Sunday, June 26, 2011

A good days work...

"Even the longest journey starts with a single step." Somebody a lot smarter than me said that but it is true. Today in the picture below you can see... the first step, in the actual construction of South Wayne's 32' Town Lattice covered bridge.
First layer (A) of the Bottom Outside Chord. This consists of a 12'  -  16'   and  8' long 2x12. Next the second layer (B) a 20'  and a 16' long 2x12 is positioned on top of layer (A). The location of the butt joints is critical to the strength of the truss chord and the placement of the necessary clamps that will be needed.

In the picture above I have placed a short piece of 2x2 above each butt joint. The spray paint can marks the center of the bridge truss. As you can see the 4x6 falsework is to either side of any butt joint.
Clamps made from 2"x3" oak and threaded rods can now be placed to hold the layers together and in proper alignment with each other. Then, with some stop blocks and a little help from a "handyman" jack the camber was put into this Outside Bottom Chord. Realize that this bend causes the butt joints to "open up" at the top edge. This must be corrected so there is no gap and the compression forces will be transfered uniformly for maximum bridge strength. In the picture below you can see this butt joint gap.

The technique to solve this little problem is called "sawing to the kerf". In the picture above you can see a handsaw cutting through a butt joint gap. The saw cut will make the ends parallel. Then a sledge hammer to the ends of the chord is used to drive the chord pieces together and close the saw kerf. If you do it right it will look like the picture below.
This is an amazing process and the results will allow the cambered chord pieces to act as a single 32' long piece of wood!
So we end the first day with the Outside Bottom Chord done and the Outside Top Chord layers (A) a 12'  -  16'   and  16' piece and layer (B) a 20'   -   16'   and  8'  piece stacked and clamped. Ready for tomorrow.

Puzzler - What do you suppose will be the next steps in the process?  A look back in the blog at a post or two when we were making the 3/8 scale model will give you some answers.

Tech Vocab - Stop Blocks - Handyman Jack - GRK screws  -  Kerf  -  Butt Joint  -  Compression

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