Foundation Uplift Resistance

[j19]

I do not typically get involved in sizing metal building footings, but I have a couple of questions after looking through several sets of foundation plans for metal buildings. When sizing a spread footing to resist an uplift load on a column, do you use the 0.6D + W load combination to determine the resistance to uplift? It seems overly conservative to only be able to use 60% of the footing dead weight.

If the column has a horizontal and uplift reaction and the foundation engineer is using hairpins, does sliding and overturning need to be checked if the column is located in the center of the footing?

 

[kslee1000]

1. It is somewhat conservative but not without merit given the uncertain nature on future changes.
2. "if the column is located in the center of the footing?".
I don't quite follow your question. Should it be "center of the building"?

 

[j19]

Thanks for the quick response.

1. Is this a code requirement or is it just typically understood by foundation designers to do this? The drawings that I am looking at appear to use the full footing weight to resist uplift, unless the foundation engineer has also taken into account some of the turn-down at the edge of the slap between footings, as well as a considerable amount of the floor slab.

2. Typically a metal building column will be within a few inches of the edge of the slab. In order for the column to be in the center of a shallow foundation, the column may be on a +/- 1' pedestal and the spread footing may extend past the edge of the slab. For example, for a 6' x 6' footing, maybe 1.5' x 6' will be outside the edge of the slab.

 

[kslee1000]

1. Yes, required by the code.
2. Yes, check on sliding and uplift are required for column pedestal placed either concentrical, or eccentrical with respect to the footing pad, and with, or without shear pin. (Shear pin should be designed for effects from horizontal load with proper safety margin.)

 

[j19]

Are you refering to a shear pin as the extension below the footing that is used to resist sliding? The hairpin that I am referring to is the reinforcing bar that is located just below the bottom of the column and is embedded in the floor slab in order to transfer the column's shear load into the slab. If the column's horizontal loads are resisted by the slab then it seems to me that overturning would not be an issue.

 

[kslee1000]

That's what I meant. The size/number of shear pin is need to be evaluated and designed for the horizontal load from column.

 

[Ron]

For overturning and uplift, the International Building Code requires considering the minimum dead load that will be in place at the time of an expected wind event, unless you are using the alternate load combinations, which then you can only consider 2/3 of the minimum load.

Other codes are similar.

I use the actual dead load of the footing.

 

[BAretired]

If the footing could be below the water table, do you use the submerged weight? What volume of soil above the footing do you consider to be resisting uplift? What is the applicable soil density?

BA

 

[abusementpark]

I used the submerged weight of the footing and do not use the 0.6 factor on the footing weight. IMO, it is way too conservative to take the submerged weight and the 0.6 factor on the footing. You still have to take the 0.6 on building floor, wall, and roof dead loads.

The geotech reports also usually give us adhesion on the sides of the footings, which helps a lot.

 

[Ron]

If below the water table, I use the submerged weight...I also consider anything I can legitimately include above the footing such as soil, slabs, walkway concrete if tied together, etc.

As for the volume of soil, I usually just take the projected area from the footing. You can argue that shear strength of the soil could be included, you could argue that in cohesionless soils the angle of repose could be used either way (+ or -), and there might be other arguments as well, but it gets down to what you feel you can do professionally to substantiate.