Truss-Tease

Over the course of a week, we assembled two trusses to lift up the timber frame. Apart from the challenge of cutting the truss members to the right size, I think we were all surprised by the number of steps required for assembly.

First we set up a low platform at each node and laid out the truss. We measured diagonals to confirm the corners were square. Each node eventually gets sandwiched by a pair of plywood gusset plates, which connect the vertical, horizontal, and diagonal members via a pattern of screws… if not for the gusset plates, there would be no transfer of forces between the members. We attached the first set of gusset plates at this stage. Then it was time to lift the truss into position.

Truss laid out and ready to lift.

Truss laid out and ready to lift.

Cillian climbed to the second floor of the timber frame with two cargo straps, wrapped them around a post, and sent the straps down to Ben who tied off the top corners of the truss. With Ben and me lifting the truss and Cillian ratcheting the straps, we slowly swung the truss into its vertical position beside the timber frame. We used a couple of Timberlock screws to hold the truss tight against the timber frame, and then attached the second set of gusset plates.

Ratcheting in progress.

Ratcheting in progress.

Timberlock screws weren’t enough to transfer loads from the timber frame to the truss. For that task I drew up a pattern of four ½-inch lag bolts centered about each node. We drilled a hole for each bolt running clear through the gusset plates and about 3 inches into the timber frame. (If the bolts went all the way through the timber they’d be called through bolts; since they only go partway in they’re called lag bolts.) Then we used an impact driver to bury the lag bolts, each 8 inches long, into our holes. The timber frame weighs about 9000 pounds, so a solid connection is critical.

A solid connection is what I was worried about when I noticed a few pieces of plywood had delaminated, with one ply separating from the next. I reinforced these locations by cutting an additional gusset plate out of some scrap Advantech and screwing it on. Nothing to worry about now.

Screw and lag bolt patterns. The node on the left has an extra gusset plate for reinforcement.

Screw and lag bolt patterns. The node on the left has an extra gusset plate for reinforcement.

The trusses will carry the weight of the timber frame out to four corners, jacked two feet off the ground. We were all set to start jacking last Thursday when we discovered to our dismay that one jack couldn’t hold a load. It was busted. Friday came and the rental store could neither fix the broken jack nor locate a working one, so our great lift was postponed for the weekend. I’ll report next time on how it went.

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