Sunday, August 14, 2011

Not for the faint of heart...

Oh brother, what a learning experience! Have spent the better part of the past two days thinking, drawing, calculating and setting up the rigging for the raising of the trusses. Once again it looked pretty simple and straight forward, get a couple of electric poles, put some pulleys on them and lift away. Right?  However, like everything else once you get down to the details you better have all your ducks in a row or you will have 1500 pounds of trouble on your hands.
Watching the experienced professionals do it in New Hampshire a few weeks ago was enlightening. I can now appreciate all the work that went into making it look that easy.

Reminds me of the ol' joke, What are the three biggest lies in the world?  #1 The check is in the mail. #2 I am from the government and I am hear to help you. #3 It's easy, it will only take a minute.

Calculating the lengths of everything was a great math application. A drawing however was in order to proof it out. Below you can see the Gin Pole as it will be set against the top chord and then how the pole will end up when the truss has been raised to vertical. Coordinating the placement of the cribbing and clearance for the poles was just as involved as it was with the false work clamp placement and drill clearance!

Above you can see the anchor point. To this will be attached the come-along to keep the bottom chord from kicking out and the block and tackle that will be attached to the top chord to keep the truss from going over center. The short piece of post against the top chord represents the Gin Pole.

Above you can see a 2x3 Oak piece notched into the pole one foot above the bottom to prevent the pole from sinking into the ground. A worm gear 40:1 ratio winch is mounted 2 feet above ground level.

With the wire rope fastened to the top of the pole and a set of pulleys between them the rigging is complete.

The calculations for the life are: The load of the 6000 pound truss in the horizontal will be split between the abutments. 3000 lbs on the abutments and 3000 lbs on the Gin poles. The Gin poles will share this 3000 pound load at 1500 lbs each. The 1500 lbs load on each Gin pole will be carried through pulleys by 3 wire ropes at 500 lbs each. The 40:1 winch ratio will require 12.5 lbs of turning effort. It should go slowly by surly!

Tomorrow we will be taking everything to the sight and begin the truss assembly.

Puzzler - What lengths will be required for each of the following: the Gin Pole, the Wire Rope, and the rope used in the block and tackle as a hold back ?

Tech Vocab - Snatch Block, Come-Along, Block and tackle, Gin Pole, Wire Rope.

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