"Flexible" pile parameter?

[VoyageofDiscovery]

Does anyone know what EI/L defines a laterally loaded "flexible" pile?

VOD

 

[Focht3]

EI/L isn't enough information. Load is an important component in that evaluation as well. And the soil stiffness is also key.

You can get a rough estimate of a pile's rigidity by looking at L/d; it's stiff if the L/d is 10 or less, flexible if it exceeds 20. "Short" piles tend to rotate as rigid bodies while "long" piles tend to flex or bend.

Relative flexibility has more to do with the deflected shape of the pile under load, hence the use of L/d as a method to estimate a pile's likely rigidity. I realize that I haven't answered your question in the form you requested; it's just that EI/L is not commonly used to measure flexibility since pile materials can vary so much.

Are you looking at a particular pile, loading, soil conditions and use - and are concerned about "flexible" behavior?

 

[VoyageofDiscovery]

Hi Focht3,

Recently there appears to be two diverging schools of thought with respect to the behaviour of piles during horizontal earthquake loads. Both refer to the Winkler differential equation, however approach group effects differently. Research by Randolph and recent research in Japan imply that most piles are practically flexible and thus the question.

Actually, I'll be designing my piles as flexible as per the Canadian Foundation Engineering Manual (CANFEM) as I wish to have no batters due to recent concerns with regards to the performance of battered piles (increased stiffness) under earthquake loads.

Regards

VOD

 

[Focht3]

Yes, avoid batter piles in earthquake areas! (That is, a mixture of piles with differing batters...)

I'm not surprised that most piles are effectively "flexible" under dynamic loading. As soil stiffness increases, 'effective' pile stiffness decreases - so piles should be "more flexible" under seismic loading. I'm not familiar with Wroth's recent work; did he refer to the EI/L ratio as an important parameter? What were his key factors?

 

[VoyageofDiscovery]

Hi Focht3,

I was trying to determine what defined flexibility. Randolph's work didn't refer to an EI/L. He just says that most piles are actually flexible.

Regards

VOD

 

[Focht3]

VoyageofDiscovery:

Okay, now I understand your question. Having said that, you may or may not like my answer...

I would suggest that you look in the 1964 ASCE Journal of Soil Mechanics and Foundation Engineering for two papers by Bengt Broms - he provided a method for predicting laterally loaded pile behavior. As I recall, the method he proposed involved calculating a relative stiffness coefficient. (Please understand that I haven't used this procedure in 15 or 20 years, so my memory may not be correct on this point.) Lymon Reese and Hudson Matlock proposed a similar idea complete with tables of factors (not graphs) that is quite easy to use. (I don't remember when or where this paper was published; I received a copy from Dr. Reese while I was a student at The University of Texas.) Both methods will require you to make some estimation of the soil; Broms' method only required the undrained shear strength, so it will probably be of more use to you. Reese and Matlock's procedure requires you to estimate the modulus of subgrade reaction, which is neither straightforward or easy.

Having said all that, you will almost certainly come to the same conclusion as Wroth (and others) have about pile behavior. Part of the reason that most piles are relatively flexible has to do with our desire to avoid catastrophic failures: we increase the pile embedment to effectively prevent the pile from experiencing a rotational failure - surely a catastrophic event for most structures.

I hope that I have answered your question -

 

[VoyageofDiscovery]

Hi Focht3

Thanks, the discussion was fruitful.

Regards

VOD

 

[adip]

Iam asking about "Soil Pile Interaction" and spring modelling for calculation of pile group

regards,

Adip

 

[Focht3]

adip:

Why don't you start a new thread? I'll look for it - and respond -