how much skin friction to assume...
how much skin friction to assume...
(OP)
I am preparing the structural framing and foundation design for a flat-roofed carport in Occidental, CA, forty miles north of San Francisco. The owner/builder wants to use drilled concrete piers of not more than 24" diameter, and he is not interested in shallow footings. The bearing loads on the post range from 3 kips to almost 8 kips.
An end bearing calculation using a 24" diameter pier and 1500psf allowable end bearing gives me an allowable load of 4700#, so I will need to try and resist the loads with a skin friction calculation.
I do not have a soils report- the building department has said it will not be required.
The IBC does not list any values for skin friction in Chapter 18 (not that I can find).
How much skin friction is a good assumption?
An end bearing calculation using a 24" diameter pier and 1500psf allowable end bearing gives me an allowable load of 4700#, so I will need to try and resist the loads with a skin friction calculation.
I do not have a soils report- the building department has said it will not be required.
The IBC does not list any values for skin friction in Chapter 18 (not that I can find).
How much skin friction is a good assumption?





RE: how much skin friction to assume...
I'd suggest some sort of soil evaluation for bearing, rather than guessing at skin friction.
RE: how much skin friction to assume...
RE: how much skin friction to assume...
RE: how much skin friction to assume...
RE: how much skin friction to assume...
RE: how much skin friction to assume...
There are several methods to estimate skin friction in cohesive soils. One is the alpha method where skin friction is equal to alpha times cohesion. Alpha varies with soil strength, but for this range alpha is 0.55. So in our example, (1500/2)x0.55 is about 400psf. I usually ignore the top 3 to 5 feet of embedment depending on soil and frost depth. For end bearing piers, skin friction is often ignored for the bottom diameter of the pier due to the bearing stress on the soil.
Reference FHWA-IF-99-025 Drilled Shafts which has a lot of good information on piers. There is also a Navy manual, NAVFAC 7.2, but I don't use it as the skin friction values are significantly different when comparing methods for very weak or very strong soils.
This was probably more information than you wanted, but it is all the same price...