Contact your local electric company and ask one of their engineers what method they use. I have some examples but I suspect there are local regulations which may apply. Who will review your design ? Ask them too since there are several ways to do these designs.
I am trying to verify the adequacy of (and min depth of) a 2' diameter drill shaft to anchor a tapered metal light pole with 300 lb of lights & bracket mounted atop pole at 50 ft above ground level. THe pole manfacturer has a chart showing adequacy of pole to suppot up to 332 lb and 16.6 sf of Effective projected area (EPA). THe lights and bracket combined weigh 303 lb and have an EPA of 13.5 sf. THis is supposed to take into account the velocity pressure of 100 mph wind. So the pole itself is fine per the manfacturer, but the foundation is left to be designed. I have found an example of a drill shaft design in sand using Broms Method. THe material on site for this application is clay at and/or below the water table. I have so far assumed bouyant sand since I have a sand example and since it seems to me that bouyant sand would be a more conservative case than clay. My trial has resulted in an 8' depth of shaft. However I have two basic questions still--
(1) I have used an excerpt from Section 6 ASCE 7-95 for wind loads on buildings to determine qz (velocity pressure and then p (wind load) on theppole and lights to come up with moment and shear at the base of the pole (top of drill shaft). It would seem to me that there should be a reduction factor to apply to the drag against the pole since it is round ands smooth, but since I am relying on a spec for buildings I have no documentation to back that up. Does ASCE 7-95 have such a factor and if so could someone lead me to it (supply a reference)?
(2) Broms Method looks at a "pile" and determines an ultimate soil reaction to the overturning forces involved (due to wind loading in my case). How do you determine the Moment capacity of a round reinforced concrete drill shaft (so I'll be able to show that the shaft is not overstressed)?
I don't have ASCE 7-95, but ASCE 7-98 has in Table 6-10 Cf for round structures.
If you need to check the concrete pier, use standard column design tables for a very small (approaching zero) vertical load and get your moment from there. Any two foot diameter reinforced concrete pier is going to be OK, but you should run the numbers.
CivilEmery: Check with vendor. Quite often, vendor do not go through the details of ASCE 7; since they sell these all over the place geographically, they want to cover all the bases, be conservative, minimize changes in design/manufacturing and stock/inventory, they may use a straight 30 or 40 psf and use the flat projected area of the pole to calculate forces and moments. If you went thru the ASCE 7, you will quite likely find lower values.
And, regarding foundations, be conservative. The single largest reason for light and flag and similar structure failures is not wind, but deflection. Who trusts a deflected pole? And the primary reason for deflection is underestimating the effects of rocking or oscillation. One must consider vibration, natural frequency, response, etc.
Not to discourage, but to educate, and make one awares that many simple things are in fact complicated. Call a local structural/civil PE who is willing to share and educate.
The California Building Code (=Uniform Building Code + amendments) has a design method for sizing the pole foundation in §1806. I recently completed a pole foundation design for moving an 85' high pole from one location on a site to another. I took the wind force on the pole and converted it to a moment at the base of the pole, then converted this moment toan eccentric load on the top of a circular column (i.e. the foundation). I have assumed that the foundation depth calculated per the CBC/UBC method created a column fixed at the bottom, and I have assumed conservatively assumed no passive resistance from the surrounding soil. Eccentrically loaded round columns are a bit of a pain by statics, but there's a neat equation in my concrete book called the Whitney-Hognestad Formula that allows you to check an eccentrically loaded column under tension control.
