The need to develop a cost-effective yet attractive bridge design led BPC engineer Arthur Hayden to devise an innovative structural system that would have a long-lasting impact on parkway and highway development throughout the nation. Conventional arch bridges were expensive to construct and not ideally suited for use as grade separation structures. They required heavy abutments along with extensive grading and/or excavation to accommodate their relatively high profile. Hayden investigated whether or not a less expensive structure could be built that resembled a true arch, yet eliminated the massive abutments and associated grading. In December 1921, Hayden presented his design for a new type of bridge that offered greater structural strength and was more economical to build. He called it the rigid-frame bridge. Hayden’s bridge derived its strength from a rigid connection between the vertical posts and horizontal beams. This eliminated the need for the heavy abutments associated with arch bridges link and produced more strength than simple girder construction, which up until that time had afforded the primary alternative for highway bridges. In the rigid-frame bridge, the structure was an integral unit with all members working together to support the structure. The strength achieved from the rigid connection of the posts and beams resulted in a bridge of slender proportions that required less concrete than conventional structural systems. Rigid frame technology was especially suited for grade separation structures because the relatively flat arch of the rigid frame provided better clearance than true arch bridges, where the arch’s pronounced curvature restricted headroom in the outside lanes unless the span was much wider than the paved roadway. The rigid frame bridge’s thin arch crown and more efficient profile also cut down on the height of the overall structure, reducing the amount of grading required to carry cross traffic over the bridge at an acceptable grade. The BPC embraced the rigid-frame bridge because it required less fill for the approaches, less excavation and concrete for the abutments, and less concrete for the bridge frame members. These economies made rigid frame construction particularly appealing to parkway commissions and highway departments interested in developing limited-access roadways but leery of the expense involved in constructing numerous grade separation structures. The simple reinforced concrete structures were also readily adaptable to a variety of architectural treatments, which further enhanced their appeal to parkway designers. Following the rigid frame’s successful introduction on the Bronx River Parkway it was rapidly adopted by other parkway and highway building agencies, appearing in such high-profile projects as the Mount Vernon Memorial Highway, the Blue Ridge Parkway, the Merrit Parkway, and, of course, subsequent Westchester County and New York State parkways.(200) Since Hayden’s bridge was the first of its type, the BPC had the design carefully checked at the testing laboratories at Columbia University. The tests checked the strength of the bridge’s knee joints and concluded that Hayden’s design was "entirely satisfactory." The Columbia engineers determined that the strength was nearly twice what Hayden’s calculations indicated. The BPC also contracted the noted bridge engineer J. A. L. Waddell to check Hayden’s design. The rigid-frame bridge was initially proposed for grade separations at Swain Street, Palmer Road, Tuckahoe Road, and Main Street. Architect Charles W. Stoughton presented his revised architectural plans for the Main Street grade separation in March 1922. His revisions incorporated Hayden’s design for a "flat arch," which allowed Stoughton to eliminate the earlier design’s center supporting columns. It is difficult to estimate how much the BPC saved by using the rigid-frame bridge. When Hayden’s design was initially presented to the BPC, Downer reported that it would save approximately $5,000 for each undercrossing. Hayden, however, reported that the rigid-frame bridge design would save between $700 to $1,000 for four undercrossings.(201) |
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| (200)Bronx
Parkway Commission, Report, 1922, 44, 47; Bronx Parkway Commission,
Minutes, January 4, 1922, 2-3; Arthur G. Hayden and Maurice Barron, The
Rigid-Frame Bridge, (New York: John Wiley & Sons, Inc., 1950),
viii, 1-3. |
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