Torsion on drilled shaft

[RRBegin]

Howdy everyone!

I am looking for some guidance on determining the torsional resistance of a drilled shaft. The shaft is not part of a group of shafts; rather, it is the sole supporting element for a traffic signal mast arm. I am trying to determine if an existing shaft is adequate to support a revised mast arm configuration. I have the forces transfered to the shaft, and have already evaluated it for moment, axial load, and lateral load. It also carries a torsional load (33 ft-kips) and I am at a loss as to how to calculate an allowable torsional resistance. If someone can point me to a reliable source or has other information, I would certainly appreciate the input.

Thanks from Rhode Island, USA

 

[JAE]

I would suggest hitting the ACI 318 code and using Chapter 11, Section 11.6 (Design for Torsion). There, you will find specifications for design of concrete sections under torsional loading - with or without axial loads.

At the top of piers, be careful that you do not depend upon the longitudinal steel as vertical pier steel usually just terminates at the top and is not developed at the pier free end (top) to help.

 

[pigdog]

RRbegin
Are you looking for a calculation to determine if the foundation is sufficient to keep the shaft from falling over? When we do investigations for drilled shaft bridges, we verify that bearing capacity and "4T" embedment is available. The "4T" depth is based on RQD blow count and shaft diameter.
roger

 

[RRBegin]

Thanks Roger (pigdog) and JAE-

I know how to design the shaft from a structural standpoint, I was looking for input into the geotechnical aspects that pigdog alluded to. Roger, can you expand on your information? What is the 4T depth?


Thanks again,
Ron

 

[breaks]

I suggest checking out the AASHTO Publication "Standard Specifications for Structural Supports for Highways Signs, Luminaries and Traffic Signals". It has a lot of foundation information that will help you out, including a section on foundations that has a table dealing with the required embedment of the Foundation Post (the drilled shaft support you refer to).

Good luck!

Matt

 

[RRBegin]

Thanks, Matt. Shame on me for not going to the obvious source.

 

[pigdog]

Rrbegin,
Well, first I have to admit to being a geologist at NCDOT. We use charts and rule of thumb material where I don't necessarily understand the engineering basis. Rocks, yes, engineering, maybe. The condition that guides us is "fixity".I quote: "This method utilizes the term 'T' which is equal to the fifth root of the quantity column stiffness (EI) divided by the coefficient of horizontal subgrade reaction (Nh). E is youngs modulus, I is the column moment of inertia, and Nh can be approximated as the SPT blow count. ...A column will act as if it were fixed whenever an embedment of 4 times 'T' is achieved.." A quick search on google for "fixity" directed me back to these forums and various higway dept. sites. Anyway you would need to know the depth of your drilled shaft and the soil or rock conditions as near to the location, (feets,not 100feets)to determine shaft diameter and depth.
Hope this helped.
roger

 

[wiktor]

Calculate the skin friction for both the shaft and the bearing end. The skin friction x the radi will give you ultimate torsional resistance of the shaft. The procedure is similar to calculation of the capacity of the friction pile or shaft.

 

[RRBegin]

Thank-you everyone, you have given me a few ideas/references. Time for me to earn my pay!

 

[mike80]

You might also try FDOT's software website:

http://www11.myflorida.com/structures/proglib.htm

They have a program you can download which analyzes drilled shafts supporting signs. Only requirement is that you need Mathcad to run the program.

 

[RRBegin]

Thanks again everyone, I found what I needed thanks to you.

 

[Kwissifar]

Would anyone know of a formulae that deals with the embedment of a drilled pier and the TORSIONAL MOMENTS. I currently have transverse loads, longitudinal and transverse moments, and torsional moments from a structural engineer, BUT from a geotechnical state of mind, I need to verify that the embedment has adequate soil friction or adhesion to counter the TORSIONAL MOMENT...
CAN ANYONE HELP ME

 

[chicopee]

Your drilled shaft which is to support an arm is stresses at its base from three factors:
1) bending from the weight of the arm and of any dangling appurtenances forming a moment thus creating a bending stress; bending also occurs from wind loading on the dilled shaft. Bending creates bending stress
2) torsion from wind load on the arm and related appurtenances. Torsion creates torsional stress
3) compression from the weights of the shaft,arm and related appurtenances. compression creates compressive stress
4) horizontal and average shear stresses which may neglected if external forces on shaft are not too high.

 

[MarkJ2]

Try the FDOT site as indicated by mike80 above. It may be of some help.

 

[BigH]

Isn't the field vane test a torsional test?