Torsional capacity in drilled piers

[FixedEarth]

I am working on a drilled pier design software. I would like to add torsional resistance and am very interested in the different equations and approcahes used worldwide. The program already has axial downward, uplift, lateral and moment loads. In addition it will have reinforcement design per ACI 318-11. Adding torsional capacity is the missing link. Thanks.

 

[Lion06]

What application of loading do you envision will put a caisson into torsion?

 

[FixedEarth]

If applied lateral load has eccentricity, this would cause torsion. This can also happen in cantilever signs and mast arm signs. rying to get several methods before deciding on the most appropriate solution for layered soils. Thanks.

 

[RacingAZ]

I don't think torsion in the pier itself would be in play unless you have a super glue bond between the concrete and soil or you extend the pier embedment to way more than the minimum required embedment that you get using the standard formula for determining the pier embedment length.

You should be checking the frictional resistance between the concrete and soil against rotation of the entire pier.

 

[Lion06]

I guess I can see that for a single cantilevered sign post. I'm not sure what other eccentric lateral load on anything other than a single cantilevered post would produce the same result. I've never seen a sign post supported by a drilled pier, although, admittedly I've not designed many. That seems like a lot of mobilization cost to get a drill rig for one or two drilled piers instead of throwing a large spread footing at it.

ACI 318 has provisions for torsion design in Chapter 11.

 

[Lion06]

Racing - I was thinking the same thing for skin friction with the soil, but I rarely see drilled piers count on skin friction with the soil. That's typically reserved for micropiles, in my experience, and drilled piers count on skin friction in the rock.

 

[RacingAZ]

Drilled pier vs. spread footing is more a contractor preference and local soil condition. Around here, a lot of contractors prefer drilled piers if they have to do a number of them, like in carport shade T-structures. I would agree if it's 1 or 2 footings, it's a hassle for the contractor to get a rig then get the holes inspected.

The skin friction value is normally in the soil report. I agree it's not a lot that's why the rotation of the entire pier would typically govern vs. the development of torsion in the pier itself.

 

[JAE]

It seems to me that there would be a magic depth where the pier would be essentially fixed. This depth would depend a lot on the torsional stiffness of the pier and the soil properties with respect to soil-conc. bond capacity and also soil stiffness.

If you assume the fixity point is too high up the pier (too shallow a depth) then you run the risk of underestimating pier rotation.
If you assume it too low, then you would overestimate the pier rotation.

But assuming a single point of fixity may not be entirely accurate either as the soil will deform to some extent along with the pier and nailing down the actual shearing stresses in the soil would be difficult.

The pier torsional design seems to be straightforward - there is an applied torque at the top of the pier and you would need to determine adequate torsional ties as well as longitudinal reinforcement for that.

 

[a2mfk]

Fixed earth got it, another example are traffic light mast arms, which also have signage to further develop torsion due to wind. This has been researched and addressed extensively in Florida by the FDOT and university research. I work with a retired geotech professor who has done a lot of research and field testing into these types of foundations, and there are some unique designs people are working on to specifically address torsion while minimizing embedment and diameter size.

It has been a long time since I have used it, but FDOT had several Mathcad templates for auger cast piles. I believe they were available to the public on their website, you can probably do some online research and find them.

 

[smb4050]

Just starting the design of a drilled pier to support a highway signalization pole that has a given design torsion of 63 k-ft, and plan to use ACI 318 for torsion design. I think AASHTO and/or FWHA have publications on torsion in drilled piers, but don't know if they address the soil mechanics, concrete design or both. Perhaps your program could address pier reinforcement for large over turning moments and torsion.

 

[FixedEarth]

Thanks for the responses. It seems that there is a geotechnical torsional capacity per Poulos 1971 ASCE paper, "Torsional responses of piles" and structural torsional capacity per ACI 318 Ch. 11, as Lion6 mentioned. As a2mfk mentioned, I just found a report by Cook et al from University of Florida, 2007 (Report #BD545 RPRWO #54) that really answered most of my questions.

JAE, you explain the plan torsion but how about the eccentric shear load torsion - see attached diagram?

 

[JAE]

FixedEarth - not sure I can read your curved arrows there or understand what your question is - I was only talking about plan torsion, as you put it - this would put a torsional load on the vertical shaft.

If you assume an X, Y, Z axis on top of the pier with Y vertical and X and Z horizontal directions, then I'm talking about My (moment about the y axis) which is pure torsion in the shaft of the pier.

If there are shear forces laterally (Fx or Fz) then they would simply apply direct shear to the shaft, not torsion. If Fx and Fz are eccentric to the center of the shaft, then they would each have My components to the shaft which would just be added to any other applied My value.

 

[FixedEarth]

JAE - I see. Thanks, that clears it up. Well put.

 

[a2mfk]

FDOT's mathcad template included torsion design of pile in the soil, which was many times the controlling factor in high wind design of cantilevered mast arm structures.

The current research is a pile system that is water jetted into the ground and then pressured out into the soil mass, instead of relying on skin friction of an auger cast concrete pile. The values in torsion and bearing are impressive.

 

[FixedEarth]

Thanks smb4050 & a2mfk for your suggestions. It seems like Florida DOT has done a lot of work in Torsion capacity of drilled piers. Anyone in FL, do you use or are you familiar with the attached PDF methodoloy?

http://www.soilstructure.com/