drilled pile structural design 2

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?

 

[ishvaaag]

I would tend to say that I agree more or less on that, but the books have collections of formulations for piles many times quite close but other times significantly different between them, implying that those that study specific types seem to think worth to make distinction.

 

[JAE]

The design of piles/piers does vary somewhat. Typically, your geotechnical engineer will provide you with design criteria. In many cases, the capacity of the drilled pile may be based on skin friction along the length. The end takes no bearing.

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.

 

Thanks for the responses. I understand that the geotechnical engineer will supply the pile capacities based on skin friction and/or bearing and the pile will need to be reinforced for the given loads. My question is whether there is any significant difference for the structural engineer depending on which construction method is used. Say, for example, that the SE has designed structures with thousands of driven piles over the years. Now, there is a project for which the owner wishes to use drilled piles in order to minimize vibrations to his adjacent building. Are there any major issues for the SE to consider in his structural design? I think that it is more of an issue for the geotech and contractor, but for the SE a pile is a pile.

 

[JAE]

When you use the term "pile" I think of smaller diameter (10" to 16&quot piles that are usually driven or augered into the soil in groups and tied together with a pile cap...concrete perhaps 2 to 4 feet thick which produces a base for your columns or walls.

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.

 

[Zulak]

I agree with JAE. Piles and piers are different in my mind, and are handled a little different in the codes, as well. Check out BOCA and/or the Massachusetts State Building Code for specific design criteria for each.
Piles implies small diameter, Piers (or caissons) implies larger diameters.

 

The responses seem to have drifted off topic. The question was NOT about the differences between piles and piers. The question was about differences between the design of buildings supported by drilled piles and driven piles.

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.

 

[emmgjld]

It is my understanding and practice that the 'structural considerations' at the foundation/building connection and above are virtually the same for driven/pushed piles and drilled piers.

The actual values provided from the Geotechnical to the Structural will be somewhat to very differant but, That is what makes designing so interesting.

 

[teasip]

The structural design of the member (usually a beam) supported by the piles must consider the stiffness of the support in calculating member deflections, moments, and stresses. The axial stiffness of the pile directly controls the deflection of the supported member. Without consideration of the deflections and the shear and moment distribution along the beam, how will the structural designer detail the rebar placement, spacing, and lengths?

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.

 

[ishvaaag]

... the problem being that for the ordinary building a complete soil-interaction analysis is not being made mainly on lack of knowledge and software power, lack of reliable data able to produce such model and unreliable prediction of the theory and models of the significantly variable behavior of the soils. I agree that the best is the best but only available to some. Not even the codes (the spanish code for buildings at least) are as demanding as your statement implies. It rather builds on past experiences, and at the office level maybe some are doing bracketing for variable conditions of the soil, still rarely the complete soil-structure interaction except for the more relevant structures. This is not to say known aspects of the rigidity of the foundations are not being taken unto account in some simplified manner, but not the state of the art way. That the extant simplified practices may be leading to some oversizing of foundations maybe then not only true, but in fact wished by the code, since it covers, as every code has been doing till today what is not known, whatever the authors or the code say. Furthermore I myself consider the foundations the better place to make some overdimensioning of parts, since from a sound foundation you can cheaperly re-build upwards. To reinforce after failure the foundations is obviously an option but not my preferred, nor I think that cheaper. And not to forget is the fact of the real capacity for proceeding the state of the art way not being in place, but at the top AEC -or specialized- firms. The practices are as they are as a consequence of the practicalities of our available means to design and build, which for no part (not only foundations) do not allow for state of the art, but standard. Standard practice may cover some or even all aspects of whay you say, but in simplified manner. Even in an ACI text of the 90's for mat foundations, the mats are analyzed separately of the structure, congruence being forced iteratively at most. For even simpler structures, it is very usual complete separation of foundation of the structure at design time, and only a guess or data derived value of the allowable pressure on the soil to proceed, and something jsut a bit more smart for piles. Then, come to piles, for some works a size differs of the other by scarce amount in cost, and even if the number of piles is to be high there might be no worth to increase the tightness of their capacity if in doubt.

 

[emmgjld]

My above comment was not quite specific enough and teasip hit the point right on the head. I should have included the 'gradebeam or other connectors from the piling to the superstructure' with the piles. I am assuming the Structural Engineer is designing the superstructure and the Geotechnical Engineer is designing the foundation.

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.