When the tip lays on firm base, the axial stiffness of the pile doesn't allow for full development of all the available friction resistance at the interface in the whole length, that only comes at higher relative displacements.
For weaker or midstrength soils where the choice is made of relying in shaft friction, it only will be just a conservative measure taken, for some tip strength will be extant.
so friction only developes with shaft displacement? but for the shaft to displace, wouldn't it need to overcome both the frictional resistance and the end bearing?
The City of Chicago, I believe, has the same rule. End bearing and frictional resistance cannot both be used in the pile capacity calculation, it has to be one or the other.
I did pile calcs in the coastal US Southeast and always used both end bearing and frictional resistance.
The soils were typically silty and clayey sands and sandy silts. The frictional capacity would be mobilized gradually and at the same time as the end resistance.
This assumption would not be appropriate if the frictional resistance could not be mobilized at the same time as the end resistance was mobilized, such as if you had a very stiff (high modulus) bearing strata or vice versa.
These could be in an area where the dominant strata were very soft/loose soils over very dense/stiff soils of rock or dense/stiff soils with underlying soft/loose soils.
Hopefully someone with more experience can chime in.
hokie66, the CBC is basically the same thing as the IBC, just slightly modified for california. And thanks jgailla, that's kind of what I figured, but it would seem that the only case where skin friction wouldnt develope with bearing strength is if the pier was resting on a very stiff rock that would shear under its maximum load without ever settling. If it was resting on a very loose soil with very dense soil along its shaft, it would seem that both friction and end bearing should still develope unless the soil below actually settles without the pier.
Ogrork1,
I've never analyzed this, but I think a dense friction on a loose base could settle excessively before mobilizing any end resistance.
Kind of theoretical anyway, since a real pile would be constructed either with the end some distance above the loose strata or sunk through it to good material below in any practical case.
All the piles I've done have hit competent material in no more than 60 or 70 feet, although there are many areas where this may not be the case.
It takes about 2 mm of movement to invoke shaft resistance. Pile compression under loading may be able to create this effect or reconsolidation of the soil around a pile after installation. However, there are certain conditions which many use to decide whether to use toe resistance only, or toe resistance and shaft resistance, or shaft resistance only. Drilled shafts on belled bases is one case which shaft resistance is often not used, this is also a function of the pile length as well and competency of the suppotrting material at the toe.
We must as well consider downdrag or settlement of the soil around the pile. This tends to place a dragload on the pile which has to be taken into consideration in the structural evaluation of the pile. This is in opposition to the support provided when the pile is moving down relative to the surrounding soil.
For straight shaft piles - drilled shafts, very often toe resistance is not relied upon and shaft resistance is used. This is generally because the pile tip cannot be guaranteed to be void of loose and disturbed material.
There are a number of situations based on soil conditions etc where the static method of analysis has to be carefully examined. In many cases if we have a very long pile greater than 20 m it is possible that shaft resistance may be able to support a significant load without toe resistance being invoked. This is based on load transfer mechanisms.
For more information, one needs to read papers by Bengt Fellenius or his red book. I would also caution that there are other literature that would have different opinions. That's the name of the game generally. These often lead to confusion so it is necessary to always spend some time in thinking about the problem at hand and deciding on whose concept fits the bill the best.
VAD gives good advice - shaft friction (adhesion) develops at small strains (movements) - toe capacity develops at much higher movements - so at ultimate, skin friction will "peak" and the toe resistance will continue. While analysis may "neglect" skin friction in cases where the design relies on base capacity, skin friction will still be present. Look up charts showing how much (or little) working loads ever reach the base of the pile.
For a single pile, the conventional safety factor is 3 (against ultimate failure). This being true whether you design for end bearing or shaft resistance. I'd think to design for end bearing if there was some measure of shaft resistance, it'd justify using a lower safety factor against the ultimate failure of the tip. This is kind of like using both and still using 3, eh?
I'm not sure I'd fully discount the shaft resistance, is kind of what I'm saying, then again, I don't design that many piles. Just thinking about it. . .
Thank you all for your responses on this thread. Fattdad, where did you get a safery factor of 3 from? The IBC section 1808.2.8.6 calls out for a factor of safety of 2 for piers. And it actually also allows for designing using both skin friction and end bearing as long as it's recommended by a soil investigation. I have a hard time understanding this because the way I see it, unless the pier is resting on some kind of very hard soil that can suddenly fail before allowing skin friction to developed, it should be able to develope both skin friction and end bearing no matter what.
from my professor. I can look for a better citation, but for single piles I use 3 for ultimate and for pile groups I use 2 or 2.5 (can't recall right now).
