Walnut Lane Concrete Arch Bridge, Philadelphia Pennsylvania
When completed in 1908, the Walnut Lane Bridge was the world's longest concrete arch bridge, with a main span measuring 233'-0". The Rocky River Bridge in Ohio exceeded it in size two years later. Taking inspiration from a 1904 masonry arch bridge in Luxembourg, Webster and Quimby's two-rib, open-spandrel design uses a minimum of reinforcing steel. The bridge crosses Wissahickon Creek in Fairmount Park.
At the turn ofthe twentieth century, Fairmount Park was the world's largest urban park. From five acres surrounding the Fairmount Water Works in 1812, the park was expanded to more than 4,000 acres, including land formerly occupied by some of the city's wealthiest residents. The park also contains fourteen miles of tributaries and the Schuylkill itself, which formerly provided water power for a variety of enterprises. The earliest industries were mills in the Wissahickon Creek valley, with Richard Townsend operating a grist mill as early as 1686. Not until after the end of the Civil War was the west bank of the Schuylkill included in the park. Following the U.S. Centennial Exposition of 1876, the park matured into a picturesque combination of landscaped parkland and natural areas. The Exposition drew thousands of visitors to West Fairmount Park, a massive array ofbuildings and landscaped grounds. Celebrating a century of U.S. history and demonstrating futuristic applications of technology, the Exposition was an instant hit with the city and the world. In the wake ofthe event, the park became a year-round destination for refuge-seeking locals and day trippers. Private ventures and various city agencies scurried to improve the urban infrastructure as West Philadelphia opened up for development. During the closing decades of the nineteenth century, modern iron and steel spans such as the Girard Avenue Bridge (1874), Falls Bridge (1895), and Strawberry Mansion Bridge (1897) were built to ensure reliable and safe access to West Fairmount Park and beyond.
Carrying Walnut Lane over Wissahickon Creek, the Walnut Lane Bridge was the first span to directly connect the neighborhoods of Roxborough and Germantown in northern Philadelphia. Densely wooded hillsides hide the steep contours of the valley below, while at either end ofthe structure are late nineteenth- and early twentieth-century residences. The Walnut Lane Bridge has six spans, all three-centered, open-spandrel concrete arches, for a total length of 585'-0". The main arch has a clear span of 233'-0" and rises 70'-3" from springing line to crown, for a total height of about 147-0" above the valley floor. Eight semi-circular spandrel arches, each 20'-0" long, support the deck above the arch ribs. On the approach spans, solid walls conceal the open-spandrel construction. Two arches on the north approach have a clear span of 53'-0", as do three arches on the south approach. All of the piers, abutments, and wing walls extend downward to bedrock, which is a mica schist. The footings had to be built around an existing 30"-diameter water main running parallel to, and 6'-0" south of, the bridge's center line. The roadway is 40'-0" wide from curb to curb, with concrete sidewalks on either side, which measure l0'-0" wide from curb to parapet.
The Walnut Lane Bridge's main span takes inspiration, both structurally and aesthetically, from long masonry arch spans in Europe, especially the Pont Adolphe, a bridge over the Patrusse River in Luxembourg. The Walnut Lane Bridge actually contains a large quantity of stone, in the form of rubble concrete. This material consists of large stones embedded in the concrete mixture, arranged in such a way to provide additional shear strength. Specifications for the bridge's construction stipulated, for example, that flat stones "not less than one man size," the size which an average worker was capable of lifting, be "placed in radial planes as close together as practicable, always edgewise to the centering."
In contrast, there is very little steel reinforcement in the main arch span. Its two arch ribs taper from 21'-6" wide by 9'-6" deep at the springing line to 18'-0" wide by 5'-6" deep at the crown. Yet there are only three 1 "-square iron bars placed below each ofthe transverse spandrel walls, making the bridge essentially an unreinforced or mass concrete structure. The ribs are incised with radial lines, creating the impression of oversized stone voussoirs. Although serving a decorative purpose, the voussoir lines also disguise joints between separate concrete pours. Each rib was poured as a series of twenty-two large blocks, each four to seven voussoir units in length. The blocks were poured symmetrically on either side of the falsework, leaving seventeen one-voussoir gaps between them. These gaps were filled with concrete after pre-compressing the arch.
Above the main arch span, four lines of spandrel arches form continuous longitudinal girders supporting the deck. Their transverse spacing is 14'-0" on center above each rib, and 20'-0" on center between the ribs. The spandrel arches are carried on posts measuring 4'-0" square and placed 24'-0" on center, with the outside pairs braced by transverse walls typically 2'-0" thick. Only the tallest spandrel posts are reinforced with six 1 "-square iron bars where they intersect the arch rib. The approach spans have a similar arrangement of transverse walls above each rib, with the spandrel posts replaced by solid spandrel walls 2'-6" thick.
The floor system consists of reinforced concrete jack arches spanning between transverse floor beams. To resist the arches' outward thrust, the floor beams are tied together by 7/8"-diameter rods. The beams are fully encased in concrete and placed 6'-0" on center, except above the transverse walls. In those locations, the floor slab is broken by a tongue-and-groove expansion joint, with the tie rods anchored into the slab on either side. One end of the slab is cast integrally with the wall, while the other is free to slide on a zinc bearing, analogous in function to the expansion roller of a truss span. The shorter beams above the ribs are 15 "-deep rolled I-beams weighing 42 pounds per foot, cantilevered outward to support the sidewalk brackets. Longer beams spanning between the ribs are 20" deep, weigh 65 pounds per foot, and sit at a slightly lower elevation. This is because the middle 20'-0" of the deck slab, between the two arch ribs, is depressed 11-1/2" to accommodate future streetcar tracks. According to construction drawings, this depression was filled with cinders before paving the entire roadway with a 6"-thick layer of bituminous concrete, followed by a 3-1/2" asphalt wearing surface. The Philadelphia Rapid Transit Company evidently planned to lay tracks across the bridge shortly after its opening, but this never occurred.
Among the bridge's notable ornamental details is the reinforced concrete sidewalk and balustrade. Supported on the cantilevered floor beams, the sidewalk is reinforced with 1" square rods running longitudinally and 1/2" square rods running transversely. The ends of the floor beams are encased in concrete brackets. At the same 24'-0" intervals as the spandrel posts, the brackets are slightly larger and emphasized by pilasters in the balustrade above. A curb at the outside edge of the sidewalk forms the bottom rail of the balustrade, onto which precast concrete balusters, 884 in all, were attached. The balusters have a hard, smooth, gray finish and were cast by using one set ofmolds repeatedly. Over the main span piers, the sidewalk and balustrade project outward an additional 3'-0" on either side, forming rectangular recesses that originally held cast-iron light standards (since replaced with modern aluminum fixtures). Were streetcar tracks installed on the bridge, the standards would have formed the bases of poles supporting overhead wires. As a commemorative detail, the bridge has four plaques, one at the base of each main span pier and one at each end of the deck, set into the balustrade. Those on the piers record the start of construction, 1906 on the west pier and 1907 on the east. The plaques at deck level reflect the official (but less accurate) dates of construction, 1907-08.