The weight is irrelevant. A skilled crane operator can move the boom as
it is being lifted leaving the section on the ground as a pivot point.
Of course if someone wishes to build a slide or use a loader to move
the base as it is being lifted that is perfectly fine, but I won't be
doing this.
John KK9A
On 2016-10-17 10:23, Grant Saviers wrote:
> 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
>>
>> _______________________________________________
>>
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