Pier footing skin friction and bearing strength
Pier footing skin friction and bearing strength
(OP)
Can anyone explain to me why skin friction and bearing strength of a pier type footing can't be used together according to the CBC 1808.2.8.4?





RE: Pier footing skin friction and bearing strength
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.
RE: Pier footing skin friction and bearing strength
RE: Pier footing skin friction and bearing strength
RE: Pier footing skin friction and bearing strength
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.
RE: Pier footing skin friction and bearing strength
RE: Pier footing skin friction and bearing strength
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.
RE: Pier footing skin friction and bearing strength
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.
RE: Pier footing skin friction and bearing strength
RE: Pier footing skin friction and bearing strength
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. . .
f-d
¡papá gordo ain't no madre flaca!
RE: Pier footing skin friction and bearing strength
RE: Pier footing skin friction and bearing strength
¡papá gordo ain't no madre flaca!