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[TowerTalk] Tower Base Construction

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Subject: [TowerTalk] Tower Base Construction
From: (Pat Thurman)
Date: 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
a direct conversion from the square (3'-6") on a side measurement to the same
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

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