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

SECTIONFOURPost World War II Technological Developmentsand improvements in welding techniques. These developments economized and enabled anincrease in the efficiency of construction of these bridge types (Hollingsworth 1975: 87-89).5.3.5 Movable Bridges1948-1960: 5 movable bridges constructed (Maryland State Highway Administration 2003).Movable bridges are put in place to fit unique situations where static span bridges would impedethe movement of objects in the waterways below the span. Vertical lift and bascule bridges arethe most frequently constructed types (Suffness 1992: 4, 11). The replacement Woodrow WilsonBridge Project, a bascule bridge, is currently underway between the States of Maryland andVirginia. This bridge links the two states, carries I-95/I-495 and spans the Potomac River(Woodrow Wilson Bridge Project: Project Overview: What and Why 2004).Many technological improvements refined movable bridge construction between the years of1924-1974. Many patented designs became available after the expiration of their owner‘s patentsin the 1940s and 1950s. Plan standardization economized fabrication. The abandonment ofriveting and the adoption of welding, use of new metal types and alloys, gearing efficiencyimprovements and improvements in bearings, lubrication and efficiency all played a vital role inthe development of this bridge type. Additionally, electrical power was incorporated into bridgemovement devices and later, remote controls of these electrical devices were adopted into thecontrol mechanisms of these bridges (Hardesty, Fischer and Christie 1975: 520, 522).5.3.6 Prestressed Concrete Bridges(See below for bridge counts.)The development of prestressed concrete broadened the use of concrete in bridge constructionnationwide (Spero and Berger & Associates 1995: 138). In 1955, the Bureau of Public Roadspublished the Criteria for Prestressed Concrete Bridges. This publication further encouragednationwide adoption of the technology (Spero and Berger & Associates 1995: 138). The WalnutStreet Bridge, in Philadelphia, Pennsylvania, which utilized post-tensioned construction in 1949,was a notable and unusual exception. Post-tensioning was much more common in Europe thanNorth America in the material‘s early years due to German and English strides in materialdevelopment (Lin and Kulka 1975: 496, 491).The development of prestressed concrete is directly related to the development of high-strengthsteel. Prestressed concrete can be created in two ways: it can be accomplished via pretensioning,in which the concrete is cast around steel cables that are already in tension; or the concrete canbe post-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 are elements that are precast in the factory; the latterfor the casting and later tensioning of members in place (Lin and Kulka 1975: 491-495).Within the United States, precast structural components were most commonly used inprestressed bridge construction. The precasting process was economical because it allowedstructural members to be fabricated off-site and for the member forms to be re-used. Themembers, if not cast on-site, needed to be transported via road or rail to the bridge constructionsite. Forms were also used in place at the construction site and it was possible for an entire span\15-SEP-11\\ 5-5