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The Walnut Street Bridge is one of the rarest and most important historic bridges in the United States. One of a mere handful of surviving through truss bridges with cast iron compression members, the Walnut Street Bridge has larger percentage of material that is cast iron than most of these bridges. In particular it is noted for its cast iron floorbeams, which is nearly unheard of, because floorbeams can be subject to tension and cast iron tends to display its brittle characteristics under tension. Finally, the bridge is also among the oldest all-metal bridges in the United States with a ca. 1860 construction date. The features the patented cast iron bottom chord connections as patented by F. C. Lowthorp, whose inventive skills were a part of several of the surviving cast iron truss bridges in the United States. The bridge itself was designed and constructed by the Beckel Iron Foundry of Bethlehem, Pennsylvania. The company was run by Charles Beckel and his son Lawrence Beckel.
The bridge was apparently originally known as Wagner's Bridge. It later developed the local name "Pony Bridge." The origin of the name is unknown. The name can be a bit confusing to bridge historians today since "pony" is usually used to describe a type of truss that has no overhead bracing. Pony truss bridges were historically called a "low truss" so the idea of a pony truss may have not been around when the bridge acquired the name "Pony Bridge." Despite the name "Pony Bridge" the Hellertown Bridge is a through truss in terms of design. Through truss bridges have overhead bracing and were historically called "high truss" bridges. At only 56 feet in length, the Walnut Street Bridge is one of the shortest through trusses known to be in existence. Generally, the shortest through truss bridges encountered are 80 feet.
Cast iron allowed for a greater deal of creativity in member design on bridges, because a casting could take a wider variety of shapes and designs than could the rolled wrought iron and steel beams that became dominant by the 1870s. The Walnut Street Bridge displays the freedoms that cast iron offers very nicely. The top chord segments, end posts, and vertical members are beautiful cylindrical column-like members that are larger in diameter at the center with a rib at the center as well. The ends of these members convert to a different shape and size that allows them to match up with the shape of the respective connection boxes. Patent dates which refer to the F. C. Lowthorp patent on the bottom chord connections are cast into some of these as well. The portal bracing which is identical in design to the stuts is truly a beautiful design, with an attractive alternating pattern of X's with curved ends and circles. The bridge's connections are formed by the use of cast iron boxes to which the compression members are locked or snapped into place on, while the tension members, which are threaded rods, are fed through holes on the connection boxes and held in place with a bolt. The connection boxes have a shaped design that combines with the shape of the ends of the compression members to complete the aesthetic detail of a series of columns like those that might be seen in a building. While most of the original railings have been lost, a cast iron beam remains on the lowest portion of the sidewalk railing. This design has the appearance of a rolled i-beam with a pattern of equally sized oval shaped holes in the web. Decorative cast iron railing posts remain as well.
The entire bridge is composed entirely of unique and distinctive details, hardly anything on this bridge could be considered standard. One of the most amazing and unique features of this bridge is its cast iron floorbeams. The idea of cast iron floorbeams is nearly unheard of because brittle cast iron does not normally stand up to tension forces that occur in floorbeams. However, Beckel was aware of this problem and they devised a clever and bold solution. For each floorbeam, they cast a single enormous casting that included the sidewalk cantilever and the lower chord connection box all in the same casting. Given the size and intricate complexity of this total casting, this is quite a feat. There was value for this effort however. Including the sidewalk cantilever in the casting helped to stiffen the floorbeam so that it had stronger resistance to bending from tension. Beckel also created angled ribs in his floorbeam casting, which further add to stiffen the beam.
The bridge appears to retain good overall integrity, although as can be expected of a bridge so old, there are alterations including replaced bolts and welded repairs. Some of the tension rods may have been replaced. The lower part of the sidewalk railings is cast iron, but the upper part is a replacement welded on. The vehicular railing is not original at all. However the key elements of the bridge including the connection boxes, cast iron compression members, floorbeams, and sidewalk cantilevers are all original. The bridge was rehabilitated in 1950, and the bridge's vehicular service ended in 1970, when it was moved off of the Saucon Creek crossing and set on land nearby. The bridge today is located near to its original crossing, today spanning a mill race on historical society property. The bridge has been largely restored (a project dating to 1999), although it appears the wooden deck needs minor repairs. The bridge is painted a beautiful silver color. Silver was once a common color for bridges several decades ago, but it is rarely used during bridge restoration projects. This is unfortunate, since silver really brings the details of the bridge out and makes photographing those details much more easy than it would be with many darker colors.
Historic American Engineering Record provides an excellent narratives that describes the history of this bridge in detail. Be sure to review their documentation. Historic American Engineering Record also provided some important historical documents, including an original plan sheet for a 55 foot Beckel Foundry through truss, a photo of the Walnut Street Bridge in the 1870s, and a photo of Lawrence Beckel himself standing in front of a similar truss in Easton, Pennsylvania. HistoricBridges.org has digitally enhanced and touched up these documents so they are more clear than what HAER provided initially. The results appear throughout this narrative. Clicking on the plan sheet and the photo of Beckel will provide a high resolution version.
Also, be sure to view the other iron Beckel bridge in the area, the Old Mill Road Bridge, which is a pony truss.
This bridge is tagged with the following special condition(s): Reused and Unorganized Photos
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