Bridges are employed to support weight over an open space, and transfer this weight to their supports, or abutments. They may be fundamentally classified by the reactions they exert upon their abutments. They may push on the abutments, pull on them, or simply rest on the abutments without horizontal forces. In general, the production of horzontal forces in the bridge structure is the cost of transferring the weight of bridge and load to the abutments. Bridges are generally, and less fundamentally, classified by the type of construction. Arch bridges push on their abutments, suspension bridges pull on them, while beams and trusses rest on their abutments without horizontal forces. The term beam is used when the material of the bridge is in a single piece, such as a log or a plate girder, while a truss is built up of pieces, called members. A truss generally has an upper chord in compression, a lower chord in tension, and web members consisting of diagonal or vertical ties (if in tension) or posts (if in compression).
Dr. Calvert's essay on the Bollman Truss bridge on the Baltimore and Ohio Railroad covers the decision to use the bridge, the reasons it was not used more widely, and a tremendously probing architectural explanation of the truss's design. However, in the process, Calvert also gives a serviceable overview of bridge construction in general, as well portions of the history of the B&O. He has compiled a superb document and a tremendous aid to anyone interested in architectural design and the history of bridge-building.