The Bridges of New Jersey: Portraits of Garden State Crossings

Movable Bridges

128 Movable bridges, as the name implies, change position in whole or in part to allow traffic to pass below or around them. The concept is not new. As early as the sixteenth century, Leonardo da Vinci designed a “very light yet rugged” movable bridge for military purposes. The basic types are the bascule, swing, and lift bridges. A familiar kind of bascule bridge is the drawbridge, in which a single leaf or each of two opposing leaves has a counterbalance at one end (bascule means “balance ” or “see-saw” in French). When the counterbalance is allowed to sink, the free end rises on a horizontal axis. A double-leaf bascule has two ends that rise and form an inverted “V” to allow marine traffic to pass. A variation of the bascule bridge is the rolling lift; as the the leaf rises into the upright position, it rolls on a bearing instead of rotating on an axis. The swing bridge rotates ninety degrees on a vertical axis on a central pivot pier, allowing marine traffic to pass on either side. Swing bridges are usually employed for railroad crossings. The movable span is supported either by a center bearing on a vertical pin or pivot, or by a rim bearing on a “drum” that is a circular girder. The latter configuration was generally used for wider and heavier bridges. Swing bridges were moved, depending upon size and date of fabrication, by hand crank, steam engine, or electric motors, with gears and rack-and-pinion MOVABLE BRIDGES drives. Although the swing bridge is often considered in a category of its own, the structure itself can employ truss or girder construction. Lichtenstein Consulting Engineers of Paramus, New Jersey, a firm that has reported on historic bridges for the Delaware Department of Transportation and for the New Jersey Historic Bridge Survey, observes that swing bridges date back to at least the seventeenth century in Europe. The firm notes that “[t]he demise of swing span bridges . . . resulted from the basic problem that the movable span and pivot pier are obstructions to navigation.” One such bridge in New Jersey that is scheduled for replacement by a high-clearance bridge is the Victory Bridge over the Raritan River near Perth Amboy and Sayreville (color plate 16). The lift bridge, as its name implies, operates like an elevator, with the entire span rising vertically between towers. In Landmarks on the Iron Road, William Middleton observes that they were preferred for railroad traffic, “particularly where long clear spans were required.” He explains how they work: “The weight of the lift span was counterbalanced by weights attached to each end through cables running over sheaves at the top of each tower. Lift spans reached some prodigious dimensions.” One now obsolete type of movable bridge was the “retractile” bridge from the mid-nineteenth century. The 1997 Contextual Study of New York State’s Pre-1961 Bridges describes it as a bridge where the span moves horizontally onto the roadway . Although efficient, it required a fair amount of land to operate, and did not survive as a popular form. A twentieth-century development was the pontoon bridge, a movable bridge that serves as a temporary crossing. The original movable bridges were the drawbridges of medieval times, raised by ropes and pulleys. The earliest timber swing bridges rotated around a pier to provide channels for passage of vessels on either side. Steel and metal replaced timber in swing bridges beginning in the nineteenth century, and eventually motorized mechanisms were used in place of manual cranking mechanisms. The bascule bridge became more popular than the swing bridge at the onset of the twentieth century, taking advantage of the efficiencies of metal girders and concrete counterweights . They could be constructed in narrower waterways and more congested urban settings than the swing bridges. The modern era of the bascule bridge is deemed to begin in 1893 with the Van Buren Street Bridge in Chicago, a rolling lift bridge built on the design patented by William Scherzer. In this type of bridge, the center of rotation shifts as the rotating part moves horizontally on a track. The Shark River Bridge in Belmar, New Jersey (discussed below with the Oceanic Bridge), was built by Scherzer’s company 129 M O V A B L E B R I D G E S but is not a rolling lift...