The newlywed DW and I purchased a house this year. It has a little less than a 2 acre lot, but only ~2/3 acre was accessible without an exciting balance beam adventure crossing a fallen tree over a 10-12ft deep ditch eroded by a small stream that cut the lot in two.
Hence, it was necessary to build a bridge to reach the rest of the lot.
First, a bridge design was necessary. 26ft unsupported span was desired to avoid a center support, so the main beams are sistered 2x12s with a Howe truss to maximize bending stiffness.
Next, a quick FE to verify the feeling that the bridge is over designed can be backed up with math. I used a 5,000 lbf load at the center joist which is a little conservative vs the local deck loading code of 60 psf (26ft span X 42" width X 60 pdf = 5,460 lbf spread across the whole structure). vM stress is below some southern pine guidance of 2,900 psi I found and deflection with the 5,000 lbf center load is ~5/8" at the deck. Looks good!
To begin construction the abutments were set. These were dug 2.5-3ft deep and set in compacted pea sized gravel. Didn't use concrete footers since the soil there had a very high clay content and I wanted to give the 4x4s the best possible chance against rotting from being set in wet ground.
DW and I constructed the initial sections of the main beams from 3 2x12x16s.
These were surprisingly flexible at 32 ft long. And VERY heavy.
Used a dog run cable, shackle, and some rope to guide the beams across the ditch. No way could I lift them back out of the ditch without machine help if they got down to the bottom; they probably weighed 250+ pounds each.
Once secured to the abutment posts, it starts to look like a bridge could go there!
Started with the joists between the abutments for stability, then started again at the center.
As might be expected, but I didn't plan for, lateral stability was on the low side with just the beam/joist ladder shape. Addition of a lateral truss beneath the 2x8 joists provided extreme lateral stability.
Next it was time for railing vertical members and the truss. Even though the bridge was what I;d consider pretty sturdy with just the beams and joists, its so solid with the truss railings that my highest jump and stomp on the deck barely moves it.
Completion of the truss, addition of the deck, and few extra safety railings finish off the main structure.
Dog enjoys the new laying area.
After a few months for the pressure treatment to dry out of the boards, its time for some paint! Used Behr Deckover with texture for the deck to add some non-skid properties. Not sure how that'll hold up over time but its fantastic when new.