IBC Presumptive Load Values for Foundation with e> L/6

[RDR89]

Hi all, from doing a search of this site, it seems that the jury is still out on whether the IBC presumptive load values in Table 1806.2 are net or gross for the vertical bearing pressure. I have a foundation where e >L/6, so my bearing area is reduced. If I assume that the IBC presumptive table values are gross (i.e. I have to include the weight of my concrete footing in my calculation of the bearing pressure), how do I account for the footing weight on a reduced area? Putting the entire footing weight to act solely over the reduced bearing area would be overly conservative in my opinion but I am curious to see how others would handle it/have handled it.

ETA: I unfortunately cannot make the footing larger, hence why e>L/6. I know it’s not ideal but trying to understand how the mechanics work in a case like this.

 

[BridgeSmith]

The footing weight is accounted for in the summation of moments. In other words, it is (or should be) part of the resisting moment used to calculate P (the force the soil has to resist) and e.

 

[RDR89]

@BridgeSmith forgive the simplicity here but I want to make sure I am understanding you correctly - in my case, I have a piece of equipment on a footing that is already offset on the footing, which further exacerbates the eccentricity (hence e > L/6). To determine what that eccentricity is, I sum the moments about a point on the end of the foundation, which includes moments due to wind acting on the equipment, and divide it by the total load and then see where that shakes out with respect to the geometric center of the pad. That total load is my P load and includes the foundation weight and the weight of the equipment. You're saying that "P" force is what acts over the reduced bearing area, is that correct?

 

[BridgeSmith]

That sounds correct to me. Including the footing weight will decrease e but increase P and the bearing pressure. The width of the soil reaction would typically have a width of twice the distance from P to the front edge, with a uniform reaction of P divided by that width. If the footing is on rock, it would generally be assumed as linearly varying (triangular) reaction with a width of 3 times distance to P.