Resistance factors for axially loaded piles
Resistance factors for axially loaded piles
(OP)
Regarding LRFD Bridge Design Specs Table 10.5.5-2, if "Wave equation analysis with assumed driving resistance" is used, the resistance factor is 0.65 * lambda-sub-v.
Our geotechnical engineer won't recommend this since he believes it is only appropriate if the drilling/sampling equipment was intstrumented with something like a pile driving analyzer, and those results are used to calibrate the soil properties or driving resistances for a wave equation analysis.
I believe that interpretation is incorrect.
I believe the 0.65 * lambda-sub-v can be used if a wave equation analysis is used with soil properties that are "assumed" based solely on SPT tests during drilling and observations of soil samples.
Who is right? Does the Barker NCHRP Report 343 discuss this? Given the price of steel right now, this makes a big difference in our project cost.
Thanks!
Our geotechnical engineer won't recommend this since he believes it is only appropriate if the drilling/sampling equipment was intstrumented with something like a pile driving analyzer, and those results are used to calibrate the soil properties or driving resistances for a wave equation analysis.
I believe that interpretation is incorrect.
I believe the 0.65 * lambda-sub-v can be used if a wave equation analysis is used with soil properties that are "assumed" based solely on SPT tests during drilling and observations of soil samples.
Who is right? Does the Barker NCHRP Report 343 discuss this? Given the price of steel right now, this makes a big difference in our project cost.
Thanks!





RE: Resistance factors for axially loaded piles
In the Standard Spec.'s, the intent of the higher Performance (LRFD Resistance) Factor for piles with wave equation and PDA testing is for this effort to be conducted during construction, not during the soil exploration or design phase.
Since you do not know the exact model of pile driver the contractor will use, or the types of cushions etc..., the wave equation analysis is only marginally helpful during the design phase. It can (and probably should) be used as a check during the design to see if the piles can be driven without damaging them using an assumed pile driver and cushion properties.
NCHRP 343 has the following text, which supports this approach:
"Performance factors can be affected by careful monitoring of pile installation and how contract documents are written to permit changes in installation procedures. For example, higher performance factors may be used if wave equation analyses are run prior to driving pile, pile driving blow counts are compared to the wave equation results, and representative piles are monitored with the Pile Driver Analyzer".
Hope this helps.
TTK
RE: Resistance factors for axially loaded piles
Good Luck!
RE: Resistance factors for axially loaded piles
RE: Resistance factors for axially loaded piles
Going from soil boring SPTs to production pile BPFs is a mighty big stretch. Some swear by it, but I think the good correlations that may occur locally is as much coincidence as anything else. Instrumented SPTs help, but they aren't a cure-all.
Choosing a good hammer-cushion-pile-soil combination isn't too hard if you have realistic static capacity estimates. (This is a problem when the geotechnical engineer has followed a very conservative design philosophy.) Then use the PDA to monitor and evaluate the system. Run static load tests if possible, then refine the system if needed.
You will end up with a very well designed and constructed project -
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"