplift should be handled with respect to the footing reinforcing.

 

[msquared48]

You have the same condition for the steel in the heel of a concrete retaining wall that is engaging the backfill to resist overturning. Other than the difference in possible load factors leading to the uplift amount to resist, what would be the difference?

Mike McCann
MMC Engineering

 

[JAE]

I don't have an example but not sure you really need one. If the anchor bolts can take the uplift tension then your footing acts like a negative moment resisting element and you do need top steel each way.

(I'm assuming a typical spread footing).

The bending would go in two directions so you have a two-way problem to solve.

 

[a2mfk]

It usually is almost a non-issue because if you just supply ACI minimum flexural reinforcement in the top of the foundation it works, since in uplift the only load is the self-weight of the foundation. The tricky part is when you are designing foundations for structures with high uplift at the edge of the building, like PEMB, and you may want to engage portions of the wall/edge footing or the slab to help you in uplift. This becomes more of an engineering judgment call, and then I may add some more top bars than the minimum.

 

[JAE]

The uplift load can be a bit higher than the footing self-weight (if there is soil over the footing) - but you are right that it isn't too severe.

 

[DaveAtkins]

Where uplift is relatively small, the footing may work unreinforced.

DaveAtkins

 

[a2mfk]

JAE- correct, I was thinking with monolithic slab-foundation systems...

 

[TDIengineer]

I have designed many footings in high-seismicity where uplift is almost a given.

I have found that applying the strength load combinations on your footings and getting the static to resolve can be challenging. A conventional footing can have no tensile forces... it can unweight, but transfers no tension. With that said, triangular soil distribution is very common, even using a static load combination.

I work with the CBC code, and it states for footing design, you can design the concrete for 1.5 times the stresses calculated from static loading. So I just check my footings for the static loads, make sure it is stable where the triangular loading isn't too steep. And then I multiple all my results by 1.5 and check moment and shears that way for strength.

You end up with negative moments on the portion of the footing that is unweighted as it has no pressure on the underside, but loading at the top from the footing self weight and whatever wall loads and column loads it is supporting.

So, that's how I get around the 0.6D+W or E combo.. For california hospitals, they require the lateral footings to be designed for overstrength load combo! Imagine trying to get stability to work for a conventional footing for (0.9D - 0.2Sds) and then with omegaE!!! it gets to be crazy sized footings.