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With potential routes of technological diffusion established, the question remaining is that of who identified the Cologne bridge from available literature. Despite credit usually given to Covell, consulting engineer T. J. Wilkerson directly took credit for the design at least once. During a discussion about whether the structures could best be classified as cantilever or suspension bridges, Wilkerson added that he had credentials, "... as I proposed the type of bridge used and am responsible for the design." [108] While Covell indeed must be credited with leading the team that transformed an idea into reality, his role in initiating the concept must be reassessed. Wilkerson's announcement to fellow engineers was not challenged at the meeting. Additionally, the recollections of an engineer intimately familiar with the bridge construction efforts of the period increases doubt about Covell's authorship and perhaps even initiation of the self-anchoring design.

George S. Richardson worked as a draftsman for the American Bridge Company before being hired in the spring of 1924 to work on Allegheny County's projects. He worked for the county for the next thirteen years. Richardson worked on bridges other than the Sixth, Seventh, and Ninth Street projects and did not know who actually proposed the self-anchored suspension design for the trio, but his knowledge of the staff's workings led him to presume that Wilkerson, the consulting engineer, was responsible. [109]

In an interview he recalled that Covell and Nutter's performance in the new Department of Public Works was competent but not inspired. Richardson suggested in his recollection "that long years of employment without any engineering work except on very minor structures had dulled their ambitions to participate in any very active way in the major programs then under way." [110] A contentious relationship with Brown may have exacerbated Covell's tendency to withdraw into bureaucratic duties. [111] Richardson's perspective suggests that the credit given Covell for the structures may refer to his ability to design the thousands of connections and parts required. Whether Covell introduced the idea of the self-anchoring system himself or supervised the project as designed by another remains in question.

Richardson said he was surprised that in his own work, which covered all major bridge structures except for the Three Sisters, Covell lacked any interest in choosing types of structures

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[108] Harrington, "Recent Developments," 68.

[109] Richardson, "Oral History," 1, 4.

[110] Richardson, "Oral History," 1, 5; quote from ibid., 6.

[111] Farrington, Highway and Bridge Program, 47, discusses inter-department tensions and provides insight into this situation. Farrington cites the Controller's Report of 1926, which records that Brown actually placed Covell's desk in a hallway and forced him to perform his duties without an office.




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or in directing those who did. Covell's particular contribution to those projects was a concern with handrail design -- a minimal portion of the bridge, albeit a difficult problem that covered making railings child-proof. Nutter showed interest in details such as expansion dams and curb types, but Richardson noted that he suggested no new designs himself, waiting instead for Richardson's advice on each matter before promoting a particular design. [112]

Wilkerson, who designed the California Avenue concrete arch bridge over Jack's Run for the county just before the 1924 construction program began, contributed more directly to the Three Sisters project. Brought on board as a consulting engineer for the concrete arch by Director of Public Works Norman S. Sprague, Wilkerson also figured prominently in decisions about the Three Sisters. [113] Richardson did not recall having seen stress calculations for the design, but from his knowledge of structural analysis, he concluded that the bridges were designed as statically determinate structures. It was unlikely that Wilkerson, who often used graphical representations for stress analysis, or his assistant, H. K. Dodge, had the ability to analyze a statically indeterminate structure. [114]

Wilkerson worked on two bridge projects in neighboring Beaver County after his contribution to the Three Sisters project, using graphical methods and requesting that Richardson verify calculations for both. The downtown bridge designs would particularly have lent themselves to this method because the immense stiffening girders and adumbrated suspension spans would make a statically determinant analysis likely to produce workable results. Further, Richardson added, it would be quite possible to use this method with a design that anticipated greater stresses than actually were likely to exist. [115]

Regardless of the exact path by which the idea arrived at the Department of Public Works, implementing such a complex project brought deserved credit to the entire staff of engineers, especially Covell, chief engineer; Nutter, design engineer; Wilkerson, consulting engineer; and Stanley L. Roush, architect. Members of sections new to the public works department also contributed specific competencies to the program's design of the Three Sisters. Chief of the Bureau of Tests Freeman added a specialty in concrete testing. His assistant, Groh, had particular expertise in a wide range of metallurgy, with exceptional depth in structural steel.

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[112] Richardson, "Oral History," 5.

[113] Richardson, "Oral History," 1.

[114] Richardson, "Oral History," 4-5. Richardson's comments provide one of the few clues available about design methods. The Allegheny County Department of Public Works no longer has information about the assumptions used. See ACDEC, "In-Depth Structural Inspection," 6:4. The report recommends for general reading about self-anchored suspension bridge theory D. B. Steinman, "A Generalized Deflection Theory for Suspension Bridges," Proceedings of the American Society of Civil Engineers 60 (1934): 323-60, and C. H. Gronquist, "Simplified Theory of the Self-anchored Suspension Bridge," Transactions of the American Society of Civil Engineers 107 (1942): 84-90.

[115] Richardson, "Oral History," 5.




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Roush headed the Department of Architecture, which consulted with engineers about bridge designs. [116]

Once the design for the self-anchored structure had been identified, the question of whether to vary bridge design solved itself: no other solution to the technical, political, and aesthetic dilemma could be found. A consensus formed among members of involved agencies that not only was the eye-bar suspension design for a self-anchored structure the only and best solution, but also that constructing a trio of bridges in such a close area would create a scenic attraction near downtown. Not to be forgotten was the potentially lower unit cost for each to be gained by awarding a single contract for all three projects. [117]


Design Issues

The crucial features of the Three Sisters' final design were the eye-bar chains, the deck stiffening girders, and vertical hangers. An eye-bar chain, draped over steel towers on the river piers, carried the floor system's weight through vertical hangers. The deck contained a stiffening girder running the bridge's entire length. Engineers designed the components to work as a system, anchoring both ends of the eye-bar chain to the stiffening girder, which resisted vertical forces through horizontal thrust. This stood in contrast to conventional suspension bridges, which used heavy shore masonry to maintain tension in the catenary. [118] The Three Sisters caused some debate within the engineering community because they confounded this aspect of the archetypical suspension form. Nonetheless, the design met the surest test of a suspension bridge: if the chains were cut, the entire structure would collapse. [119]

In a suspension bridge, catenary elements carry loads in tension rather than compression. Cables or chains are draped over towers and anchored at bridge ends. They function like inverted arches, with the roadway hung from the catenaries. [120] New materials like heat-treated eye-bars and drawn steel wire offered greater strength in suspension bridges. With the greater span lengths and elegant catenary lines, a suspension bridge would satisfy both navigational and aesthetic considerations. Calculating stress required in stiffening trusses was difficult, however. In 1930 this remained a point of contention among engineers who could not explain why some

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[116] Richardson, "Oral History," 3.

[117] Roush, "Sixth, Seventh and Ninth Street," 196.

[118] Watson, "Bridge Architecture," 91

[119] Watson, "Bridge Architecture," 94.

[120] Walter C Kidney, "Bridge Anatomy for the Layman," in James D. Van Trump Library, Pennsylvania History and Landmarks Foundation, Pittsburgh, Pa.




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stiffening trusses used in suspension bridges worked in practice while failing to meet theoretical stress thresholds. [121]

Although the Three Sisters essentially behaved as suspension bridges when complete, they were erected by cantilever methods. The 1889 Firth of Forth Bridge inspired renewed interest in cantilever erection, which had great application on long-span work. Generally, cantilever erection does not save money when used on spans less than 700'-0" in length if falsework can be contracted for without exceptional cost. When deciding whether to use cantilever erection, engineers had to consider the difficulty of using falsework, the length of the span, and whether navigation must be maintained under the structure. Cantilever methods made erection possible without falsework, for longer spans, and with free navigation. [122]


Contractors

Erecting suspension bridges using cantilever methods was a complex job demanding only the most experienced of contractors. After advertising the bids, the county awarded the contract for the three bridges' superstructure to the American Bridge Company and the substructure to the Foundation Company of New York. County commissioners, anxious to assure the public that contracts were in no way marred by corrupt deals, publicized the savings provided by awarding the contracts to the same company. By building all three bridges at a single price, the county shaved about $0.5 million off of the project budget. [123]

The American Bridge Company, perhaps more than any other company in the world, was best qualified to work on a bridge with a continuous stiffening component. The company was then constructing the Florianopolis Bridge in Brazil, working from 1922 to 1926 on the longest eye-bar suspension span in existence at that time. The 1113'-0"-long design featured towers with rocker bearings. The company brought to that project its own experimental heat-treated eye-bars, and as project contractor pioneered stiffening techniques that saved materials and money while providing greater rigidity. [124]

H. D. Robinson, who had worked with Steinman on the Florianopolis Bridge, had also consulted on towers for the Cologne bridge's design. Robinson used similar towers on the Rondout Bridge at Kingston, New York, which was constructed in 1922 and used a stiffening truss that functioned apart from the towers. By 1924, the American Bridge Company had many opportunities to acquire a base of knowledge that specifically would have helped in building a bridge such as that specified by Allegheny County engineers. Whether the firm advertised its

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[121] Harrington, "Recent Developments," 55; see ibid.,64-65 for further information.

[122] Harrington, "Recent Developments," 60.

[123] "Parade Marks Sixth Street Span Opening," Pittsburgh Post-Gazette, 20 Oct.1928, in Clippings Files, Pittsburgh Bridges -- Sixth Street, Pennsylvania Room, Carnegie Library, Pittsburgh, Pa.

[124] D. B. Steinman, A Practical Treatise on Suspension Bridges: Their Design, Construction and Erection, 2nd ed. (New York: John Wiley & Sons,1929),198-227.




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services to the county before specifications were devised is not known. The American Bridge Company used its knowledge from the Florianopolis Bridge to prepare alternative designs for bridges over the Ohio River in Point Pleasant and in St. Marys, West Virginia, however, securing both projects. [125]

The American Bridge Company also accumulated vast experience by company acquisition. In 1900, J. P. Morgan consolidated two dozen bridge-building companies under the umbrella of the American Bridge Company, which a year later became a subsidiary of Morgan's U.S. Steel. Thereby he created an industrial giant that controlled half of the country's bridge-building capacity. The acquisition united large Pittsburgh competitors, like Carnegie's Keystone Bridge Works, and smaller area concerns, such as Schultz Bridge & Iron Works of nearby McKee's Rocks. Morgan's reach also encompassed New York's Union Bridge Company, Gillette-Hezon Manufacturing Company of Minneapolis, and Berlin Iron Bridge Company of Connecticut in that first year, as well as other operations in coming years. [126]

The Foundation Company of New York brought its own wide experience in to Pittsburgh. Incorporated as the Foundation & Contracting Company in 1902 and specializing in "design and construction of difficult foundations," the firm shortened its name to the Foundation Company to reflect its specialized focus. Known for constructing foundations for skyscrapers at Manhattan Island's southern tip where bedrock was exceptionally deep, the company also expanded its operations to Canada and the Midwest. The Foundation Company's projects in the first decades of the twentieth century included bridges, mine shafts, tunnels, dams, and sea-walls. The firm used a variety of techniques, such as pneumatic caisson work, pile driving, and coffer-dam construction. The most prominent of its projects included sinking foundations for the Woolworth Building in New York City, at the time the tallest building in the world, 750'-0" above street level, and weighing 136,000 tons. The company also constructed foundations for the Miramachi and Pitt River bridges in Canada and the Penn Bridge in Coshocton, Ohio. [127] With rock Iying 50'-0" to 60'-0" under the Allegheny River's vigorous current, an experienced firm was needed for the Three Sisters Bridges. [128]


The Three Sisters, 1924-28

The Sixth, Seventh, and Ninth Street bridges form a striking trio of structures crossing the Allegheny River in downtown Pittsburgh. These bridges are unique in the United States for the clustering of three similar bridges in such close proximity, their place as the first American self-anchored suspension bridges, and as significant examples of eye-bar suspension bridges.

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[125] Steinman, Practical Treatise, 2nd ed., 225, 230-31.

[126] Darnell, Directory, 85-86.

[127] The Foundation Company, "Difficult Underground Construction," promotional brochure, n.d., in Trade Catalogs File, Technology and Science Room, Carnegie Library, Pittsburgh, Pa.

[128] Engineering News-Record, "Three New Bridges," 995.




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HAER Text: Haven Hawley, August 1998; Pennsylvania Historic Bridges Recording Project - II
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