Deck truss bridges have triangular beams that distribute weight to the piers, with steel being the most common material. They need to be straight and regularly inspected for tension and signs of deterioration. Truss bridges were first built in the mid-1800s and are still commonly used for both highway and railroad bridges.
A deck truss is a type of bridge on which the road deck sits on top of the structural parts. The supporting beams of the truss structure are arranged in triangular patterns to distribute the loads and ensure the stability of the bridge. A truss bridge can have wooden, fiberglass, or steel beams, with steel being the most commonly used material for major engineering projects in the transportation industry. In general, truss bridges need to be straight, because the deck truss is designed to handle shear and thrust forces. A bending motion could cause this type of bridge to fail.
The weight loads of the bridge and the cars and trucks that cross it are transferred through the triangular shape of the girders. The overall structure then carries the forces to the bridge piers. Although different in shape, a truss works similarly to an arch shape, except that arch bridges support loads through the arch shape at the sides with no central support. According to the US Department of Transportation, girder bridges are built to last approximately 50 years. Signs of deterioration include corrosion, poorly welded seams, fatigue cracks, and bearings that won’t move under heavy truck loads to keep the bridge secure.
Typically, bridges with this type of truss need to be regularly inspected for truss tension. At first, the deformation of the metal beams and fasteners is not dangerous, but if it is not repaired, it can cause other parts to fail and the bridge to collapse. The I-35 bridge in Minneapolis which collapsed in 2007 was a truss bridge. A truss crack can appear suddenly when enough stress is placed on the structure, especially if corrosion and weld failure add to the structural deficiency. When no cracks were found in the truss, the experts only recommended further inspections for that bridge in specific time increments.
Bridges with a truss design were first built in the mid-1800s. They were used for level crossings because the design was strong enough to carry heavy iron, steam trains. The bridge on top of the trestle is a design, but similar designs have the road going through the trestle. A through truss design can have the supports both vertical and diagonal towards the center, so they can be thin yet resist strong tensile forces. Another type has vertical columns and diagonal beams that slope towards the ends of the bridge and must be thicker to resist compressive forces. Bridge truss designs are still commonly used for both highway and railroad bridges.
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