huangyhg

light pole foundation design
i need to design a spread/shallow foundation for a laterally loaded pole. hi wind loads create a considerable moment. any ideas?
thanks
we usually use a drilled concrete pier for light pole foundations. this requires analysis of a pile/pier in the ground with lateral shear and bending moment applied at the top of the pier.
you can model the pier in a 2d analysis program. another proprietary program is "l-pile". not sure where you can find it online. there are also some nomographs and charts for laterally loaded piers in the structural engineering handbook by gaylord & gaylord.
jae, thanks for the info, however my dilema is in regards to the posibility of a spread footing design under such conditions where the eccentricity falls way out of the kern and way above the l/6.....how can this issue be address when there is a limitation of the width of the foundation in the direccion of the moment...
agree with jae. if possible, use a drilled pier. we usually hire a local geotech engineer to run l-pile for us, and give us the required depth of the shaft and the shear and moments, then we design the required reinforcing.
is there some reason a drilled pier is not possible on this site?
if a drilled shaft is not possible, are drilled helical anchors an option on this site? have designed a couple of sign foundations, where drilled shafts were not possible, using helical anchors as 'hold downs' on the edges of the footing.
i agree with previous posts. use a drilled concrete pier if there are no boulders or other reasons one could not physically drill such a foundation element.
get the aashto design manual for signs and luminaires. there are cookie cutter formulae for the pier embedment based on moment and soil lateral stiffness. i just did one of these for a small custom welded road sign and compared the aashto vs ibc formulae (since my knowledge of aashto codes is neglible) . the ibc formula yielded one more foot of embed than the aashto formula...
these formulae do not (if i recall) tell you anything about the bending moment in the pier. you can look up some solved equations for beam on elastic foundation stuff with the boundary conditions you may have. the upper bound moment is the moment at the base of the pole.
l-pile is nice and is the most suitable software, but my opinion is that a material non-linear fea is perhaps over-analysis given the "lateral soil stiffness modulus" is known to maybe one significant figure. my old boss (geotechnical) would groan, roll his eyes and simply look up presumptive lateral modulii like this and i suspect that there is great degree of uncertainty with any such lateral soil stiffness determination. a 2d fea model with linearly elastic soil springs is probably a reasonable simplification for the amount of lateral deflection most structural engineers would deem ok.
the ibc formulae that sundale mentioned are found in section 1805.7. this approach is very simple and is typically sufficient for light pole or sign bases. for more complex and important uses (building foundations, etc.), l-pile or similar analysis may be warranted.
guys, thanks so much..i will proceed with checking aashto design manual for signs and luminaires and try drilled conrete pier.
i appreciate your help
thanks
you can use a spread footing, of course. but if you do, i would be very tempted to deepen it and widen it such that the footing resultant is still withing the kern. if not, i would short term winds might create long term loss of plumbness of the pole and footing. as the footing lifts, debris and dirt fill in (along with water getting sucked in with a clay condition perhaps). i just wouldn't feel good about the design.
here are some good references for pole foundations:
1. design of concrete foundation piers, by frank randall; portland cement association (pca), skokie, il, may 1968
2. resistance to overturning of single, short piles" - by eli czerniak; asce journal of the structural division, vol. 83, no. st2, paper 1188, march 1957
3. 1997 uniform building code (ubc), section 1806.8, page 2-45;
4. outdoor advertising association of america (oaaa) - new york, ny
5. tapered steel poles - caisson foundation design; prepared for united states steel corporation by teng and associates, july 1969
regards,
lutfi
fdot has a mathcad program that designs light poles, from foundation to anchorage to pole properties. if you have mathcad it's fun to play around with.
dude... get a copy of enercalc. it does pole design, footing design, etc. and do whatever jae, lutfi, and taro tell you because they are legends of the game, so to speak... although their names are a bit indecipherable. ah, anonynimity...
but perhaps that is just so they don't get sued by some yahoo on eng-tips.
mike

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