Depth of footing for basement

[eric1037]

Hey all,

In calculating bearing capacity and/or settlement, there is a variable of Df or depth of footing.

If you have a basement that is 10 feet deep with a concrete floor slab on the inside with the top of slab 2 feet above the bottom of footing, is the depth of footing 10 feet or 2 feet?

 

[fndn]

The depth of footing is from the top of existing ground or backfill to the bottom of the footing. So with a 10 ft basement, the foundation wall is 10 ft, the footing thickness say 1 ft, which is 11 ft in total. Then subtract the height of the foundation wall sticking out of the ground. If this 2 ft, then your depth of foundation is 9 ft.

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[eric1037]

fndn:

Thanks for the reply. Your answer is what I thought would be correct.

 

[BigH]

I'm not so sure that the answer is that straightforward -for bearing capacity. If you are in a basement, the outer footers will have the high soil on one side and little on the other. Traditional bearing capacity assumes certain geometric similarities on both sides of the footing for developing the sliding planes. Without one on the inside, the bearing factors do not have fully developed planes and I would expect the Nq values to differ from a normal footer. All said and done, bearing capacity seldom governs the design anyway - it is settlement and you can compute the various stresses beneath the footing. For inside footers, the "depth" for bearing capcity would be the basement floor level.
For instance, on a wide reinforced earth wall embankment -walls on both sides of the carriageways, the BS codes and others talk about traditional bearing capacity. A good many authorities, though, permit the safety factor to be 2 on shear bearing capacity rather than 3. I personally favour doing a global slope stability analysis to see if the situation is safe or not.
Before I get jumped on, I am making the distinction between shear bearing capacity and allowable bearing pressure (net). See other threads about it.

 

[AW12]

Clearly, with regard to bearing capacity, footings within the basement have a reduced depth of overburden, which no longer contributes to the bearing resistance, hence the foundation depth should be taken from basement level. Since failure would occur in the weakest direction then foundations at the edge of the basement should be designed in the same way - take foundation depth from basement floor level.

With regard to settlement, the correct approach will be dependant on other factors such as whether the underlying soil is cohesive or granular and whether you are dealing with re-development of an old existing basement or constructing a foundation within a new excavation.

 

[Ron]

eric1037...I agree with BigH...it isn't as simple as it appears. The outside condition is easy. The inside condition requires computing an "equivalent" depth based on the soil, the concrete, and the "confinement" of the concrete. If the concrete is reinforced and will handle flexure, then its "confinement" equivalence might be high. Your net depth will be either the outside or the inside equivalent, whichever is less.

...but, the bearing capacity is not going to be your critical parameter as BigH said. Go ahead and compute it though, for the exercise.

 

[GeoPaveTraffic]

eric1037,

I agree with BigH, use 2 ft.