Torsion on drilled shaft
Torsion on drilled shaft
(OP)
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
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






RE: Torsion on drilled shaft
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.
RE: Torsion on drilled shaft
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
RE: Torsion on drilled shaft
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
RE: Torsion on drilled shaft
Good luck!
Matt
RE: Torsion on drilled shaft
RE: Torsion on drilled shaft
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
RE: Torsion on drilled shaft
RE: Torsion on drilled shaft
RE: Torsion on drilled shaft
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.
RE: Torsion on drilled shaft
RE: Torsion on drilled shaft
CAN ANYONE HELP ME
RE: Torsion on drilled shaft
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.
RE: Torsion on drilled shaft
RE: Torsion on drilled shaft