Large Pratt Truss Bridge Video
By Garrett Boon on November 4, 2009
This is a Pratt truss bridge that looks very well built. It is much larger than most of the models here. Notice the guys loading it are not wearing safety glasses, which is not recommended. I wonder why the bridge failed. It looked as if the right side of the bridge, the last two diagonal members were bending. That probably could have been fixed by making the beams into an L or T beam shape.
The bridge most likely failed because there was little torsional bracing in the truss. This led to the bridge swaying to the side during the collapse, which could have been prevented by adding cross bracing between the top and bottom chords. The straight bracing they have between the trusses is not enough to prevent overall torsion. With X-bracing on the top and bottom, the bridge is much stronger laterally (real bridges utilize this for wind bracing).