the pratt truss was invented in 1844 by thomas and caleb pratt. a pratt truss has vertical members and diagonals that slope down towards the center. the interior diagonals are under tension, and the vertical elements are under compression. the pennsylvania truss is a variation on the pratt truss. while the pratt truss has braced diagonal members in all panels, the pennsylvania truss has half-length struts or ties in the top, bottom, or both parts of the panels. the pennsylvania truss is named after the pennsylvania railroad, which pioneered this design. the pennsylvania truss was once used for hundreds of bridges in the united states but the design fell out of favor in the 1930s and only a few such bridges remain. the edz irigary bridge, with a span of , has the longest clear span of any county bridge still in use in wyoming, and is one of the most important historic bridges in wyoming.