Glossary of Bridge Terms

All-Welded Construction Bridge: A bridge on which all the connections are welded—no rivets or bolts are used. All-welded construction helped THD cut down on using expensive rivets between 1945 and 1965.

Beam/Girder Bridge: A type of bridge structure. Girders and beams are both long pieces of iron, steel, or concrete that are used to build the support structure of a bridge. Girders are bigger, longer, and deeper than beams.

Bearing Plate: A component of a bridge’s supporting structure. It is located between the deck of a bridge and its piers, or upright supports. The plate covers the ends of the piers and lets the different parts of the bridge move slightly against one another without causing damage.

Concrete: A building material made out of stone mixed with other components such as gravel. In Texas it is common to mix limestone, a local material, along with ash and gravel.

Concrete Box Girder: A bridge type that uses hollow concrete boxes as its main support. This type of bridge was not frequently used between 1945 and 1965 in Texas.

Concrete Pan-Formed Girder: THD developed this bridge type after World War II. Concrete was poured directly into large steel molds called pans to make girders and beams (instead of using steel or iron). The concrete was poured on-site (cast-in-place). Pans were reusable, and concrete is inexpensive. Many farm to market road bridges between 1945 and 1965 are concrete pan-formed girders.

Farm to Market (FM) and Ranch to Market (RM) Road: Roads in rural areas. Also known as a secondary road system. Texas is the only state in the Union with its particular system of farm to market roads.

FS Slab Bridge: A bridge type developed by THD after World War II. These reinforced concrete slab bridges have high curbs that are poured in one unit with the slab, which allows the curbs to act as small girders and help carry the load. Also known as concrete slab. Fun fact: despite being created in Texas, no one today knows for sure what “FS” stands for.

High Tensile Bolts: A cheaper and easier to install alternative to rivets. These bolts are made of carbon steel and are heat treated to increase their strength.

Interchange: A series of roads passing over others on bridges with ramps to connect them. They allow vehicles to move from one road to another without having to stop. Interchanges can be two-, three-, or four-levels high.

Prestressed Concrete: Concrete with taut steel wires, known as tendons, running through it to improve its compressive strength. Prestressed concrete uses a limited amount of steel but is incredibly strong, which means bridges can be longer and sustain more weight. Prestressed concrete can be created with either post-tensioning or pre-tensioning. Post-tensioned concrete requires stretching the steel and attaching it to an anchor on site. To create pre-tensioned concrete, the steel is stretched and held under stress until the concrete is poured. Pre-tensioned concrete was precast before arriving at the job site and was more commonly used in THD bridges.

Neoprene Pads: Also known as neoprene bearing plates. Constructed of high-grade synthetic rubber to support heavy loads on longer bridges. Developed by a THD engineer in 1958, they are used on bridges all over the world today because they are more durable and economical than steel bearings.

Rivets: A metal pin that holds together two plates of metal. Its headless end is beaten with a hammer to spread out to a size larger than the diameter of the hole to make a second head. By the early 1950s, welding and high tensile bolts began to replace rivets because these technologies were less time and labor intensive than riveting.

Span: A distance between two bridge supports.

Steel: An extremely hard building material. It is an alloy, or combination, of iron, carbon, and other elements.

Texas Department of Transportation: Known today as TxDOT, it was previously known as the Texas Highway Department. Founded in 1917, TxDOT is charged with the construction and maintenance of roads, rail, aviation, and public transport throughout Texas.

Welding: The process of joining pieces of metal. The surface of the metal is heated to an extremely high point and then hammered or pressed together.