LRFD Footing Design

[ztengguy]

I dont have a full geotech report, but I do have a ultimate bearing capacity (strength limit state) of 12.5+1.5Bf. What is the 1.5Bf? 1.5*Width of footing?

I have never come across that before. Any ideas?

 

[TehMightyEngineer]

Never seen that either.

Maine EIT, Civil/Structural.

 

[ztengguy]

I found out it means that the bearing is 12.5KSF plus 1.5*width. So 10 foot wide footing, is 27.5 KSF bearing

 

[manstrom]

Way to simplify things, geotech.

 

[ztengguy]

Well I know with LRFD there is an iterative process, we usually get a range for the different widths, I guess this does help a bit.

 

[TehMightyEngineer]

Wow, 12.5 KSF. Is that bedrock you're putting that on?

Maine EIT, Civil/Structural.

 

[allgoodnamestaken]

It makes sense in the context of the calculation for finding the stress of the footing on the soil. Both Terzaghi's and Vesic's formulas use B in the final term for calculating ultimate bearing capacity.

 

[MrHershey]

That's the highest allowable pressure I've ever seen.

A 'good' pressure here is 5 ksf.

 

[jayrod12]

We get values like that for bearing on bedrock (and I mean actual bedrock, not till, as many contractors seem to use those terms interchangably along with hardpan).

 

[ztengguy]

Thats not an allowable pressure, its the ultimate bearing capacity. Factored is somewhere around 5 or 6 KSF