[TowerTalk] Engineering closer-in guys

[jimlux]

On 9/2/20 6:52 PM, Matthew Kaufman wrote:
> TL; DR: fire got my tower, can’t replace at same height with stock guy
> anchor distance.
> 
> I had a Rohn 55 tower that was built in 1971 as 150’ high with 4 levels of
> guys and anchors 120’ from the base. Some ten years later they put two more
> sections on, making it 170’, and added a 5th guy level (and borrowed the
> 4th level’s spot on the guy anchor plate for it)
> 
> A couple weeks ago the anchors got melted in the CZU Lightning complex
> fires (the camera on top caught the fire:
> http://www.alertwildfire.org/southeastbay/index.html?camera=Axis-BonnyDoon&v=81e002f
> before the camera and microwave cables melted)\

OK, you win the Impressive Picture from my Tower award...




> 
> I want to replace the tower with 170’ (or 180’) of Rohn 65... but due to
> property lines, I can’t get more than a couple more feet out past 120’ from
> the base(the default for a 150’) Has anyone on the list gone through the
> engineering for guys that are closer to the base than the stock design
> calls for?
> 
> Due to trees, the first 90-100’ of the tower is virtually unusable, so
> every foot counts.

Even though the trees burned? (not seriously).

Any decent engineer can calculate the loads and figure out what you'll 
need to do.  Ultimately, it's a 1/cos(theta) kind of thing, so as the 
angle gets past a certain point, it gets really bad really fast, in 
terms of the tension and loads. Kind of like how it's impossible to pull 
a string with a weight in the middle perfectly horizontal.  You'll need 
to make sure the anchors can take the higher vertical loads, as well.

  Probably a bigger issue might be whether the county will let you do 
it, or whether they'll use this as an excuse to remove things that were 
grandfathered in.

You need to find an PE with two things:
1) experience with doing towers and things like that
2) experience with your local "authorities having jurisdiction"

The latter is probably more important.