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[TowerTalk] Tower Base Construction

 To: [TowerTalk] Tower Base Construction thurman@primenet.com (Pat Thurman) Tue, 20 Mar 2001 12:38:10 -0700
 ```"Lonberg, Hank" wrote: > The Spread footing type of foundation uses the geometric properties of the > shape, usually a square or rectangle, to distribute the loads , vertical and > overturning moment to the soil., in this case the maximum force delivered > needs to equal or less than the vertical bearing capacity of the soil. > As an example I reviewed an used 67' tri-ex tower for an 85 mph wind load > for a client and developed 2 different foundations for this tower. One a > post type, and one a spread footing type. By the way the tower itself did > not calculate for the 85 mph wind since it was originally designed for 50 > mph. > The overturning moment was 28,000 ft-lbs, the horizontal shear was 762 lbs > and the vertical load was 870 lbs. With this loading a post-type of > foundation of 3' by 3' by 7 ' deep was adequate, given the worst case soil > in the UBC. (table 18-I-A, 97 ed., class 5 soil) > The spread footing was 7' by 7' by 2' thick. This is 3.63 cy and the post > type is 2.33 cy. The spread footing is more concrete but is easier to dig > and place. There is a trade off but concrete is relatively cheap. You should > have the spread footing designed by someone who knows how to do it or go > with the manufacturer's recommendation. > Sorry to be so long winded. > 73 > Hank Lonberg P.E. / KR7X > I agree completely with your comments. I did notice that someone mentioned using a direct conversion from the square (3'-6") on a side measurement to the same diameter if doing a drilled foundation. IE, they wanted to suggest that 3'-6" diameter can be used in lieu of 3.5' x 3.5' foundation. This is incorrect for nonconstrained foundations per UBC. One must use the diagonal measurement for the equivalent diameter. In this case, the correct diameter for a nonconstrained foundation is actually 4.95' or say, 5' diameter. -- 73, Pat Thurman P.E. K7KR -- FAQ on WWW: http://www.contesting.com/FAQ/towertalk Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com ```
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