## Thursday, February 24, 2011

### A funny thing happens on the way to the top of the arc

In high school everyone in track knew that when it came to the longer races the outside lane was the best one to be in because you always started out way ahead of the rest of the runners. I made it a point to get the outside lane as often as I could. I must not have been that good of a runner because it seemed like they always caught up to me by the second or third turn of the race. Still it puzzled me why there wasn't more of a fight from the other runners to get this lane with this obvious advantage!

Well bridge building and putting the camber into the chords of the truss has shed considerable light on my thinking. Daha! The geometry involved is tricky but the up shot is that lattice spacing distance on an arc requires some adjustment on the top chord.  Milton Graton, a man of few words, in his book "The Last of the Covered Bridge Builders", explained it like this. "The base intersections of the lattice having been laid out for 4' on centers, a radius is projected from the extreme intersections to the top of the fourth chord. A measurement along the top chord will determine the amount of excess to be added to each 4' multiple without the inaccuracy of projecting additional radii." I have to admit I didn't even catch this little detail until the forth time I read his book!
In the picture above if you look close the white nail to the right of the crossed tape measures is where the 4' center comes out if you just mark it off with a ruler on the top edge of the chord. Doing that really messes up the placement of the web members, especially as you get closer to the ends of the truss. It is an additive thing. Maybe it is like track lanes 2 through 5?
Follow Milton's instructions and all is well. Web members nice and parallel. Remember there is a 1.125" camber in both chords.
And here you have it. What next? Two more chords with the same 1.125" camber. Any ideas on how to do it? In the book,  Milton explains it in one sentence. So how hard can it be? We'll see.

Puzzler - How do you think the camber will be put in the next two chords?

Tech Vocab - lattice, annual ring, parallel clamp