[TowerTalk] Fall Zone

[jimlux]

On 7/8/19 10:38 AM, Glenn Pritchard wrote:
> Boy is there a lot of misinformation here.
> 
> Glenn, VA7UO
> 
> Sent from my iPhone
> 
>> On Jul 8, 2019, at 10:36 AM, jimlux <jimlux at earthlink.net> wrote:
>>
>>> On 7/8/19 9:34 AM, Jim Thomson wrote:
>>> Date: Mon, 8 Jul 2019 11:20:31 -0400 (EDT)
>>> From: Wilson  Lamb <infomet at embarqmail.com>
>>> To: undefined <towertalk at contesting.com>
>>> Subject: [TowerTalk] Fall Zone
>>> <I wouldn't want a neighbor's tower/Yagi looming over my backyard...and I love towers!
>>> <The fall zone idea seems like simple good manners.
>>> <I have been loosely involved in dropping 200' BC towers, dropped by cutting the rods at one guy anchor, thus losing all guys on that side.
>>> <They fell absolutely full length, with a few sections not even bent!
>>> <I think a foundation failure (soil, bolt, gin pole) would drop a crankup to full length.
>>> <Is there any experience available on this?
>>> <WL
>>> ##  per software,  Trylon self support  towers will  fail  at the junction  of the 4th  and  5th  section..... 40’  above ground.
>>> UST  designs a weak  spot.... which  is  midway  up  the  3rd  section...  46.5  feet  above ground.
>>> ##  Never seen  a pix  of  either  tower folded  over... as  depicted  above.    40  ft...and  46.5 ft.   So dont  know if  their
>>> design theory  works....or not.
>>
>> It doesn't have to be a specially designed weak spot. Towers tend to bend/buckle in the middle anyway.
>>
>> _



Well.. there's plenty of pictures of fallen towers out there. There are 
towers that have failed due to wind loads (typically due to ice buildup 
from a casual look) - free standing towers break somewhere in the 
middle, or are hung up by external supports (power line transmission 
towers are sort of a special case, they're free standing, but there's a 
power line holding them up, or at least constraining where they fail.

Crankups have all sorts of stress concentrations - where the sections 
overlap, for instance,

Guyed towers fail by crumpling up - a lot depends on what failed. If it 
was intentionally brought down in a particular direction, they do tend 
to lay out straight, because that makes it easier to haul away.

A guyed tower that fails because the anchor pulls out of the ground 
tends to fall and break, but sometimes "lays out" and sometimes doesn't, 
but there's almost always a "kink" in the middle somewhere (because of 
the physics of falling).

I think also that there's a substantial difference between a 1000 ft 
broadcast tower and a 100 ft ham tower. The broadcast tower is going to 
have multiple guy tiers and is probably a lot more "slender" (and 
flexible) than a ham tower.

Ham towers tend to have a lot of aerodynamic drag and mass at the top 
compared to broadcast towers. That's going to affect how it fails and falls.


But overall, I would think a building code type requirement for "must be 
at least the height from property line or structures" is a pessimistic 
worst case requirement. (and would prevent a lot of commercial 
structures from being built)