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# Settlement-based bearing capacity

## Settlement-based bearing capacity

(OP)
As you know, the bearing capacity of large foundations is controlled by settlement, rather than strength. I have produced graphs that shows the bearing pressure corresponding to a specific settlement (say one-inch) for a range of foundation sizes.

Q. Should a safety factor be applied to the bearing pressures to have the allowable bearing pressure? Can we argue that since the settlement curves correspond to service limit state, they don’t need a safety factor applied to them?

Please refer to Figure 1.2 of the FHWA-GA-16-1426 for further insight. I am not sure it is applicable as I am no expert in LRFD design.

Thank you!

Replies continue below

### RE: Settlement-based bearing capacity

You shouldn't need an additional factor of safety on the allowable bearing pressure determined by the calculated settlement, assuming that it fits within your typical range for the subsurface conditions based on your experience. That last bit is important. Don't blindly trust the calculations, as the value of a great engineer comes from their experience and not just plugging and chugging.

### RE: Settlement-based bearing capacity

Yep you’re getting at the LRFD approach of bearing capacity. You calc a service limit state bearing resistance for your desired settlement, and a strength limit state bearing resistance, which is the nominal bearing capacity multiplied by a resistance factor (<1). Then you check that your applied factored bearing pressure is less than both of the limit states.

### RE: Settlement-based bearing capacity

Just a side note, I typically use the terminology "Bearing Pressure based on Settlement Criteria" and "Bearing Capacity based on shear failure".

You should be very explicit when you discuss the two as alot of people mis sue terminology and mix them up.

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