Pile Lateral Load

[tq3610]

A question on the evaluation of pile capacity. In general, the pile size is calculated based on the vertical load capacity required. Then often an available lateral load on that pile is calculated and provided for design.

For a vertical pile, when checking the available capacity of the pile, would it be the correct process to check the combined vertical compression along with bending capacity (ie pile subject to compression and bending) if we are considering wind/seismic loads? Ie, if we have a battered pile that is taking vertical loads it is subject also to bending, so there is need to check combined compression/bending. Would it be typical to limit the compression or lateral capacity of the pile in the same way for a vertical pile subject to transient lateral loads?

 

[Retrograde]

Yes you need to check combined compression/bending (or combined tension/bending) for wind and seismic loads.

Would it be typical to limit the compression or lateral capacity of the pile in the same way for a vertical pile subject to transient lateral loads?

I'm not sure what you mean by this.

 

[tq3610]

Per IBC: "Where required by the design, the lateral load capacity of a single deep foundation element or a group thereof shall be determined by an approved method of analysis or by lateral load tests to not less than twice the proposed design working load. The resulting allowable load shall not be more than one-half of the load that produces a gross lateral movement of 1 inch..."

My question stems from the code statement above. The code doesn't say based on combined loading.

So, if I were to size a pile for compression and ignore from lateral loads, then I may achieve a higher lateral load capacity than if I based it on the combined loading.

Example: If checking the combined axial bending capacity using (Pc/Pc-all) + (Mmax/Mall) <= 1.0, and I'm using the full Pc-all for axial compression design (Pc = Pc-all), then I would have no capacity left to assign to lateral load/bending (Mmax would need to = 0). But if I follow "my interpretation" of IBC reprinted above and assign the pile 1/2 of the lateral load determined though a p-y analysis resulting in 1-inch deflection, then I would exceed what is allowable by checking the combined compression/bending.

This isn't an attempt to design an unsafe structure, just to understand how others manage this issue.

 

[tq3610]

Possibly another interpretation considering AASHTO LRFD 7:

C10.7.3.12: The strength limit state for lateral resistance is only structural...this limit state is reached...when the nominal combined bending an axial resistance is reached."

For serviceability the code essentially leaves the design to limit horizontal deflections to the tolerance of the structure.

Realizing this is mixing up design methods, but based on AASHTO above this seems to suggest the combined compression/bending equation should be modified to replace Pc,all and M,all with Pc,ult and M,ult.

 

[Settingsun]

I think that IBC section you quoted is referring to geotechnical capacity. For structural, use the applicable design code for the pile material, eg AISC for steel, ACI for concrete.

 

[tq3610]

Thanks Steve - that makes sense.

 

[Retrograde]

(Pc/Pc-all) + (Mmax/Mall) <= 1.0 - note that this is not correct for concrete piles. You need to develop the interaction curve as per the ACI code.

 

[tq3610]

Thanks Retro. I was looking at h pile/micropile but I’ll note this for future reference.