Gross vs Net Allowable Soil Bearing Pressure

[StrEng007]

If a geotechnical report lists the soil's "allowable bearing pressure" but doesn't not specify 'gross' or 'net', how would you answer the following.

1. Spread footing cast on top of the grade
a. Include self weight of the footing (I think this one is self explanatory)


2. Spread footing with top of footing 2ft below grade, with soil on top of footing =
a. Include the self weight of the soil above the footing.
b. Include the self weight of the footing.
c. Include the self weight of the footing as a function of footing volume x (Conc. density - soil density)
d. Both a and b
e. Both a and c
f. Ignore any weight contribution from the soil on top of the footing and the footing self weight.

 

[JLNJ]

I would ignore the footing weight. I suppose you could add in the difference between the concrete weight and the soil weight for a footing, but no need to split hairs when dealing with dirt. Maybe if I had a deep drilled shaft I'd ask the geotech for the specifics. He/she would probably say that you don't need to add in the difference between the excavated soil and the added concrete.

Gross and Net mostly comes into play at the bottom of an excavation where soil is permanently removed (as in a basement).

 

[slickdeals]

I would first ask the geotechnical engineer to clarify net vs gross.
When it comes to net bearing pressures, typical practice is to ignore the weight of the soil and footing and only consider the applied loads to it.

 

[StrEng007]

Do you mind answering question 2 in regarding to the choices, a, b, c, d, e, or f.

 

[StrEng007]

slickdeals,
You're basically saying... if it was net bearing pressure your answer to question 2 would be 'd'?

And if was allowable bearing pressure, what would you answer?

 

[slickdeals]

If it was net bearing pressure, the answer would be f. Only consider the column reaction.

 

[JAE]

Net bearing pressure would be the pressure induced on the soil by the footing above or below that which it originally had during the geotechnical study.

a) Think of a little discreet pieced of soil happily resting with soil above it.
b) Now the contractor removes that soil directly above it and casts a footing on that little discreet piece of soil.
c) Then the contractor loads soil over the top of the footing.
d) All that adds to the total weight that the footing feels.
e) But the NET pressure is that of d) less what originally was there.