Note
About 2 hours of pretty strenuous work. We built a bridge at Hodges Village Dam, Oxford, Ma, over a little trickle (in O-speak, maybe, a seasonal stream?) that was behind a beaver dam and a pond. The other guys working were mountain bikers from the local bike shop. We bought materials from Home Depot, loaded them onto the ranger's truck, and hauled them down the slope from the paved road to the where the trickle crosses the trails. The bridge isn't even intended for mountain bikes -- the ranger intends that the dirt bikes will use the sturdy bridge rather than erode further the banks of the stream.
We used 3 16-foot long 2x10" PT boards for the long members, 2x10" PT boards (24' of them) sawn to 18" length for the cross pieces, and some decking made from that plasticky stuff that's supposed to look like wood. After we had finally bought, sawn, hauled, and prepared the materials and were ready to start attaching stuff, the whole bridge went together in about an hour. We were all amazed that it could go together from scratch to completion in about 2 hours with 3-4 guys, a couple of cordless drills and a cordless saw.
Hodges Village Dam rec area has changed quite a bit since I last ran a trail race there. I only saw the northern part of it, near the birthplace of Clara Barton, but there had been some significant logging in places. They said that Hodges is one of the better places for novice MTBing, but parts of it looked challenging to me. The good part is that there's a variety of trails, so that if one is too rocky or rooted, there might be another option.