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Steel Pipe Pile Foundation Design

Steel Pipe Pile Foundation Design

(OP)
I am designing a pile foundation for a new building based on information provided by the geotechnical engineer for the project. The pile foundation is required due to the high probability of liquefaction of the underlying soil during a seismic event. The pile system proposed consists of 8" dia. hollow steel pipes that will resist both vertical and lateral loads from the structure. Many questions have been raised by the city plan reviewer in which I cannot find direct code answers or answers from the common engineering practice for similar systems.

The three questions are:
1. Is the table from IBC 1810.3.2.6 commonly used that requires the allowable pile stress to be limited to 0.35Fy? None of the calculations I have from other foundation designs in the area include this reduction but it seems like this should apply according the the IBC code language.

2. Does the R value used in the base shear equation for lateral loading to the individual piles need to be reduced to 2.5 (special cantilevered steel column resisting system) instead of the value used for the lateral system for the building above the pile system (R=5 for special reinforced masonry shear walls). Again, the designs I have seen in the area do not appear to change the R value for the foundation design but the plan reviewer thinks that should be done for this system.

3. Does the ASCE 7-10 requirement in section 12.2.5.2 for cantilevered column systems that limits the axial strength in the column to 15 percent of the available axial strength apply for pile foundation systems? I cannot find any references to using this reduction in any pile foundation literature or design examples. The plan reviewer thinks this reduction should be applied on top of the 35% reduction for allowable stress which would effectively only allow for 5.3% of the steel stress to be used for design.

I cannot find any design examples or literature that addresses the above questions. Has anybody run into this before and is there any literature available that addresses any of these questions?

Thanks in advance for any help or direction.

RE: Steel Pipe Pile Foundation Design

Hi bgssmith,

try to give my opinions:

Quote (1. Is the table from IBC 1810.3.2.6 commonly used that requires the allowable pile stress to be limited to 0.35Fy? None of the calculations I have from other foundation designs in the area include this reduction but it seems like this should apply according the the IBC code language.)


I think this is correct, no other way to escape.

Quote (2. Does the R value used in the base shear equation for lateral loading to the individual piles need to be reduced to 2.5 (special cantilevered steel column resisting system) instead of the value used for the lateral system for the building above the pile system (R=5 for special reinforced masonry shear walls). Again, the designs I have seen in the area do not appear to change the R value for the foundation design but the plan reviewer thinks that should be done for this system.)


I do not agree that the R is changed, but rather you should consider the overstrength (omega) factor when designing the foundation, this is to ensure that the building will not become stronger than the foundation.

Quote (3. Does the ASCE 7-10 requirement in section 12.2.5.2 for cantilevered column systems that limits the axial strength in the column to 15 percent of the available axial strength apply for pile foundation systems? I cannot find any references to using this reduction in any pile foundation literature or design examples. The plan reviewer thinks this reduction should be applied on top of the 35% reduction for allowable stress which would effectively only allow for 5.3% of the steel stress to be used for design.)


I don't think so. As this ASCE is for upperground structure, then it won't be accurately same as design for foundation.
The fact that the steel pile is not really cantilever structure. The soil resistance will give some support to steel.
is this cantilever column system conclusion is coming from the fact that there will be liquefaction effect?

last:
I think if you are designing a very big foundation that can eliminate the liquefaction, this is not a proper approach.
Soil improvement (surrounding the project) is one of the solution to reduce the effect of liquefaction rather than designing a very big foundation that resist liquefaction as this result with very2 big design/dimension.

RE: Steel Pipe Pile Foundation Design

(OP)
rbudiman: Thanks for the responses. Do you have a code reference that indicates the overstrength factor requirement for the foundation design? It is clearly noted for battered pile systems in ASCE 7-10 but I could not find that requirement for typical pile systems.

I don't know what is triggering the approach to review the piles as cantilevered columns. It was a comment by the city plan reviewer that we are trying to work through. I haven't come across any methodologies that use this analysis but wanted to see if others had.

The depth of the soil that has a high probability for liquefaction is very large. We discussed soil improvements but the geotechnical engineer decided that was not an option for this project due to deep extents of "bad" soil -- aprox. 60 feet.

RE: Steel Pipe Pile Foundation Design

bgsmith,

Some years back we were involved in the design of a new building in Charleston, SC (high seismic, high wind, and deep liquefiable soil strata). The geotechnical engineer suggested that we use groups of minipiles under each pile cap. This allowed the closely-spaced minipiles to condense the soil between and around them, improving it somewhat. However, he did suggest we design the piles based on their being laterally unsupported during liquefaction. The soil provided some limited dampening to the pile movement, but not much lateral support. Thus, we analyzed them as cantilevered columns, just as you were considering.

Thaidavid

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