Thursday, February 02, 2006

Raising (2/3's of) Bent D

First we aligned, tweaked, and assembled Bent D on the ground (upper left picture).

Because there was only one girt going from post D2 to post D3 (seen laying on the ground in the upper right picture), all three posts of bent D were not stable to "fly" with the crane as a single assembly (unlike previous bents A,B, and C). Of course, Bent D is a much more rigid assembly if the truss (rafters+collar tie+king post) is included and we seriously considered flying the whole shebang, but I did a quick calculation and estimated that this bent weighs somewhere close to 4,000 pounds with the truss included. Then I looked at the load chart for my 1974 model Grove crane, and realized that at the distance we were going to be lifting from (about 45 feet) this load was on the hairy edge of barely possible. (Although this is a "15 ton crane," you can't lift even 1 ton when boomed out 60 feet!) Better safe than sorry, so we decided not to even try and lift the whole thing.

So, we drilled all of the peg holes for this entire bent while it was on the ground, but we drove pegs only in the subassembly that we intended to raise first. This subassembly flew quite nicely (lower right picture), even though it was assymetric. Why? Because the big tall post on the left is yellow poplar (D2), whereas the shorter post on the right (D1) is oak, and even with the big maple braces at the top of it, the poplar post did not weigh more than the oak post. (For what it's worth, the top girt in this assembly is also hard maple and the lower two braces are walnut - everything else is oak).

In the final picture (lower left) you can see that everything came together by the end of the day. All we need to do now is to fly in post D3 (and the 3 girts that connect it to the rest of the house), and then lower the bent D truss on the tops of the bent D posts. We have put very healthy chamfers on the tenons on the tops of those posts - in the hopes that the truss will just sit down nicely when we get to that point. Posted by Picasa


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