Pole footing 3

[scottiesei]

It's been awhile since I have designed cylindrical footings, such as those formed from sono tubes and filled with RC. I have a 12' light pole and am coming up with a 16"diax36"d footing sized for overturning. This seems a tad large too me....any comments?

 

[JAE]

Doesn't seem too large to me.

The 16" may be required to allow cover over the anchor bolts and the depth is what we see as a minimum to get below frost depth.

 

[civilperson]

The top 12" or so of soil is not counted for stability in most software. A provision to hold the top of the footing rigid in pavement will lessen the requirement but the depth must be down to frost level. Try varying the horizontal sub-grade modulus to see if this variable has much effect.

 

[msquared48]

16" Dia X 36" Depth does not seem too large to me either. Normally I have a minimum burial depth of four feet and go deeper or wider if needed. With the frost level here of 12" to 18", that does not come into play for my projects with these footings. The center of rotation is normally about 2/3 of the burial depth (+-) for the unrestrained (no slab) condition.

Mike McCann
McCann Engineering

 

[scottiesei]

Thank you everyone.

 

[Lutfi]

I really can not comment if the footing is large enough or not since critical information is missing. Here is a short list of data that I may want to evaluate before responding:

1. Wind load/speed
2. Estimated Projected Area (EPA) and its location
3. Soil type and strength
4. Is the pole footing restrained at surface?

I use a rule of thumb for initial estimation, “footing depth is estimated to be about 1/3 the height above ground”, for this type of foundation

I also would suggest that the pile footing be few inches above grade to reduce effects of corrosion to the pole base.

Good luck


Regards,
Lutfi

 

[scottiesei]

1. 100mph
2. 10.0 at pole top, 8.6 18" above
3. sand, using 110 pcf and 40deg
4. more than likely yes, but we are analyzing with no restraint.

This is an al light pole....