If I was in the military and was tasked with this, I would make a
sandwich like construction, with 1/2" plate on top, 2x4 ribs
underneath, and then a 1/4 plate welded to the bottom. Drill some
holes in the 1/4" plate to weld it to the inner beams.

This has two advantages, one is rigidity, and another is that it is
not easy to place an IED under the bridge between ribs that could
easily me missed during a quick visual check.

i