High Mast Arm Design 1

[GOTEK]

Our Agency has completed very few high mast arm designs, a limiting bearing capacity of 3 ksf is typicaly used, except for those foudations constructed in poor soils I am designing four mast arm foundations using drilled shaft design and FBPIER in combination with LPILE to quantify deformation under lateral loads. I have also used BROM's Method outlined in Section 13 of AASHTO's Stadard Specifications for Structural Supports for Highway Signs, Luminaries, and Traffic Signals to confirm embedment depth due to the overturning moment. I believe that I am lacking a standard method for analyzing torsion.
Has anyone performed a torsional stability check on these structures if so what method did you utilize? Was a factor of safety of 3.0 utilized or was the factor of safety more like 1.3? I performed a crude analysis using the skin frictional resistance around the shaft diameter and multiplying it by a lever arm - which I assumed to be half the shaft diameter.
One other coment, the torsional force appears to be governing the design rather than the moment, is this typical?

 

[eric1037]

GOTEK:

Check out the following reference:

"Determination of Optimum Depths of Drilled Shafts Subject to Combined Torsion and Lateral Loads Using Centrifuge Testing." By M.C. McVay. Florida DOT Report BC354-09, March 2003.

 

[Panars]

Florida DOT has a mathcad worksheet that analyzes overturning and torsion in drilled shaft foundations for sign and signal structures. The website is:

http://www.dot.state.fl.us/structures/proglib.htm