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Joined: Oct 2002
Location: Ra-cha-cha, NY
I don't design in wood, but if it were me, and I were doing my own design, I'd assume the full loaded weight at the center of the span, in two locations, one for each wheel. Essentially two point loads. Plus half the dead load of the bridge structure, of course. For bending stress on the beams, that is.
For the foundation loads, assume the same two point loads directly on each bearing point, plus the dead load of half the bridge structure, of course.
That's overdesigned, but it's a) out in the weather, and b) not likely to be expensive anyway, and if you have to double or triple up the beams under the wheel path, that's just another couple of boards.
Assuming PT framing lumber is even acceptable for this. As I said, I don't design in wood, and really don't have much feel for how much load you can put on framing lumber of a give size and span. But that's what span tables are for, I guess. But that will yield a very conservative design, as you're not taking into account the spread of the axles, which can greatly reduce the actual bending moments on the beams on such a short span bridge.
Tom
“This place smells like that odd combination of flop sweat, hopelessness, aaaand feet"