Axial Load capacity for an partiall encased pile

[Awre]

Hi,
I have an HP pile in the water, 29 feet exposed height and assuming fixity at 20 feet below mud line.

http://www.eng-tips.com/

The pile is encased by 24" diameter concrete from top until 15 feet down (almost half the exposed height is encased)
How to calculate axial load? How to calculate the P critical for buckling.
I appreciate if sample calculation or spreadsheet is available..

Thanks

 

[KootK]

I'd treat it as a 49' bare steel column fixed at the bottom. kL = 0.7 x 49' assuming a pinned condition at the top. Given that the concrete encasement is only effective over the upper 1/3 of the unbraced length, it won't do squat to improve buckling capacity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Lomarandil]

I agree with KootK's conclusion (it won't do much, only being on the top 1/3), but you can look for stepped column solutions (Dalal published some) if you really need to sharpen the pencil.

Make sure that you're adequately accounting for any cracking in the concrete section.

 

[Awre]

Thanks KootK and Lomarandil..

Can I reasonably assume the same with the following 2 scenarios:

- the same 49' long HP with 30' encasement (~full of the exposed height and 60% of the total height)
- 35' long HP at shallower areas (15' exposed + 20' to fixity) with 15' encasement (full of the exposed height ~ 50% of the pile)

Thanks

 

[KootK]

These are certainly more likely candidates for improvement. Personally, I'd probably still just go with bare steel. As Lomarandil mentioned, it all depends on how badly you want the extra capacity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Agent666]

Do a buckling check in any capable commercial analysis software accounting for your boundary conditions top and bottom. It's clear the the effective length may be less than the full bare steel length if the concrete effectively provides restraint to the buckling forcing a higher mode shape for buckling.

It's usually a relatively easy thing to do and it either gives you a load factor by which you can increase the loads, of it gives you an effective length factor so you can go off and design it. It's usual practice to decrease this by a factor of say 0.8 so it's consistent with code values as the code length factors have some conservatism when compared to the exact Euler buckling length factor. For example a fixed/fixed ended column is potentially taken as 0.7 in a code, but the true Euler solution is 0.5 for the same scenario. This difference usually accounts for imperfections, for example initial variations in the straightness for the members.

 

[SlideRuleEra]

...assuming fixity at 20 feet below mud line.

The key to getting an accurate answer is how "good" is that assumption (especially at the pile's full loading). Soils must be "poor" to have fixity that deep. Assuming a point of fixity is reasonable for piles with conservative loading (and generous safety factors). Since the intent is to maximize loading based on pile buckling, IMHO, assigning an estimated depth to full load fixity is not wise.

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