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Re: [TowerTalk] Fwd: HDBX-48 figures for 90 mph zone

Subject: Re: [TowerTalk] Fwd: HDBX-48 figures for 90 mph zone
From: Edwin Karl <>
Date: Tue, 06 Dec 2011 23:41:44 -0600
List-post: <">>
When computing windload, tubular surfaces are 66% of flat also a 30# 
tower is talking about 86 mph
(as I recall, been a while since I did this) a 40# is 105 mph. Again, I 
am rusty on this stuff, but I know you take
the cross section of a leg or element compute the impact pressure and 
multiply by .66 if it's round.

I also recall towers are calculated at 150 (0r 175%) for a safety factor.

Not sure if this gibberish means anything, but to do this right you need 
an engineer and data on the
steel the tower was fabricated from. The rivets, not bolts or welding 
that connect the pieces need
to factor in.

Remember wind load goes all the way up the tower, not just on the 
antenna. The mast and rotor are a factor.
Even the transmission line should be tucked into a corner so as not to 
increase the surface area.

Overturn moment gets to be real big on taller towers and sections. 
(Think foot pounds). Failure of the bolts
connecting to the stubs coming out of the ground could lead to a bad day.

Of note, I have a pretty substantial tower and antenna (90' AN Wireless 
and 22 El Optibeam for HF)
A recent windstorm bend the steel mast for a DB-224 used by a local 
repeater, my stuff stayed up and in
one piece. If you are that concerned about the strength of the tower, 
maybe the installation should be re evaluated.


ed K0KL

On 12/6/2011 10:13 PM, wrote:
> In a message dated 12/6/2011 4:43:51 P.M. Eastern Standard Time,
> writes:
> As I  recall wind pressure "on a flat surface" at 70 mph is 20.6 lbs/sq
> ft.,
> so  if the  tower is rated for
> 18 sq ft at 70 mph, would it be too much of  a stretch to say to the zoning
> board that since wind pressure at 90 mph is  34 lbs/sq ft, by simple math,
> the tower should be good for 10.9 sq ft at  90 mph ?
> Gene / W2LU
> Nop! It is not that easy. You have to take the wind surface of the  tower
> into the calculation as well which makes it "a little more  complicated".
> Hans - N2JFS
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