Formula For Calculating a spread footing

6/2/2011

Question:
Hello. I have an 18'-0" wide x 26'-0" High x 3'-0" Wide Pylon to Install. How would I calculate the size of footing I would need for this pylon. Please let me know the formula you would use. Thank You.

Answer:
Hi ,
The footing you describe is similar to sign post footing or electrical light pole footing, I would not characterize it as spread footing.



In this type of footing design, the most important load to consider is wind load, you don't want the pylon blown over and cause damages to other properties. Not knowing your state and city make it difficult to come up with exact numbers. For illustrative purposes a few assumptions were made to come up with the numbers shown in the table above.

The factors that influence the wind effects on the pylon include contour of the land, buildings, trees, surroundings, etc. The assumption in the above table was the land is flat and very open or exposure C with 80 miles per hour wind.

From the table column P is the wind pressure in pounds per square feet resulted from multiplying columns Ce,Cq,qs,Iw. The A column is the area of the pylon affected by the wind which increase with height. F is the force applied on the pylon caused by the wind.

As you can see there is 11,874 pounds pushing on the pylon during 80 mph wind storm. This force will cause 154,356 lb-ft of overturning moment at the top of the footing. The footing need to be able to resist this overturning moment.

This will be trial and error design, i.e. keep trying a footing size till you find one that works. Due to the 18 ft span of the pylon, 2 or more footing will be the best solution, assuming the pylon has two point of support about 18 ft apart.

Two - 3 ft diameter drilled shaft by minimum 6 ft deep buried in the ground reinf. w/ 10#8 vert & #4 ties @ 12" o.c. After the footing is poured backfill, compact really well and pour concrete apron at surface 6ft square, 6" thick reinf w/ #4 @ 12 EA WY. The concrete apron will help prevent water from weakening the soil at perimeter of the footing.

When calculating the resisting moment, use the soil pressure against the footing. The resisting moment due to the weight of the footing itself is pretty small compared to the resisting moment provided by soil pushing against the footing.

If the pylon is continuously supported in the 18 ft span than you will need a continuous rectangular footing. Divide the 11,874 by 18 will give you wind force per foot which is 660 plf. Multiply that by 13(half of 26ft, height of pylon) will give you your overturning moment per foot. Use that to figure the depth of soil that you need to resist overturning. A good start is 14 inches wide wall by 3 ft deep reinf. w/ #5 @ 10" o.c. EA WY, EA Face

Before finalizing your footing, hire a licensed engineer in your area to verify the information before construction.

Good luck
Tonga