had disagreement over Concrete Light Bases

[WE WERE THE MULVANEYS]

I got into an argument with another engineer over the embedment depth of concrete light bases. A contractor wanted to cut 18" off the bottom of the base, that measured 5'-0" and he wanted to make them 3'-6". I told him no because the stability is met by the embedment depth and concrete weight to resist overturning. If you chop off 18" of concrete you are losing that weight. Another engineer said I was wrong because once these are buried and if you have sufficient embedment depth, the weight of the base has no effect on the stability of the light pole. I don't agree because although depth and compaction around the base is important, if the base is too light the light pole could potentially want to have a "rocking effect inside the hole". What I mean by this is say you have a 16" hole with a 16" diameter base, you could back in a year and the light pole may still be standing, but there is now an annular space around the circumference of the concrete and the soil backfill due to slight movement over time. I just wanted to get some other opinions here.

 

[ptatohed]

It should be 5'.  Here is our city's standard.

 

[youngmotivatedengineer]

Depth will vary by depending on what is the reasoning of the concrete base.  If there is a cantilevered light at the top of the pole, the manufacturer will probably have specific requirements needed for the base to resist the moment and the end of the light. If it is just a way to keep the post straight in the ground, depth usually depends on the frost level. In NJ, footings only need to be 30"below grade. If the base is designed for stability of a cantilevered light, you may be able to compensate for the shorter depth by making footing wider and using steel reinforcement to increase the weight.

 

[ptatohed]

Good point yme, WWTM didn't state what kind of 'light' but I assumed he meant a street light with a typical 24'-28' pole height and an ~8' mast arm.  

 

[geomane]

Passive earth pressure would likely govern the design.

 

[OHBridgeGuy]

I agree with JMcc06, the embedment is almost always governed by lateral resistance provided by the passive earth pressure.  The weight of the base does not provide much in the way of overturning resistance and is really not that much of a factor for minimum embedment, but depending on the size of pole and wind criteria uplift resistance could be a factor.

You can always go shorter, but normally this means going to a much larger diameter base.  For a rough calculation you can use IBC 1807.3.2, equation 18-1 with presumptive capacity values from IBC 1806.2.

https://up.codes/viewer/general/int_building_code_2015/chapter/18#1807

Remember that lateral resistance is essentially a cubic function since the force F = 0.5*gamma*H^2, and the moment resistance = H/3 * F, so the best way to optimize the foundation is to get it deeper.  It's hard to effectively make up for the loss in depth with more width economically.  

 

[WE WERE THE MULVANEYS]

Thanks