Permissible Lateral Deflections for Steel Piles

[StructuralNomad]

Hi,

Are there any reference materials on design of pile foundations that specify permissible lateral deflections for steel piles at the pilehead? I vaguely remember about an article specifying permissible lateral deflection as function of pile diameter (Can't recall the Authors name).

Best Regards,

StructuralNomad

 

[Riggly]

Are you looking at the structural aspect of the pile, or the soil response? Typically I would limit the pile head movement based on rotation, lateral movement, and vertical movement, depending on the structure and recommendatons from the structural engineer, and that would be independent of the pile diameter. You also have to consider the soil type. When it comes to the soil aspect, I would be a little concerned about basing allowable deflection at the pile head solely on pile diameter. The diameter-to-length ratio can also be used to determine whether the member is rigid or flexible- and may theoretically affect the soil response, according to Bowles.

 

[StructuralNomad]

Hi Riggly,

Thanks for your response.

The structure that I am designing is supported on piles in a ground that is very soft in the top five meters, turns harder and terminates in strong granite down 25 m below ground level. The intent is to limit the lateral deflection at pilehead to a nominal acceptable value that minimises bending in pile and structual displacement at its highest point.

My quesion was based on the soil response rather the structural response. As mentioned above the soil is very soft in the top five meters and allows a large lateral displacement. The structure and the pile (for a given size) by themselves can absorb that value of displacement.

In view of the above, can the pile be treated as a structural member/column (cantilever or propped cantilever) and subject it to lateral displacement limit and then assess the soil response for the selected pile size? Is there something I am missing in the design of pile foundations?

Best Regards,

StructuralNomad

 

[FixedEarth]

The soils engineer can provide you the lateral subgrade modulus for the various soil layers. You can then take your shear and applied moment at the pile head and use beam on elastic foundation equations to get the response of the pile (shear, moment and deflection with depth). So for example, you may have 50 kips/cu.ft for the soft soils and 500 kips/cu.ft for the granite.

The equations are either for free head connection or fixed head connection. Based on research, often, a pile has to be embedded inside a pile cap 18 inch minimum so as to get a fixed head condition, These Hetenyi equations are in "Geotechnical Engineering: Foundation Design" 1995 by John Cernica and many other handbooks like Roark's formulas for stress and strain.

The response of your pile to a combined shear & moment is not that dependent on the lateral subgrade modulus value of the soil and the pile embedment as much as you would expect. It is mostly governed by pile's E*I value. So if your deflection is over say 0.3 inch, for example, use a pile with a higher moment of Inertia The best and most current reference on this topic is "single piles and pile groups under lateral loading" by Reese and Van Impe, 2011. While Bowles covers it, the above book has everything you need.

Alternatively, you can use the soil's passive resistance to resist the applied top of pile shear and moment loads.

 

[StructuralNomad]

Hi FixedEarth,

Thanks for your response and please bear with me for a delayed response. Ended up with a resonably sized pile to conrol lateral deflection at the pile head. With the soil being slushy in the top few meters, passive resistance did not help much.

Best Regards,

StructuralNomad

 

[FixedEarth]

Thanks for the closure! Since the original post, I discovered Author V N S Murthy also has his own method of laterally loaded vertical piles. For more information, see his Advanced Foundation Engineering Text.