Soil Perscriptive Bearing Capacity 1

[OHIOMatt]

I routinely design foundations based upon the provision in the 2006 IBC for perscriptive bearing capacity based upon soil profile. Typically this ends up being on the low end at 15oo psf.

I have always used this as gross bearing capacity simply because I cannot find anything in the code that would allow this to be considered "net". By net, I mean being able to neglect the weight of the foundation itself.

The frost depth in the area varies between 3' and 4'. The columns are wood, so we do not want them to be in contact with the soil. A 4' deep footing weighs 600 psf, and essentially uses 40% of the bearing capacity self weight.

Does anyone know if the code allows for the use of net bearing capacity as I described above?

 

[DaveAtkins]

No, I don't--but it makes sense that this would be NET allowable bearing pressure, for at least a few reasons:
* Geotechnical reports almost always give the net allowable bearing pressure
* 1500 psf (or 2000 psf, for that matter), are conservative allowable bearing pressure values, compared to what you get when you have a geotechnical investigation done
* If 1500 psf is GROSS allowable bearing pressure, any soil deeper than about 12' below grade could not even carry the weight of the soil above.

DaveAtkins

 

[JLNJ]

Be wary of applying code-prescribed allowable bearing pressure. No matter how low the presumptive bearing pressure you can still get into trouble with settlement.

 

[woodman88]

Section 1808.3 (IBC 2009) states that the dead load is permitted to include the weight of the foundation. with no bar on the side to show that it has been changed from the 2006 ed.

Garth Dreger PE
AZ Phoenix area

 

[woodman88]

DAveArtkins, Every soil report I have read, has given a increase in the allowable soil bearing the deeper one goes. At a 12' depth, typically, I have seen 5000 psf or higher for the bearing allowable.

Garth Dreger PE
AZ Phoenix area

 

[Lion06]

Regardless of what you use for the foundation design, you need to have a special inspector look at the bearing surface, and do whatever test he does, to verify the bearing capacity. This is true even if you have a geotech report.

 

[BigH]

Doesn't building code presumptive bearing values take into account "normal" settlement limits? From Peck Hanson and Thornburn: " . . . for each soil there existed a specific pressure under which the settlements of the various footings would not exceed reasonable values. This pressure, known as the allowable soil pressure, was generall specified in the building code or ordinacnes of the city in which the construction was to be located." Can't put my hand on it now, but I have seen comparison tables of various codes and they note the above comment.

 

[Lion06]

The IBC doesn't have such a note associated with the prescriptive soil bearing values.

 

[ToadJones]

OHIO-
You could consider using a spread footing with a pier, no?
Of course, the soil overburden still adds weight.