The bridge members are divided into two major categories: the superstructure and the substructure.
The superstructure is the upper portion of the bridge above the beam seats where you drive or walk. Members include:
Deck wearing surface
The substructure is under the superstructure and supports all of the bridge loads. Members include:
Some of the main materials found on a bridge are steel, concrete, stone and asphalt. Other materials include iron, timber, aluminum, rubber and other joint materials. Below is a description of some typical uses for these materials in a bridge.
Concrete is commonly used for many bridge superstructure members such as decks, pre-stressed concrete beams, curbs, sidewalks and parapets (side traffic barrier walls). It is used extensively in new construction for the entire abutment, including the footings, stem (main front wall), wingwalls, cheek walls, backwalls, endwalls (for traffic barrier connection), beam seats, and the piers with similar members. It can also be used for cast-in-place or precast concrete piles to support the abutments and piers.
Steel is commonly used in the bridge superstructure for armoring expansion joints, beams, bearings, floor beams, girders, reinforcing bars in concrete, traffic barriers and trusses. It is used in the substructure for the reinforcing bars in concrete, armoring for expansion joints, anchor bolts, etc. It is also used for piles to support the abutments and piers.
Stone was commonly used for building the abutments and piers in the 1940s and earlier. This is particularly true where local field stone was readily available. Many spectacular stone arch bridges were built for the B&O railroad system in the 1800s. In Harford County, the remains of some MA & PA Railroad bridge abutments and piers are still standing.
Asphalt is the material that has been used extensively for the wearing surfaces on corrugated metal decks, timber decks and concrete decks in Harford County.
Iron was used typically in beams and trusses that were built before 1900. Steel replaced iron because it has more tensile strength than iron and is less brittle. There are almost no uses for iron in today’s bridge designs.
Timber is used for several decks and traffic barriers in Harford County. It is also used for the beams on one bridge and the abutments and piles on another bridge.
Aluminum is sometimes used in fabricating bridge railings.
Rubber and synthetic rubber products are used for bearings and for expansion joint material.