[TowerTalk] Legs of a 60' piece of tower being dragged while being lifted

[Grant Saviers]

Perhaps a non-issue for your situation, but for me it wasn't.  20' to 
60' sections of Rohn 65 with K0XG rings weighed in between 600# and 1800#.

A free body drawing shows the problem.  A crane cable hook is a pivot 
point, virtually no resistance to torque.  Same with slings. The tower 
section(s) being lifted have a center of gravity somewhere along its 
length.  Where depends on what other than a plain tower section is the 
section.  With two rings on some sections, I could find the balance 
point only by trial and error, ie single sling on a leg until it balanced.

The further the center of gravity is from the hook at the top, the 
larger the moment is on the section, trying to swing it under the hook 
point.  So even if the crane op swings the boom tip directly above the 
tower top as he lifts, there is still that moment. Perhaps by swinging 
the boom tip further towards the section base a force can be added to 
cancel what gravity is doing.  However, that offset will be different if 
the sections are different re their center of gravity.  The geometry of 
boom tip vs hook position also varies as the boom is swung, so the boom 
top height would need to change.  This could be pretty difficult to pull 
off.

Rings on towers really complicate the lifts since the tower section 
needs to be assembled on the ground, off the ground on heavy duty 
supports, not HD or HF saw horses.  My 4400# capacity fork lift with a 
boom extension could not lift some single sections to vertical for 
testing mounted rings, so I rented a 8,000# shooting boom fork lift.  My 
regular forklift was used to manage the bottom of the sections as they 
swung.    So as soon as the lift starts, the section swing force wants 
to tip over the far support.  To avoid that the section was first 
suspended with slings off the supports at both ends.  We got this done 
slowly and carefully since the shooting boom fork position (lifting and 
extending at the same time to keep the forks over the section top) and 
forklift motion needs to be in sync.  Once vertical the lower slings 
were removed and the section set vertically with the shooting boom, but 
with some tension as a safety support.  After the rings were tested (can 
only do this with the section vertical), the process was reversed to 
place the section back on the supports.  Not a "no sweat" process.

The sure/safe way to go for larger towers is to keep the boom tip as 
directly over the section top as possible and rig for the base to swing 
or drag.

Grant KZ1W


On 10/17/2016 5:09 AM, john at kk9a.com wrote:
> I have used a crane many times and I still say that this is a non-issue.
> Is it addressed in K7LXC's book?  Please read my post:
> http://lists.contesting.com/archives//html/Towertalk/2016-10/msg00141.html
>
> John KK9A
>
> To:	"tower" <towertalk at contesting.com>
> Subject:	[TowerTalk] Legs of a 60' piece of tower being dragged while
> being	lifted
> From:	"StellarCAT" <rxdesign at ssvecnet.com>
> Date:	Fri, 14 Oct 2016 07:58:44 -0400
>
>
> So Fred pointed this out ... I hadn’t considered it up until then... is it a
> problem to just allow the legs drag in the dirt while the 60’ piece with
> rotating ring attached (~850#) is lifted? Is there ANY chance the legs will
> deform making it impossible to mate it to the tower? I don’t have access
> to an
> end loader or any other piece of heavy eqmt ... I thought, and this might
> sound
> silly, a dolly ... those cheap(er) ones – seem to be rated at 600# ... if I
> could get enough guys to lift the end and put that under the end – then we
> could pull it along as the crane goes up.... although the ground is really
> rough so that is doubtful... it would probably get stuck and the legs drop
> off
> which would be far worse than just having them drag on the ground ...
> experiences anyone?
>
> Gary
>
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