historic context of maryland highway bridges built between 1948 ...

SECTIONFOURPost World War II Technological DevelopmentsWelding and bolting eliminated the need for the slow and tedious work of the rivet gangs.Improved falsework increased construction efficiency. Wood falsework was replaced by metalfalsework sections after 1930 (Bigelow 1975, 242-243). The production of large prefabricatedbridge sections necessitated advances in delivery strategy and improved construction efficiency.Large sections were delivered by road or railroad. Crane technology continued to improvethroughout the period and the utility of the bridge deck as an integral structural componentincreased span strength (Burroughs 1975: 463-465, 474).The use of prestressed (and post-tensioned) concrete in bridges in the 1950s was the mostnotable technological advance in Maryland and North America. A series of prestressed concretebridges were built between 1950 and 1954 that spanned Tampa Bay between St. Petersburg andPalmetto, Florida. At the time, it was one of the world‘s longest crossings, with an overall lengthof 15 miles (Hammond 1960:85 – 88). According to noted American bridge designer T. Y. Lin:―The most important development [in concrete bridge development between 1925 and 1975] iscertainly the introduction and expansion of prestressed concrete for bridge construction‖ (Linand Kulka 1995:491).Historian David J. Brown summarizes the theory of prestressing and its attractiveness to bridgedesigners as follows:Any building (or bridge) material can be ―pre-stressed.‖ A single rope across ariver, stretched from one tree to another, has tensile stresses locked into it toreduce its flexibility. The principle of prestressing concrete, expressed at itssimplest, is much the same: longitudinal steel strands in a concrete beam arestretched or tensioned and then anchored to the ends of the beam. Thisneutralizes the tensile forces created by dead, live, and environmental loads—andthus the propensity of concrete for cracking—by the application of greatercompressive forces. Far more effectively than with simple reinforcement,prestressing maximizes concrete‘s strength in compression and compensates forits weakness in tension.Prestressed concrete is economical because of its sturdiness and also because it islighter than conventional reinforced concrete (1998:124).Prestressed concrete can be created in two ways: it can be accomplished via pretensioning, inwhich the concrete is cast around steel cables that are already in tension; or the concrete can bepost-tensioned. In post-tensioning, concrete is poured with intentional voids that allow for thethreading, stretching, and anchoring of cables after the concrete has hardened. Both means areutilized in bridge construction—the former at elements precast in the factory; the latter for thecasting and later tensioning of members in place.American engineer P. A. Jackson was likely the first to formulate the idea of prestressingconcrete in 1872. French engineer Eugène Freyssinet pioneered and propelled the use ofprestressed concrete in bridges during the second quarter of the twentieth century (Brown1998:124). It was not until after World War II, however, that the technique crossed (or recrossed)the Atlantic. The first prestressed concrete bridge erected in the United States was theWalnut Street Bridge, erected in Philadelphia in 1950 (Barker and Puckett 1997:20). Prior to1975, virtually all precast beam and girder bridges erected in the United States utilizedprestressed concrete. The Walnut Street Bridge, which utilized post-tensioned construction, was\15-SEP-11\\ 5-2