[TowerTalk] HBX tower rotational loading

[Roger K8RI on Tower]

>
> I've been reluctant to continue this guying thread...
> but this really is slightly different...it's hbx/hbdx related:

Something to remember about the BX through the HDBX series.  They are made 
from fat stamped steel and like aircraft structures derive their strength 
from the shape of the stamped material.  Anything that distorts the shape 
causes a loss of strength.  How much distortion is created by the twisting 
it the important thing.  That is why they limit the boom length even though 
the tower is rated at a rather high figure for wind loading.

The shape factor also makes the proper atachment of guys (when used) 
important.  I'd not want to resort to the time honored way most of us have 
gone in the past, by just looping the guy wire around the leg and I'd guess 
that probably happens more with the lighter and less expensive towers.

Guys should be atached using a backing plate and eyebolt if guying hardware 
is not provided. Even better would be a backing plate the same shape as the 
tower legs that would nest inside. I'd want a backing plate at least the 
thickness of the tower leg and about 6" to a foot long.

Properly atached, I don't see how guys would hurt in this case, but the 
"loop around the leg" will exert distorting forces on the leg.  I'm sure we 
have at least a few on there who could calculate the actual values.  I 
belive those legs are galvanized, mild steel (according to my very old 
manual) so the tensile strength figures should be available

Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com
>
> I saw an hbdx48 come apart from torsional loading from a KT34A...a 16'
> boom.  It was guyed at 48', and around 24'.  The top section torqued,
> tore, crumpled, and crushed the second section down.  From the mid-point
> down, the tower was bent, but not crushed.
>
> And that ended thoughts of a small lpda with a 14' boom on that tower.
> For the record, I don't think the guying hurt or caused the failure...
> just that the tower wasn't very strong to begin with.
>
> N2EA
> jimjarvis at ieee.org
>
>
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