Here is a bridge design we (the Town of Groton Massachusetts Trails Committee) used recently that seems to work. It feels plenty strong and sturdy when walking on it. The first bridge of this kind was only installed two months ago, so we don't yet have any direct evidence how long it lasts. However, we were generally pleased with the outcome, and are planning a second bridge of this kind in a different location now.
The basic idea is planks across two stringers (the 4x4s). However, those alone would sag too much and be too weak over a reasonable span. The 2x8 pieces are bolted to the side of each stringer, and add significant stiffness and strength. The bridge feels very solid and hardly sags, even when jumping up and down in the middle.
The basic span is 10 feet (3 meters), but the 4x4 stringers are 1 foot longer at each end. These rest on solid masonry blocks. This not only provides a solid footing, but keeps the wood from direct contact with the soil. The masonry blocks are dug into the ground to keep them anchored, which also allows for some adjustment to level the bridge.
The 2x8 pieces are arranged to extend both past the planks above and the stringers below. On the bottom side, they provide a stop against the masonry blocks at the ends to keep the bridge from sliding lengthwise. On top, they provide a little lip to hopefully decrease the chance of someone's foot slipping off the bridge. It also gives a more secure feeling, even if you don't slip.
The planks have a little space between them so that rainwater can drain without getting trapped. We used the same nails that fasten the planks to the 4x4 stringers as temporary spacers. There is therefore a one nail-width gap between each plank.
It is important to only use one nail at the end of each plank. The planks will swell and contract as the humidity changes, so there would always be stress on two or more nails. Since these nails don't take any of the load and only keep the planks from sliding around, a single nail at each end is plenty sufficient.
Here is a picture of the first bridge as it was being constructed:
It took 2 hours for 4 volunteers to carry the parts to the site and to assemble and install the bridge. In this case we were fortunate that the parts only had to be carried by hand about 100 feet. It also helped that the ditch this bridge spans was dry and walkable that time of year. The work included installing a post and some signs not shown in this picture.
Here is a breakdown of the cost:
The values are in US dollars, and were reasonable prices at the time shown.
The 4 pieces or rebar where driven into the ground at the corners, then the bridge attached to each of them with a short piece of hanger strap and two nails. This was done because the Conservation Commission insisted that the bridge be "anchored" to keep it from floating away in case the whole area floods (it is within the floodplane of a nearby river). In hindsight, I think the rebar pieces should have been longer. They went into the ground too easily, and I am not sure how much they will really keep the bridge in place in the unlikely event of a large flood and the water moving fast enough to actually push the structure. I am also not so sure how well the hanger straps and the two nails fastening the bridge to each piece of rebar would hold. Here is a closeup showing one corner anchored to the rebar driven into the ground:
Many nails and most of the hanger strap are left over. These won't therefore be expenses for the next few bridges.
The carriage bolts were longer than necessary. This does no harm, but 6" long bolts would have worked and cost less. That's what we'll use next bridge.
The cost above does not include the four masonry footing blocks. That is because we found (yes, sometimes you get lucky) six suitable blocks buried in the woods on another trail construction project. Next bridge we'll have to buy two blocks, but I don't know yet what they cost.
In any such project, it is important to consider administrative costs. Here in Massachusetts, any construction project within 100 feet of a wetland or waterway, even a small seasonal one as in this case, must be approved by the local Conservation Commission. This required filing a form and getting 4 copies of it printed. One copy has to be mailed to the state, 2 filed with the Conservation Commission, and we made one for ourselves for reference during the Conservation Commission hearing. Even though we are a town body, the $53 fee could not be waived. This fee is to advertise the hearing in the local paper, which is required by law. The Conservation Commission does not actually get any money from the application. Overall, the administrative requiremets added almost 50% to the cost of the project. This surely varies by jurisdiction, but it is something anyone contemplating such a project should investigate and include in the cost up front.