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@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@contesting.com>
Subject: [TowerTalk] Legs of a 60' piece of tower being dragged while
being lifted
From: "StellarCAT" <rxdesign@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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|