The extra support is used to carry the lighter, but stronger deck. The Warren relies on the triangle shape to carry stress. Some types have vertical members, and some do not. A few designs employ a polygonal upper chord to increase strength, as in the Parker truss. For concrete girders, the roadway rests on a concrete slab or on concrete floor beams.
Plate girders rely on metal floor beams to do the same.
From wood to iron A rebirth of the Iron Age occurred in the nineteenth century as new techniques were developed for crafting and shaping this ancient material. The rebirth was perhaps most evident in the great advances in engineering technology between 1870 and 1900. One by-product of the engineering advances was the iron bridge. The ungainly, skeletal frames of iron bridges would come to dominate American roads, and in time would become a beloved and valuable part of the cultural landscape. Covered bridges were among the first bridge structures to employ iron.