What Is a Deck Truss?

A deck truss is a type of bridge on which the road deck lies above the structural parts. The supporting beams of the truss structure are arranged in triangular patterns to distribute loads and ensure the bridge stays stable. A truss bridge can have wood, fiberglass, or steel beams, with steel as the most commonly used material for major engineering projects in transportation. In general, truss bridges need to be straight, because the deck truss is designed to handle stretching and pushing forces. A bending motion could cause this kind of bridge to fail.

Loads from the weight of the bridge, and cars and trucks that pass over it, are transferred through the triangle-shape of the beams. The overall structure then carries the forces to the piers of the bridge. Although different in shape, a truss operates similar to an arch shape, except arch bridges support loads through the shape of the arch to the sides without any center support. Bridges with a deck truss are built to last about 50 years, according to the U.S. Department of Transportation. Signs of deterioration include corrosion, poorly welded spots, fatigue cracks, and bearings that don’t move with the loads of heavy trucks to keep the bridge safe.

Typically, bridges with this type of truss have to be regularly inspected for truss stress. Strain on the metal beams and joining elements isn’t dangerous at first, but if not repaired, can eventually cause other parts to fail and the bridge to collapse. The I-35 bridge in Minneapolis, which collapsed in 2007, was a deck truss bridge. A truss crack can appear suddenly when enough stress is put on the structure, especially if corrosion and failed welding add to the structural deficiency. When no cracks in the truss were found, experts only recommended additional inspections for that bridge in specific increments of time.

Bridges with a deck truss design were first built in the middle 1800’s. They were used for railroad crossings because the design was strong enough to carry heavy iron, steam engine trains. The deck on top of the truss is one design, but similar designs have the road pass through the truss. A through-truss design can have the supports both vertical and diagonal toward the center, so they can be thin but resist strong tension forces. Another type has vertical columns and diagonal beams slanted toward the ends of the bridge, and which must be thicker to withstand compression forces. Deck truss designs are still commonly used for both highway and railroad bridges.