A beam bridge, or girder bridge, consists of a horizontal crossing beam supported by piers or pillars. It is structurally the simplest kind of bridge and the least expensive. It began as a simple log laid out between opposite banks of a river. The workings of a beam bridge include the principles of tension and compression. A sturdy beam is required to support a crossing load (pedestrians or vehicles) and to prevent the beam from snapping under excessive compression. Beam bridges are typically made from steel and concrete. Types of beam bridges are distinguished based on the composition, location and design of the employed trusses, or connecting elements.
Howe Kingpost Beam Bridge
The Howe kingpost bridge, or Howe truss bridge, was designed and patented in 1840 by millwright William Howe. It comprises a single beam that is supported by diagonal and vertical trusses that slope upward to meet at the centre. A Howe kingpost bridge employs construction material economically and was popularly used in the 19th and early 20th centuries. Examples of the Howe kingpost bridge include the Sandy Creek Covered Bridge north of Hillsboro in Jefferson County, Missouri, and the Jay Bridge, located in Jay, New York.
Warren Truss Beam Bridge
The Warren truss bridge was designed and patented by Willoughby Theobald Monzani and James Warren in 1848. It consists of a single beam held up by longitudinal supports joined by angled cross-members, which are stressed from compression and tension. The beams make alternating equilateral triangles; the entire structure is lightweight and easy to construct. The Warren truss bridge is ideal for two-way traffic, trains or slot cars. One example of the Warren bridge is the historic Foxburg Bridge located in Foxburg, Pennsylvania.
Pratt Truss Beam Bridge
The Pratt truss bridge can span hundreds of feet--more than most other types of beam bridges. Its design is straightforward and comprises a compressed upper supporting cord and a lower cord in tension. The two cords are connected with the horizontal beam via diagonal and vertical members. The Four Points Bridge
located in Emmitsburg, Frederick County, Maryland, is an example of the Pratt bridge.
The Bollman Truss Beam Bridge
The Bollman Truss Railroad Bridge in Savage, Maryland, is an example of the Bollman truss beam bridge. It was the first metal bridge adopted for railroad connections. Its design comprises two trusses, made of cast and wrought iron, that rest on a granite pier and granite abutments. The Bollman bridge is also referred to as a suspension truss, for it suspends the beam by members that are under tension.
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