Skin friction resisting torsion for a drilled concrete pier 1

[MartinOsier]

Hello All,

We provided some pier recommendations for support of a power line. The structural asked if he could use our skin friction values provided for axial loads to resist torsion. Been only working in the Geotechnical field for five years and haven't been asked this question before. I would think there would be no problem with that, but would like some opinions.

Thank you.

 

[SlideRuleEra]

...asked if he could use our skin friction values provided for axial loads to resist torsion.

Not with confidence:

See "Torsional Resistance of Drilled Shaft Foundations" (October 2018)

http://www.SlideRuleEra.net

 

[Lomarandil]

What sort of transmission line structure could deliver a significant torsion to the foundation? Some sort of heavy angle monopole?

I'd be really tempted to calc out what the shear friction would be if distributed over the entire surface area of the drilled shaft... because I'm having a hard time imagining that being anything more than a rounding error.

----
just call me Lo.

 

[Ron]

Lo...when a conductor breaks it introduces an unbalanced load on a typical "T" configuration including torsion. Wind loads can do similar.

 

[Lomarandil]

Yep, I stand corrected! Thanks Ron

----
just call me Lo.

 

[oldestguy]

OK let's say a break occurs, what will be the ground water level? Does friction depend on the density (submerged or not)? Likely yes. What do you design for? Then how do you get more torsion resistance from a standard caisson diameter and how likely is that cable break going to happen against a bunch of enlarged caissons built to handle that break, maybe? How to allow for varying soil condition's from one to the next? Didn't help did I.

 

[skeletron]

Thanks for the paper @SlideRuleEra. I'm going to dig into it.

I know ALLPILE does allow you to input a torsion force and it computes the allowable torsional force of the soil. I'm not sure of the magnitude of your forces or the scale of your pile. In my experience the torsion force is significant, but generally less than 20% of the calculated capacity from the software. Not too sure of the particulars of the design method (off the top of my head), though, and maybe that is addressed in literature somewhere...