What Is a Deck Bridge?

Paul Reed

A deck bridge is a design in which the road or walkway of a bridge, called the deck, sits cleanly atop any supporting structures, which are usually made of concrete, steel, or wood. Different support designs are used, but the advantage of a deck bridge is having no visible support above the roadway. This design gives an open roadway with no steel work to obstruct views or create any hazards for helicopters or nearby aircraft.

Scientist with beakers
Scientist with beakers

Pre-cast flat deck bridges are perhaps the simplest design, but have limitations. These bridges are built with rectangles of steel-reinforced concrete connected end-to-end and placed on vertical supports called pillars. Less steel is used on pre-cast deck designs, and pillars must be placed relatively close together to provide support. They are best suited for pedestrian bridges or short connecting sections where there is room underneath for more pillars.

A different type of pre-cast bridge is a box girder design. This bridge is built using a reinforced concrete box with a flat roadway section on top. Concrete box girder designs can span greater distances than a flat concrete deck, because the box will support greater loads without sagging or failing.

Another type of deck bridge is a beam-style design. Horizontal beams, typically steel, are connected end-to-end across the length of the bridge and supported by pillars placed beneath each connection point. A steel and concrete roadway can then be placed above the beam supports. The steel beams can flex up and down if the span is too long, which will weaken the bridge. Beam deck bridges are useful for pedestrian bridges or roadways where multiple pillars will not create any space or clearance problems beneath them.

A truss deck uses a series of steel beams with a square or triangular design to create a box that rests on pillars at each end. The steel supports of a truss system can be more difficult to build than a beam bridge, but can be built on the ground and then lifted into position with a crane. They can span a greater distance than a beam deck, which makes them useful where clearance is needed underneath. A truss deck bridge requires more height than a beam deck for the clearance needed under it, because the truss system has a height of about 12 percent of the bridge length.

Arches can be used in a deck bridge design, if the arch is placed under the roadway deck. In this design, a steel arch is built that is supported by foundations on both ends of the arch. The roadway is built above the arch, with the highest point of the arch at the center of the road span. Arch designs are useful where higher clearance is needed underneath, such as over rivers, harbor waterways or another roadway.

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