Shallow Foundations

[steellion]

When is the weight of the footings themselves applied to the soil in bearing pressure calculations?

My understanding is that for ASD allowable bearing pressures, you do not include the weight of the footing as part of the pressure on the soil because that is factored in as part of the safety factor. Is that correct?

Reading a design manual this weekend, the foundation design was based on LRFD ultimate soil bearing capacity, and the weight of the footing was included. Is that correct?

 

[Lion06]

I guess technically you should add in (unit weight of concrete - unit weight of soil)*(thickness of footing) to the check since this is the net increase in weight from the footing and is increasing the soil pressure by that difference divided by the plan area of the footing.

Our geotech reports typically give net allowable bearing pressures for which I neglect the weight of the footing competely (admittedly in contradiction with my idea above). As for LRFD combinations (I'm assuming used for the footing checks and not the soil checks), I would not include the footing weight as this isn't a concentrated load like the applied column load and won't cause any bending in the footing.

Both of the above statements are true only for gravity footings or lateral footings with no moment. If you have moment at the top of the footing, then I add the weight of the footing (if I need to) to help with overturning.

 

[ToadJones]

I have always included the weight of the footing.

 

[swm25]

I generally add the weight of the footing if I'm using a prescriptive code given allowable bearing pressure, and that seems to be the standard of practice.

If I am using a soils report frequently the soils report says that the weight of the footing can be ignored.

The concrete is generally around 150 pcf, and is replacing soil that is around 110 pcf. If I ever really wanted to get something to work I suppose I could justify just adding the difference of the concrete weight - soil weight as that is the net increase to the soil bearing, but I've never done this yet, and it does not seem to be the standard of practice for the engineers I've been around.

 

[strucguy]

If we are checking soil pressures against net allowable bearing pressure, weight of the footing need not be included in the calculation of soil stresses. But, for cases where overturning is an issue, the weight of the footing and the soil above should be considered in the evaluation of soil stresses. But, the net allowable soil pressure is increased by (density of soil x depth of footing)to be compared with the calculated soil stresses.

 

[csd72]

I believe what the normal justification for this is that you are removing soil and replacing it with concrete and loose fill over so the load is similar.

Depends on what data the bearing pressure is based on.

 

[archeng59]

Look at the geotech report. If they give you a "net" allowable bearing pressure, you do not need to include the weight of the footing. If it is not clear, I contact the geotech engineer for clarification before including the footing weight.

 

[hokie66]

Agree with SEIT, strucguy...fiddling around with the increased pressure due to typical concrete footings is silly when both the allowable soil bearing and the loading from the building above are so approximate.

 

[Ron]

I always add the weight of the footing, since I use the weight of the footing for deadload overturning moment resistance. Too lazy to take it out of one equation and leave it in another!