Simply defined, a bridge deck is the portion of a bridge that acts as the roadway in the support of vehicular or pedestrian traffic. While deck parts like trusses, girders, rails, arches, posts and cantilevers assume a number of forms and types, there are relatively few bridge deck types given the utilitarian nature of the component. Deck types are defined by the materials from which they are made and the manner in which those materials are fit together.

...
Bridge decks must be sturdy enough to withstand constant pressure and extreme weight.

Concrete Decks

Concrete bridge decks are much what they sound like: flat roadway surfaces made of poured concrete that is laid in sections on large bridges or in one fell swoop on small bridges. Thin steel beams are laid into the concrete to provide both strength and flexibility. Concrete is a rigid material that will break if under overburdening pressure. Steel laid into concrete bridge decks permit the deck to bow and sway gently under the pressure of weight or atmospheric elements without fracturing.

Reinforced Decks

Reinforced bridge decks are similar to concrete decks in that they are poured of reinforced concrete and interwoven with steel support bars for added strength and flexibility. Such decks differ from standard concrete decks in that they apply further support systems beneath the concrete for additional support. A typical reinforced bridge deck consists of a thick layer of concrete to form the roadway and solid steel sheets laid beneath the concrete to offer additional relief from the pressure of weight. Distribution bars and support beams are laid beneath the steel sheets. Distribution bars relieve pressure from the deck incrementally and transfer weight-bearing responsibility to support beams, much like flying buttresses do for buildings.

Beam and Girder Decks

Beam and girder decks are those that employ side reinforcement. Girders are support beams made from steel that are divided into sections, or plates, and affixed to the sides of a concrete bridge deck for added support. Beams work much in the same manner but are generally one enormous piece of steel that provides structural support by helping a bridge maintain its shape in the face of elements that may cause the concrete to bow or flex. Girders help alleviate deck weight by segmenting the deck and relieving pressure from the structure as a whole.

Wooden Decks

Wood is a material commonly used in pedestrian bridge construction. In such applications wood is most often laid as planks. Wood bridge decks are almost uniformly constructed with metal support systems to both reinforce the strength of the material and provide insurance against possible weakening and failure of wood in the face of extreme heat and moisture. Heat can cause wood to buckle and morph while constant rain or snow weakens and swells wood such that it is easily broken.