Saturday, August 25, 2018

Bridge Building

I sketched the design, wrote out a bill of materials. I let the drawing sit, revised it, added detail, thought it through, looked at models. I designed a buttress of rock and rebar, one for either end, and measured the gap. It was 20 feet across the pond inlet, bank to bank.

I debated what type lumber. What type stress will my bridge support? Grandkids jumping up and down? (I dream.) A lawn mower? Snow and ice … Most lumber only goes so long as sixteen feet. How to form a 20-foot-truss? I work alone. How to build, then carry the bridge? I could build it in place, over the water. Maybe. Maybe not. How do I haul lumber to the site? It could be delivered. I asked an expert, a builder friend, Robert, how wide should I make the bridge? “The wider the better,” he said. I chose 32 inches, so I could cut three deck pieces from each eight-foot board, about fifteen long boards in all.

It was a delightful cogitation, which I stretched and chewed for about six months. I decided to bind two ten-foot wood girders with a sixteen-foot piece. I debated types of stock — 4x4, 2x6 or 2x8 inch treated — the types of bolts, binders, screws and fasteners, and the type of glue. I pulled up bridge designs for the Appalachian Trail and challenged my specs. I considered features like a toe rail but said not. I decided on 2x8 inch pine for the truss frame, topped by 5/4x6 inch deck boards. I fastened the truss beams using brown-enameled carriage bolts, washers and nuts, with a finger-thick smear of all-weather glue snaked between bolted boards.