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RE: torsion in circular foundations

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Short and simple, you can definitely resist torsion in a pile. I had a professor in college who got a grant to study this for when boats hit bridge piles, and I saw them testing them to failure in torsion. Pretty neat (for engineering).
From limited DOT experience, I know that pile foundations used to support traffic structures such as cantilevered signs, traffic lights, etc. ("mast arms"- like the one you have) are often controlled by torsion from a geotechnical analysis. This is of course in FL, so high wind is the issue. Sandy soil perhaps does not provide the greatest amount of lateral pressure and friction but it does provide restraint against torsion. The sizes are usually per DOT specs and are predesigned for specific structures and wind speeds. The piles are a minimum diameter to support the mounting of the structure above it, so min As controls the concrete design. This is really a geotech problem, I highly suggest coordinating with them, they will do the dirty work (pun intended), and you can concentrate on the concrete design and anchor bolts. We did have a problem once where a pile foundation was poured too small and it was failing in torsion (soil) and we had them come back and dowel into the pile at the top and pour "fins" which acted as keys (like someone else suggested), but we got the soil info from the geotech so we only had to do concrete design. If this is a new pile it is likely cheaper to size the pile for all forces from the begining. Don't forget your proper FS.
When we did do some analysis we used a Mathcad program, I believe you can download it for free from FDOT website. It is very in depth and covers all the bases.
On a side note, torsion and overturning in sign structures is a big issue in FL due to high wind loads. You have signs on interstate highways that cantilever out over many lanes and have huge signs attached to them. Also, many old traffic lights here that were hung on cables are now being remounted to large "mast arms" which are large HSS members cantilevering up from the ground which are bolted to piles, then the "mast arm" or arm that supports the lights cantilevers off this. When you think about a 6+ lane intersection with lights and signs mounted to it, you can see why torsion becomes an issue. This is for aeshtetic reasons (they do look better then cables), and they don't sway in strong breezes and hopefully don't fail in high winds. I wonder if this type of changeover is happening in other parts of the country.
Andrew Kester, EI
Longwood, FL