drilled pile structural design
drilled pile structural design
(OP)
It's my understanding that there is no major difference in the structural design of various types of drilled piles. Whether it's a traditional drilled pile, augercast pile, slurry-tremie, etc., the structural design should be the same. The method of construction may be different, but the end result for the structural design would be essentially the same. Is this correct, or is there something else to it?





RE: drilled pile structural design
RE: drilled pile structural design
In other situations, you may be resting on a hard surface some distance down and be totally end bearing. In other cases it may be a combination of end bearing and skin friction.
As far as the physical design of the shaft, the reinforcing required in the pier/pile depends on the axial load, lateral loads, and bending at the top of the shaft. Also, in expansive clay regions, the geotech will give you design parameters to design a minimum amount of reinforcing to resist uplift forces on the shaft due to expanding clay.
RE: drilled pile structural design
RE: drilled pile structural design
With driven piers, you are usually dealing with a single pier drilled under each column. Pier shaft diameters vary from 18 to 96 inches...usually for end bearing you use smaller shafts with belled bottoms. The shaft diameter is usually set at a minimum 1/3 of the bell diameter. A pier cap is cast over the pier to facilitate setting anchor bolts or column dowels. ACI 336.3 provides design guidelines for drilled piers.
RE: drilled pile structural design
Piles implies small diameter, Piers (or caissons) implies larger diameters.
RE: drilled pile structural design
Say you have a building supported by 16" diameter driven concrete piles and another identical building supported by 16" diameter augercast concrete piles. Is there any major difference in the structural design of the two buildings? The design of the piles themselves may be different (skin friction vs. end bearing, driving stresses, etc.), but this is usually the geotechnical engineer's domain. I believe there is no major difference for the structural engineer.
RE: drilled pile structural design
The actual values provided from the Geotechnical to the Structural will be somewhat to very differant but, That is what makes designing so interesting.
RE: drilled pile structural design
Steel piles and concrete-filled steel shells are usually much stiffer than grouted piles, allowing less deflection at the support point. They also provide more resistance to rotation (moment restraint).
Auger-cast piles usually have only a single rebar through the center of the shaft, thus provide little bending stiffness (EI). These members are also less stiff axially, allowing the supported member to deflect more under the load. The concrete grout is usually mixed with the soil from the hole, resulting in a much weaker and softer grout than drilled and grouted piles.
For identical piles that are supported by either side friction or end-bearing, the friction pile will be much stiffer than the end-bearing pile (remember PL/AE ?).
There are quite a number of issues that must be considered in pile design and analysis, and in the design of the supported member, than merely capacity. A competent structural designer ought to include the load-deformation behavior of the supporting piles in the analysis of the supported member, else the calculated deflections, moments, and stresses will be greatly in error, leading to overdesign or to cracked and unsafe conditions.
RE: drilled pile structural design
RE: drilled pile structural design
Now I understand that such a demarcation of responsibilities does not often occur in the U.S. but, the result is that building design occurs which does not really consider the soil/foundation interaction.
I must admit that I have different 'stiffness' considerations for my foundation designs when using different deep elements and I often assume that others also do the same.
I also agree with ishvaaag that the subtle considerations are very seldom considered, nor are they often asked for. The only times I have been asked for more than basic information is when a space in software requires filling. I have been left with the impression that no understanding of the required value, how the final design may be influenced By the value or (dare I say it?)sometimes whether the designer has any understanding of the actual design theory.
It is usually easier to 'overdesign'. Probably a desirable thing considering the type of geotechnical data most clients will pay for (not understanding the cost of overdesign), the oftentimes poor geotechnical data produced and our litigious society.