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This bridge was a heritage bridge that was demolished and replaced in 1981 with a modern structure. As a modern structure, the current bridge cannot be considered to have heritage value. However, it does have some interpretive value since the main movable span of the heritage bridge was at least partly simulated in the replacement bridge, as are the piers and bridge tender house. The approach spans visually simulate the heritage bridge, but underneath it are what appear to be pre-stressed concrete spread box beams with the arch-shaped parts of the approach spans merely a facade. The dimensions HistoricBridges.org is giving for this bridge are for the heritage bridge. Also, the listed engineer, Strauss Bascule Bridge Company designed the heritage bridge, but did not design the replacement bridge. However, since the design ideas of the Strauss Bascule Bridge Company were reused to design the main span of the replacement bridge, it seems appropriate to give credit to the original inventor of this design.
The heritage bridge was one of the only surviving examples in the world of a Strauss direct-lift type of vertical lift bridge. Joseph Strauss's typical direct-lift bridges used two counterweighted arms that rotated around a trunnion (or axle), each resting on a tower. The rotation of these arms pulled a truss span up vertically. The novelty of the design is it is a vertical lift bridge without cables. The Pretoria Avenue Bridge is unique however because Strauss apparently adapted the design to omit the towers, and instead hid the counterweights in the piers.
Otis Hovey's 1926 publication Movable Bridges describes the bridge as follows. "The bridge "is an ingenious adaptation of the type. The counterbalancing and operating mechanisms are below the bridge floor. The counterweight trusses are placed at right angles to the longitudinal centerline of the bridge, thus utilizing the piers to support and conceal the counterweights and mechanism."
In operation, the bridge has the appearance that is similar to that of the Erie Canal lift bridges in New York State. However the Pretoria Avenue Bridge reportedly does not operate like the Erie Canal Bridges, which use a set of cables and a rack and pinion to cause the bridge to rise up from the ground.
The existing 1981 structure does simulate the motion of the heritage bridge: it rises up from the deck vertically. It also retains the truss configuration and built-up beams, however bolts have been used instead of rivets. It is not known if the internal mechanics of the 1981 bridge simulate the original design of the bridge or if they have been altered.
Video of the bridge in motion is available here.
Video interview of a bridge tender is available here.
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