Tuesday, February 22, 2011

When do we get to work?

Throughout my career as a high school shop teacher this question always popped up within the first few days of the school year. My answer, "When you are ready. When you are, let me know. Tell me what work you will be doing (tools, machines, materials and processes you will be using) so I will know what to expect. Then go to work." Simple as that.

As with most complex projects there are several preparation steps that need to be performed before the "fun" begins. Building camber into the truss chords will be a process. Study the pictures for awhile and see what you can discover about this process. All input is welcome as this is new learning for me.

Here you can see the camber built into the bottom truss chord. The small blocks on the edges are holding the cambered chord in place on the false work and preventing it from springing back straight. This camber will offset the dead load weight of the bridge so there is no sag in the truss and bridge deck when finished and placed between the abutments.

 This would be much less complicated (for the builder) if I just get 40 foot long 4" by 12" timbers (nightmare for the sawyer). Instead good'ol Ithiel Town decided that each chord be made up of more available, common, regular size pieces of timber. His design calls for using several 2"x12" - 16 and 20 foot long planks. Staggered and lapped for maximum strength. Also, with this design almost any length truss chord could be built! Above you can see a pair of vertical clamps keeping the edges lined up and a single clamp going across the top and bottom to hold the pieces tight together. When the wood is arched to form the camber the butt ends will form a vee (wider at the top and tight at the bottom. To correct this a hand saw is ran between the joint. The saw kerf will cut the ends parallel to each other so they can be driven together for a tight fit top to bottom. Very important for the compression strength of the chord. 

Puzzler - How is this going to be kept in place during the assembly of the web and inside chord layers?

Tech Vocab - Jig,  Fixture,  False work


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