Resistance factors for axially loaded piles

[gopher2]

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!

 

[TTK]

I don't have the LRFD Specifications in front of me, but I believe you are both wrong.

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

 

[DRC1]

I agree with TTK. In the design phase you can (and should) do a static analysis of the pile. Here the soil properties are very important to the quality of the answer. However, since there many assumptions present, this is generralyy the least accurate method. You can also do a wave equation for some possible hammer pile configurations to get a feel for how the static matches. Although a part of the input, the quality of the soil property data does not have to be as high. SPT are driven with PDA attachments, but this is more for research and esoteric projects than our everyday projects. This level of sophistication is not needed for wave equation. Personally, I think you would be better off doing SPT's without PDAs and doig some lab tests on some samples. Once a contractor has been selected, he can give you the details on his pile driving set up. You can then modify your wave equation runs based on on his equipment. Note that the cushion details are very important to getting good results. This will give you a bearing graph showing the pile capacity for a given set at the approximate bearing level. This is much more accurate than the static analysis alone, and therefore qualifies for a better reduction factor. To really ensure you have capacity, run a static load test. This will give you an even better factor. For information on Wave Equation go to

http://www.pile.com

For Static anlysis and load test analysis, I like unisoft products. Go to

http://www.unisoftltd.com

Good Luck!

 

[DRC1]

One thing I forgot to add is that you can perform a Pile Driving Analyzer (PDA) test on a percentage of the piles as they are driven. The PDA measures the force delivered to the pile by the hammer and the pile's resistance to the blow. This technology has been around for a while, and as part of a Q/A program, can improve reliability of the pile and therefore reduce the allowable F.S. This technology has only recently been adapted to SPT testing This may be the source of your confusion

 

[Focht3]

This LRFD spec is new to me. But I can talk about "wave equation analysis with assumed driving resistance." Been there, done that. A big waste of time. It was worth a try - we had over 900 piles to install. But the big problem was getting from the static capacity analyses to the probable dynamic resistance values of the soil. And the SPTs were all over the place. Very frustrating.

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?"