Caisson/Pier Frictional Resistance

[outatime2002]

IBC 1805.7: For caissons and short piles (<12' embedment), IBC 1805.7.2.3 states that the resistance to vertical loads shall be determined by the allowable soil-bearing pressure set forth in Table 1804.2. (I assume this means end bearing resistance) IBC makes no mention of skin friction to resist vertical loads for caissons and short piles.

IBC 1808.2: As you look further into the code, IBC 1808.2.8.4 states that frictional resistance may be used for piers and cast-in-place piles, but the engineer shall not assume that frictional resistance and bearing resistance act simultaneously. Hence, you can only use one or the other.

Why does IBC constrain the engineer to use only end-bearing resistance for caissons and short piles in 1805.7? This stipulation is forcing me to use large diameter piles in order to achieve adequate vertical resistance. In practice, I've seen engineers use frictional resistance in lieu of end-bearing resistance because it typically results in a more economical design. It seems to me that frictional resistance should be present regardless of whether we are talking about a caisson or a cast-in-place pile. Am I missing something here?

 

[jamie]

i would assume it's to end up with a larger factor of safety. if you only use one or the other, when in reality you would have some of both in most cases, then you deliberately overdesign. for short piles, they may just be neglecting skin friction, since in some soils it is considered to increase linearly.

 

[Badger]

Are these short piles driven?

The last time work with piles, and was complying the pile driving, I beleive the design engineer required that the piles be driven at least 15 feet before refusal or the prescribed minimum hammer blows per foot.

It is my guess for soft or loose, or for cohesionless soils, skin friction would be negligible for less than 12 feet.