[TowerTalk] Base equivalent

Jim Lux jimlux at earthlink.net
Fri Jan 10 16:49:21 EST 2014

On 1/10/14 1:28 PM, Mike Reublin NF4L wrote:
> The specs for my crank up call for a hole 4x4x7.5 feet. That works out to 4.4 yds of concrete. The smaller backhoes that I can get to the site can go to a depth of 6 feet. Because of the water table, I consider it an unacceptable risk to put a man in the hole to dig out to 7.5 feet deep.
> A hole 5x5x6 is 5.5 yds.
> Would that be a safe equivalent to hold the tower up with the same wind load?

You really need to look at the analysis for the original design: what 
bearing strength for the soil did they assume?

Think about the original design as a sort of pier that sticks down, 
resisting the overturning from wind loads.. At a very simple level, it's 
a 4x7.5 foot surface pushing on the dirt.  30 square feet with an 
average radius arm of 3.75 ft.

Your question is whether a 5x6 foot (also 30 square feet), would resist 
the same overturning moment.  At first glance, you'd say "no" because 
the average lever arm is only 2.5 ft.

BUT, you also really need to consider the "bottom of the cube"   That, 
is, the forces are acting on something like an L shaped thing.

There are ways to do a base that is shallow and wide instead of deep and 
narrow, but I don't know how to convert between them. I just know enough 
to know that it's non-trivial.

The answer is "ask a local Engineer"... for a few hundred bucks (or 
maybe less) they can give you a real answer, probably with the local 
knowledge of soils and stuff factored in. (you didn't say if it was for 
topband, and you're erecting it in the middle of a saltwater swamp)

Free standing towers are harder to analyze than guyed towers.. with 
guyed towers all you worry about is the lateral force at the base trying 
to skid the bottom (which is small, usually) and whether the base will 
sink into the soil.  Lots of people put up guyed towers with NO concrete 
base and basically a big tent stake into the ground.  And that works.

But with free standing, you're worried about the bearing strength of the 
soil with a combination of forces.  It's like asking how far to stick 
the straw  into the milkshake or frozen daiquiri so it doesn't fall over 
when you carry it from the bar.

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