how much skin friction to assume... 1

[ampersand]

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?

 

[oldestguy]

If 1,500 psf is a true indication of the allowable bearing, then the available skin friction is a very low number. Lacking any soil info I'd not go over 100 psf. It may be even lower.

I'd suggest some sort of soil evaluation for bearing, rather than guessing at skin friction.

 

[hokie66]

I would agree with oldestguy in using 100psf without any other information. Add the two together, and you only need the piers to be about 6' deep. How much uplift do you have? Probably not too bad in northern California, but in high wind areas, piers have been known to pull out of the ground.

 

[Ron]

I agree with hokie66 and OG...your overturning and uplift dead load might control....so you might need deeper pier just to get additional mass and friction to compensate for the uplift/OT.

 

[MikeDB]

1500 psf seems like a very conservative number for a pier foundation. Normally the allowable bearing for an individual footing is larger than for a continuous footing. Anyway, if it is a cohesive native soil (not fill) then qa=1500 psf and cohesion =q/2 = 750 psf and allowable skin friction would be about 400 psf below the frost depth.

 

[ampersand]

Thanks, all... MikeDB, you seem to have a comfortable system to go from the allowable bearing pressure, to the cohesion, to the skin friction. Is this based on an equation, or are you just quoting typical numbers from projects you have worked on?

 

[MikeDB]

By definition the cohesion is 1/2 the unconfined compressive strength (qu), so using 1/2 of qa should already include the safety factor.

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...