The Greatest Bridge Never Built?
It would have bestridden three big rivers like a Y-shaped Brooklyn Bridge, decades before that span was built
The greatest American bridge builder of the nineteenth century was John Roebling, and he is celebrated above all for designing the magnificent Brooklyn Bridge. His bridge-building career began in Pittsburgh, the city of rivers, where he constructed his first two suspension bridges and made some of his most critical innovations, including the technique of spinning wire suspension cables and using inclined stays for stiffening. It was for Pittsburgh that he designed one of his most audacious constructions—the Tripartite Bridge. This Y-shaped bridge would have spanned the Monongahela and Allegheny rivers where they flow together to form the Ohio, with a third branch extending from a central pier to the Pittsburgh Point. It would have been one of the chief landmarks of American bridge history. But it was never built.
Today more than forty bridges cross the three rivers that define Pittsburgh, and hundreds of others provide transit over the city’s numerous ravines and valleys. The city, founded in the 1750s, grew into an important commercial and industrial center during the first half of the nineteenth century. It was peninsular, limited to the wedge of land between the Monongahela and Allegheny rivers, and agitation for bridges across its boundary rivers began early. By the beginning of the 1840s, when John Roebling began his career, four wooden covered bridges and one aqueduct spanned the Monongahela and Allegheny rivers in Pittsburgh.
Roebling, born in Germany in 1806, had graduated from the Prussian Royal Polytechnic School, in Berlin, and then emigrated to America, inspired by the philosopher Hegel, who saw America as the land of the future. In 1831 he founded a Utopian farming community, Saxonburg, twenty-five miles outside Pittsburgh. By the end of the decade, weary of hard and unproductive farming, he became a surveyor for the new Pennsylvania Main Line canal system, which used a system of inclined planes known as the Portage Railroad to pull barges over the Allegheny Mountains. Aroused by the repeated accidents caused when the thick ropes used to haul the barges broke, he suggested the use of wire rope and devised techniques for producing such rope.
Above all, Roebling aspired to be a bridge builder, and the wire rope gave him a means of building bridges of unprecedented length that could accommodate heavy loads. His first opportunity came in the winter of 1844, when ice badly damaged the aqueduct over the Allegheny. He daringly proposed to rebuild the ruined truss as a suspension structure, using wire cables. In the summer of 1844 he received the contract, probably as much for his low bid of sixty-two thousand dollars as for his design. By May 1845 the aqueduct was completed, the only structure of its kind in the world. Next Roebling prepared to use his suspension principles on a bridge.
His opportunity was provided by a catastrophic blaze in April 1845 that destroyed about a third of the city and burned all of the Monongahela covered bridge except its seven piers and two abutments. He proposed to rebuild the bridge by constructing a wire suspension structure atop the old piers and abutments, for the bargain price of fifty-five thousand dollars. Again he won a contract.
Work began in May 1845. Roebling hung the cables from two-and-a-half-foot-long pendulums suspended from towers. The pendulums served to balance the cable tensions. Stiffening was provided by handrails and by a system of inclined stays made of iron rods. The fifteen-hundred-foot bridge opened in February 1846.
Pittsburgh’s newspapers greeted the new bridge and its builder enthusiastically. “It is in every way a most wonderful work,” editorialized the Morning Post . The American Railroad Journal called it a “triumph of art,” and its successful completion led to proposals for more Roebling bridges in Pittsburgh and in other river cities, such as Cincinnati. The most dramatic proposal was the one for the Tripartite Bridge. A group of Pittsburgh businessmen joined together to advocate a threepart bridge spanning the rivers at the Point, to be built by John Roebling at an estimated cost of $250,000. In February 1846 a bill was introduced in the state legislature to authorize the incorporation of a Pittsburgh Tripartite Bridge Company.
The idea offered a solution to several transportation and communication problems that vexed Pittsburgh. It would provide access to the flatlands north and west of the city, for residential and manufacturing growth, and to the rich coal deposits across the Monongahela. In addition, it would give cross-river communities direct communication with the city, freeing them from dependence on ferries that relied on good weather.
Several sketches of the bridge, maps showing its location, and copies of the legislation incorporating the bridge company survive at the Roebling archives at Rensselaer Polytechnic Institute. They show a bridge with suspended spans crossing the mouths of the Allegheny and Monongahela rivers and meeting at a central pier in the Ohio River, from which a third branch extends to the Point, between the two rivers. The bridge’s dimensions appear in Roebling’s hand, specifying that the Point and Allegheny branches would be 900 feet long each, and the Monongahela branch 750 feet. The bridge was to rise 65 feet above low water at the center pier and 80 feet at the center of the Monongahela span.
An elevation (side) view drawn by Roebling (shown on pages 28 and 29) depicts the intersection of the three branches as the bridge’s focal point. Above the three intersecting roadways rise three hexagonal masonry towers, framed by three arches and capped by short pyramids, which support the cables. The six-hundred-foot main suspension span of the Point branch extends between the center pier and a matching masonry pier, also with hexagonal towers. A three-hundred-foot side span extends from there to the anchorage.
The Allegheny branch of the bridge, not shown in the drawing, is presumably similar to the Point branch. The most spectacular element in the design is the Monongahela branch. Its 750-foot main span starts from the center pier as a suspension structure stabilized with diagonal stays, like the other two branches, but in the far half of the span, the two support systems separate. From the top of the far tower radiate diagonal stays supporting the roadway, but the main cable itself rises right past the tower in a magnificent curve with a half-span of 850 feet and a rise of 160 feet, to end at a massive masonry support structure near the top of the cliff of Coal Hill (now Mount Washington). Two slender towers, about 170 feet apart, support and brace the cable along the way.
The proposal for the bridge stimulated sharp controversy in the city. Three of Pittsburgh’s daily papers supported it; a fourth strongly opposed it. The main objection was that the bridge would block steamboats with tall chimneys, especially when the water was high. The bridge’s most vigorous opponent, the Pittsburgh Commercial Journal , made the tortuous argument that the structure not only would interfere with river traffic but also would set a precedent for bridge building that would be followed in other cities, thus threatening Pittsburgh’s commercial supremacy. The paper recommended building a suspension bridge over the Allegheny alone, with steam and horse ferries to serve the Ohio side of the Point. Letters to the editor agreed that blocking the river would “prove highly injurious to the permanent prosperity of this city.” On February 23, 1846, a group of Pittsburghers addressed a petition to the state legislature opposing authorization of the bridge. “It is a matter of great and vital importance,” they said, that the waters “be preserved open and without obstruction.”
Several very long letters signed “Civis” appeared in two of Pittsburgh’s newspapers, defending the bridge plan and offering detailed refutations of the “perversions” of the bridge’s opponents. “Civis” was never identified in print, but the writing style and the arguments used, especially against wasteful tall smokestacks, are very similar to those presented by Roebling himself. Civis wrote that “old style” bridges, built without concern “for the navigation, or the commercial interests of the city,” were indeed objectionable, but new suspension bridges, planned in full cooperation with “builders and owners of steamboats and commercial men of intelligence,” would prove “pregnant with advantage.”
In both Pittsburgh and the neighboring city of Allegheny (now a part of Pittsburgh), the pro-bridge forces sought to pass resolutions favoring its construction; opponents attempted to amend the resolutions to require clearance heights as great as a hundred feet. Ultimately, however, a majority in the councils of both cities approved resolutions favorable to the bridge.
In early April 1846 a bill passed creating the Tripartite Bridge Company and was signed by the governor. It provided for the sale of five hundred shares of stock at fifty dollars each; the selling was to begin on July 13. A flurry of forecasts greeted the legislation: Supporters claimed the bridge would be the most popular in Pittsburgh, capturing a large part “of the travel by land to and fro”; opponents argued that the stock would never sell. On the appointed day, the stock went on sale at several locations around the region. Not a share was sold. The project was successfully blocked.
There were a number of reasons for this. The fear that the bridge might block river traffic clearly weighed heavily; in (act, it became a convenient disguise for the concerns of ferry operators, the owners of the other bridges, and property owners who worried that city businesses and land values might decline as outlying areas became more accessible. Another factor was the bridge’s demand for substantial capital at a time when cash was also needed for several railroad projects and for rebuilding some of the city destroyed by the 1845 fire.
Further complicating the picture was agitation for a free bridge over the Allegheny, which started up in early April, when the Old Allegheny Bridge Company raised its tolls. The Tripartite, like all the existing bridges, was to pay for itself with tolls; the citizens of the city of Allegheny formed an association to push for a free bridge and to purchase all the existing bridges. No free bridges were built over the Allegheny until this century, but the controversy may have been the fatal blow for the Tripartite, putting in doubt as it did the hope of any dividends for bridge stockholders.
Would the bridge have worked had it been built? The plan presents several real puzzles. It appears, for instance, that the intersection between the three branches would have produced massive traffic jams even in horse-and-buggy days. A major use for the bridge would have been to transport coal into Pittsburgh, using lorries drawn by teams of six or more horses. These teams would have had to make sixty-degree turns inside the tower, an obvious difficulty. Roebling apparently failed to take such traffic problems into consideration.
The cables presented other problems. Roebling did not specify how they were to be spun, particularly the unprecedented 1,225-foot main cable on the Monongahela branch. His experience before 1846 had consisted of the Pittsburgh Aqueduct, where a continuous cable was spun in place, and the Monongahela Bridge, where the cables were spun on the shore and lifted in one piece. Presumably the cables for the Tripartite would have been spun in place, but unless all three pairs of cables were spun in exact synchronism, their unbalanced tensions would have applied huge overturning forces on the center pier.
The complicated design presented problems of support as well. The intersection of the cables at the top of the center pier, for instance, would have created enormous horizontal forces, and the stone arches connecting the three towers could hardly have resisted the tensions imposed by the cables radiating from their tops. In addition, the load of the Monongahela branch needed to be distributed between the two independent support systems— the stays and the cable—without overstressing either system. Such a balancing act would be a challenge even to a modern bridge designer equipped with the most comprehensive analysis software; it would have been formidable with the relatively primitive analytical tools of the 1840s.
Additional stability problems could have been caused by the very shallow suspension spans— which would have been quite flexible, producing substantial deflections and large unbalanced forces on the towers under load—and by the long southern cables, which were braced at only two points. Roebling probably would have corrected these weaknesses somehow during construction; he had earlier pioneered the use of diagonal stays for stiffening while building the Monongahela Bridge.
The archives contain tantalizing hints of subsequent Roebling designs for a conventional Point bridge. One depicts a conventional suspension span across the Monongahela; another boldly presents a one-tower version with a thousand-foot main span supported by cables from the tower to a cliffside anchorage.
Despite this activity, the Tripartite Bridge was not yet dead. In 1871 a group of Pittsburgh businessmen once again formed a company to build a Tripartite Bridge. The engineer for the company was Charles Davis, a wellknown Pittsburgh civil engineer, but the design of the bridge appears to have been the work of Washington A. Roebling, John Roebling’s son. John Roebling had died in 1869, during the building of the Brooklyn Bridge, and Washington had assumed direction of the family firm and of that job. In June 1871 the president of the new Tripartite bridge company wrote to Washington Roebling, “We propose building a bridge at the Point, known as the Tripartite bridge, to cross both rivers at their mouths, and I was anxious to have seen you about possible costs & c[onstruction].”
The proposal that followed represented a far more mature phase of suspension-bridge design than the earlier version. Its overall dimensions are almost identical, but the design is entirely different. The Point arm was to consist of a series of truss spans; the arms across the Monongahela and the Allegheny were to be suspension structures, but they would have used the central pier as a common anchorage (as is the case with the San Francisco-Oakland Bay Bridge). Such a design would have solved all the puzzles raised by the 1846 design: the central intersection could have been much more spacious; the anchorages could have acted independently or have been interconnected to resist horizontal forces; and the center pier could have been made to resist unbalanced forces even if the sets of cables were spun at different times.
In spite of the superiority of the design, the bridge again failed to attract capital. It never even became much of a cause. The scanty newspaper record shows that bids were scheduled for February 1872 and were expected to run as high as a million dollars, but formal bids appear never to have been taken. No mention of the bridge can be found after this date, so the Tripartite Bridge passed finally into history, as a bridge never built.
What would have happened if it had been built? Clearly the 1872 bridge, freed of the impractical elements in the 1846 plan, would have been a monumental structure, rivaling the Brooklyn Bridge in scale and grandeur.
Either version would have provided a spectacular entrance to the Pittsburgh Point—one that might have stimulated its redevelopment long before the Pittsburgh Renaissance of the postWorld War II period. In the Tripartite’s absence, bridges were constructed crossing the Allegheny and Monongahela rivers at the Point in 1876 and 1877, and bridges remained there until 1966 and 1970. They were removed then to provide a clear view of a newly constructed park and fountain. Today there is open space where the Tripartite Bridge might have been.