Footing self weight to resist uplift

[BSPE90]

When checking against the uplift reaction from a column, can I use a portion of the adjacent strip footing and foundation wall (integrated with column pilaster and spread footing) to help resist the uplift forces? If so, is there a rule of thumb of what length of the foundation wall to use?

 

[CANPRO]

When I was designing foundations for pre-eng buildings, I was pretty generous with the amount of wall I would include in my uplift resistance. I suggest looking at your foundation wall as a beam - supported at the column in question and cantilevering out from there on each side. The portion of wall you can count on is the distance the cantilever can extend under its own self-weight. If you have some minimal horizontal steel at the top of your wall, you can count on a decent amount of wall in uplift.

 

[BSPE90]

Ah, that would be helpful if we went with our original concrete walls. Along the way, our architect decided to provide an alternate detail (fully grouted CMU wall instead of concrete foundation wall) which my boss OK'd. As you can see from the attached, there is no horizontal reinf. at the top of the fully grouted portion of the CMU wall.

In this case, should I ignore the CMU portion and only count the strip footing?

 

[EngineeringEric]

Your foundation (concrete footing) can still act as a beam. Plus you seem to have a heavy wall on it as well.

 

[WARose]

Usually for uplift, my decision on how much of the foundation to use depends on a number of things: strength, crack control, serviceability, etc. In other words, it's like any other design problem. In the wall footing (for example) in uplift, if you have to grab a lot of it to hold it down.....that will mean some flexural forces that will have to be accounted for. The wall could possibly help if it has the strength.

 

[ARKeng]

Yes of course you can use that weight in the proper load combination (e.g., 0.6D + W/E)... as long as you can justify the load path works for the concerns WARose mentioned.

 

[Old Eng]

Looking at the detail and considering the situation when that uplift resistance is needed, if the ground it saturated and temporary or permanent high water table is resent, reduce all materials weight if below the water table due to buoyancy. That reduction is the unit density of water displaced times the volume displaced.