The parts of a bridge are separated into two categories: the Superstructure or the part on which you drive or walk (i.e. beams, deck, curb, sidewalk, railings, expansion joints, bearings, etc.); and the Substructure or the parts supporting the superstructure (i.e. abutments, backwalls, wingwalls, piers, footings, etc.).

The main superstructure members are generally made of reinforced concrete, steel or timber. The substructure members are generally made of reinforced concrete and stone, although some bridges substructures have been built with timber and steel. The Foundation part of the Substructure is generally built with reinforced concrete or stone.

Harford County’s bridge inventory also includes drainage culverts, that allow streams to flow under roads. Culverts are used for narrower roadways and generally have shorter spans or distance between supports than bridges. These shorter spans allow more options for different structures that can be used.

All types of culverts have a headwall, a concrete or stone "collar", at the unstream entrance and downstream exit to keep the roadway side slopes from sliding into the stream.

Bridge Materials

Concrete is a widely used material in superstructure and substructure member because it is very durable, can be formed into many shapes, and is extremely strong when reinforced with steel bars. The main uses for concrete in the superstructure are in the deck, sidewalks, curbs and parapets (side walls at the edge of the road). The main uses for concrete in the substructure are in the walls and footings of members such as abutments, wingwalls, and piers.

Special high strength concrete is used to make precast (factory built), prestressed (special wire is embedded under tension in the concrete) concrete beams. The precast beams are shipped to the site and placed on the substructure members.

Steel is mainly used in the superstructure as beams and trusses to span the long distances from one substructure member to another. Steel is used because it can be rolled and fabricated into very strong, efficient members. The weight of a steel member needed to span a given distance is much less than that of a concrete member to span the same distance. This can be very important as bridge spans are built longer.

Steel is generally not used in the exposed parts of substructure members because substructure members are in contact with or exposed to the ground and water. This causes rapid corrosion (rust) of the steel, even if it is painted. Painted steel beams in the superstructure generally last a long time, especially when the beams are shielded from direct water (rain) contact by a concrete deck .

Timber is used mostly for decks, although the county has one timber covered bridge and one bridge with timber beams. Timber is a relatively lightweight material that can be easily cut into different size and length members. Timber members must be treated with preservatives to prevent them from decaying and to ward off attack from insects. Some older bridges over railroads have been built with timber trestles (piers), but there are none in Harford County.

Stone is used mostly for abutments, wingwalls and headwalls and as a veneer (exterior decorative layer) on walls. Stone is a very durable material that can support a lot of weight. Some original B&O railroad bridges, west of Baltimore City, were built with stone in the form of an arch and still stand today. Cut stone can be built without mortared joints.

Harford County Bridges

Harford County maintains 204 bridges that provide continuous transportation on County maintained roadways. Most of these roadways are across streams.

Below are the various types of bridges found in Harford County:

TYPES OF BRIDGES

Posted Bridges

Many bridges in Harford County were built before large modern trucks were readily available for use. Therefore, bridges were designed for smaller loads or weight limits than we now see on the road. Today, most bridges are designed for a maximum allowable vehicular load to prevent large trucks from traveling across and damaging the bridges. As older bridges are repaired or rehabilitated through the Bridge Program, the maximum allowable vehicular load for that bridge may be upgraded. Some modern bridges are not posted because they are designed to support maximum highways loads.