Best joint for gin pole

[grweldon]

I need to construct a 30' long gin pole out of two 15' long timbers.  I'd like to use 6x6s due to the extreme weight of the finished pole.  I am wondering what would be the best type of joinery to connect the two halves.  I'm thinking that I'm going to reinforce the joint with steel flat stock and lag bolts.

Anybody have a good idea what would be best?

[Heartwood]

Hi grweldon,
I don't think any joint would be acceptable in a gin pole, especially right in the center where the buckling stresses are greatest. Even with no joint, the slenderness ratio of a 30' long 6x6 is pushing the limit. Given that the Actual compressive stress is less than the Allowable, then the slenderness ratio (length/diameter) rules, which should be kept under 50 for safety, with no joint.
The Army rigging manuals are a free download and may have the calculations. I think there would be too many variables with a joint in the middle for even an engineer to suggest something; maybe one lag was drilled funny or one guy line was tighter than another. I guess a steel channel sleeve of some length would work.

[1938farmall]

best would be no joint at all - but you already know that.  i would forget about a joint in the wood & simply butt them.  get pieces of 3/8" x 3" angle x 6' long for each corner to be thru-bolted to the opposite side.  make a sketch & i.d. top/bottom of each angle and layout 6 holes on each flange 2" away from the corner & drill 17/32" dia. for 1/2" bolts or threaded rod (that will be 48 holes).  use two "story sticks" & measure all from the same end (top or bottom) to offset the hole pattern about 1" so the crossing bolts clear each other.  round the corners on the 6x6's so the angle will fit tight, assemble the angles per your sketch, line up the holes with a square, and wrap/strap the whole mess together.  then use a 1/2" spade & drill the wood half way from each side carefully aiming for opposite hole.  at his point you could insert 2 foot-long chunks of rod to judge what you have to do to make the holes line up, then ream out the holes until you can get a bolt through.  do this 24 times and you're done   before you get here you will be thinking lag bolts, but i would not trust them - thru bolts are much stronger.

i know there will be some "horse laughs" from the forum about "overkill" but methinks safe is way better than sorry.

[1938farmall]

p.s.  i agree with heartwood, 6x6 is pretty skinny at 30' - 8x8 would be better.  of course, the whole experiment depends on the weight you are trying to lift.

[S.Hyland]

My 32' pole is a 7x7 Larch, and works quite well. One can also use a round pole, if it is hard to find sawn timber in that length.  I got that sizing from the Army rigging manual that Heartwood referenced. That is absolutely the best single resource out there for this, and it's free to download!
I would also not trust a joined pole of any sort. Too hard to predict reliably. There are complex forces going on during a lift, and you don't want your pole to fail. If something fails during a lift, it will not just fail a little, it will be sudden and catastrophic.
Don't get me wrong, I love my gin poles, but one has to be very circumspect.
What are you trying to lift with the pole? If we knew what you are trying to tackle we might be more help.

[grweldon]

Thank you all for your replies.  I basically figured I would get answers that told me not to use joined timbers.  1938farmall has the right idea, I thought about something similar myself.  I was just tossing the question out there to see if anybody had successfully done what I was asking about.

I will be lifting a pair of Perka frame trusses.  The total length will be 39' and approximate weight will be about 300-400 lbs.  The peak of the bolted-together trusses will be about 21' off the slab.  I will be using a 20' 4 x 4 spreader with 2 cables attached to the truss.  The center of the spreader will be attached to the gin pole line.

I haven't completely decided that I'm going to use this method.  Renting a manual lift or an all-terrain forklift (lull) is not out of the question and is looking more attractive all the time...

[Roger Nair]

Of the options mentioned, I would opt for the Lull, it will easily handle the task of setting trusses and any material handling all within a half day rental.

If however you choose the gin pole route, reconsider the spreader rigged from the center, seems risky, a likely bendy snappy deal.  And look for a suitable pole tree, all square sawn material will have grain runout, a tree stem will be intact.

[Roger Nair]

I should add, when rigging and you want a two point lift, rig a cable wye from the hook to the spreader ends then cable to the truss.  The spreader prevents the wye from drawing together and forcing the truss to fold.  Get a spec from the truss manufacturer on pickup points and spreader length, 20 feet seems way to long for a balanced pickup.

[Roger Nair]

Last word, you may not need a spreader.  Again get specs from manufacturer.

[jander3]

Not a fan of a joint in a lifting pole.  I would cut some poles and use a shear. 

Lifting with a couple of Spruce poles (lifting shear). 



 



 



 

Poles in these photos are 25 or 30 foot spruce.   Big end is about 5 or 6 inches, the small end was about 3. 

[grweldon]

I've got plenty of SYP that size.  I might give it a go just as an experiment.  I check on the price of a lull today... about $560 for 8 hours but I could keep it the whole weekend.  I hate to part with that kind of cash that I'll never see again but I know that the lifting shear won't be anywhere near as easy.  Besides that the rental place said they would throw in a working platform for free.  I DO have a bunch of 2x8s that I'll have to screw to the metal frames.  I have no other way to do it besides constructing or renting a scaffold.  Hmmm.  I didn't ask about the cost of one of those...