Block Footing Foundation for a Self-Support Tower 1

[DataAB]

I am looking for information regarding footing designs for self-support communication towers.

The foundation design (by others) consists of a reinforced concrete block foundation (i.e. there is no pad footing, just a block) with dimensions 6 ft x 6 ft x 8 ft deep. The self-weight of the tower and antennas is small but the overturning moment (due to wind) is substantial. I cannot find any literature (i.e. Bowles, Das,...etc.) on this type of foundation subjected to substantial overturning moments.

The problems I am having with verification of its capacity are as follows:
1) If I assume that the base of the block footing is taking all of the loading, then
a) the kern limit is exceeded, and taking this into account,
b) the maximum calculated bearing pressure at the edge of the footing (assuming that all of the bearing area is NOT effective) exceeds the allowable bearing pressure of 3500 psf.
2) The base must obviously not be assumed to take all of the loading, therefore the sides of the footing must be assumed to take out some of the overturning moment. Is one side (i.e. vertical face) considered to experience passive pressure and the opposite side considered to be in a state of active pressure? Also, where would the axis of rotation of the block be located?

Comments or suggestions would be appreciated.

 

[civilperson]

See the method of pile designed footings for poles using soil pressure from the Horizontal Subgrade Modulus on the opposite sides of the pile,(rigid pile assumed).

 

[kslee1000]

You will need to consult with your geotech to work out the soil parameters. The focusing question should be the stiffness of the surrounding soil, from which you may derive the equivalent soil springs, which can then be applied to the analytical model as supports.

It is prudent to ignore at least 1' of soil layer at the finished ground level, unless there is pavement/slab surrounding the foundation block.

 

[azcats]

Could the engineer have used the lateral bearing design based on IBC 1805.7?