which stiffness of piles is used in engieneering?

[peterman722]

I am going to do some research work on piles' stiffness. would you tell me ,in th engineering,which stiffness, initial stiffness or the secant siffness under design load, is used more frequently? and in what situations are they used,respectively?

 

[Focht3]

Stiffness is a function of load (for non-linear materials), that is, the initial stiffness generally applies for low loads while the secant modulus may be more appropriate at working loads. Keep in mind that tangent stiffness, rather than initial or secant stiffness, may be more appropriate for some dynamic problems.

There is no simple answer.

What are you trying to research, anyway?

 

[peterman722]

some engineers analyzing pile-raft foundations of high-rise buildings is going to take the piles as springs under the raft in their models and one of their concerns is the axial stiffness of piles. If field load tests data are not available, how to estimate the stiffness under work loading according to the soil stratum?
My goal is trying to get a practical method to calculate the axial stiffness of single pile in sand.
thanks for reply.

 

[Focht3]

There are a lot of methods available...look in the Offshore Technology Conference (OTC) proceedings. Search for papers by Kraft, Reese, Matlock, Kagawi (sp?), O'Neill. That should get you started.

Also look in the book, Offshore Engineering, and read the chapter on axial pile design by Focht & Kraft. Focht wrote the practical part; Kraft wrote the theoretical part. (You will understand when you read it.) Kraft's work is good, just a little too specific for me. Things change over time. But it is probably far more detailed than you want...

No, I did not co-author the chapter. But I know the guy who did!

 

[BantrelStructural]

This is a problem that lends it self to sensitivity analysis, where various pile stiffnesses are examined to see what the impact is of over/under estimating the stiffnesses.

Of course easy to say, but lots of work to execute. Still it may be necessary to determine potential problems....

 

[Focht3]

Yes - you need sensitivity analyses for this class of problem. It is a lot of work, but it's necessary.

By the way, My book citation was wrong - it's Planning and Design of Fixed Offshore Platforms edited by Bramlette McClelland and Michael D. Reifel. Look at Chapter 21 for pile stiffness considerations -