[TowerTalk] Thoughts on towers suitable for my difficult location?

[Jim Lux]

On 10/23/13 9:38 AM, Your Name wrote:
> I am trying to determine if it would be practical to safely install a 40
> to 50 foot free-standing or bracketed (not guyed) tower in my location.
> I would like to hear from anyone who has erected a tower in a similar area.
>

You need to define "practical"  I'll bet there are 50 foot high 
flagpoles and highway light standards around, but they may be larger or 
have bigger foundations that you would like or can afford.

>
> One final feature of the site: it is subject to tidal flooding during
> severe tropical storms and nor’easters. Every 2 or 3 years, the base of
> any tower at this site will be immersed for several hours in seawater,
> to a height of up to 4 feet. (Most houses here, if you are wondering,
> are elevated on pilings or perimeter walls.)

I'm thinking that if you were not cost constrained, drilling a suitable 
hole and filling it with a reinforced concrete pier might be a decent 
solution. having the forms stick up 4 feet isn't all that big a deal, 
and the extra concrete isn't all that expensive.

What do they do for traffic lights and advertising signs around your 
area?  Especially if its the traffic lights with a big cantilever 
sticking out, that's a huge overturning moment, and what they do for 
that will work for you.



>
> My initial idea is to simply overengineer - select a tower rated for 120
> mph at *double* my planned windload, and go up a step or two on the
> usual foundation size. I'm also inclining toward an aluminium, rather
> than steel, tower due to the saltwater corrosion issue.
>
What you describe is not overengineering.  It's overbuilding, without 
actually looking at the engineering.

Bear in mind that for many towers, the wind induced loads on the tower 
are as big as the wind loads on the antennas.  You need to look at the 
design information for your candidate tower to be sure.

But it's the base that's going to be your big deal.  The tower is just a 
big stick. The "standard design" for the base assumes fairly good soil 
(because the base is all that keeps it from falling over). If you're in 
mushy sand, "going up a step" may not be a good strategy: you might need 
more, or a different kind of technique (deep caisson, horizontal grade 
beams, etc.)

> Any thoughts would be much appreciated. I might end up turning to a
> Professional Engineer for final planning, but I'd like to check to see
> if this is even remotely feasible before I incur that expense.
>

If you know someone locally who has an antenna and tower system similar 
to what you want to do that has been up for some number of years, that 
might be useful.