From the first truss bridge, engineers experimented with different forms of truss bridges trying to find better shape and the one that will suit them for the particular problems. Because of that we have today many forms of truss bridges. Truss bridge can have deck (roadbed) on top (deck truss), in the middle (through truss), or at the bottom of the truss. If the sides of the truss extend above the roadbed but are not connected, it is called a pony truss or half-through truss.
The common concept design used in a truss bridges are Basic Warren Truss Bridge Basic Pratt Truss Bridge , Howe Truss Bridge & K-Truss Bridge . All of these bridges have their own pros and cons which we will further discuss and explain.
1.Basic Pratt Truss Bridge
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The interior diagonals are under tension under balanced loading and vertical elements under compression. If pure tension elements are used in the diagonals then crossing elements may be needed near the center to accept concentrated live loads as they traverse the span. It can be subdivided, creating Y- and K-shaped patterns. The Pratt truss is practical for use with spans up to 250 feet (76 m) and was a common configuration for railroad bridges as truss bridges moved from wood to metal. They are statically denominate bridges, which lend themselves well to long spans.
Example of Pratt Truss Bridges are Governor’s Bridge in Maryland, Dearborn River High Bridge in Montanna & Fair Oaks Bridge in Fair Oaks