Pittsburgh's bridges and the Roebling connection
Donald L. Gibbon, Ph.D.
Senior Materials Analyst
A suspension bridge is surely one of engineering's loveliest and most visible public works icons. And John Roebling is almost certainly the patron saint of American bridge builders. But as a young man, this German-trained immigrant engineer temporarily gave up his profession to escape the stultifying conformity of the European office. In about 1830 he came to America to try farming. Ending up near Pittsburgh, Roebling's creative mind and passionate ambition couldn't long be kept down on the farm, and he was soon back into his beloved engineering. Back then there was little distinction between civil and mechanical engineering, and Roebling was never one to let artificial barriers keep him down.
His first big engineering breakthrough in America was a contract to replace a large hemp haul rope on the Allegheny Portage Railway with a homemade wire rope. The Railway was an integral part of the Pennsylvania Mainline Canal, from Philadelphia to Pittsburgh, 400 miles right across the Allegheny Mountains. To make the rope, he first had to design it, create the equipment with which to lay it up, and then physically put it together. There was great controversy about rope design and he tried numerous solutions over his lifetime. But this first one turned out to be a great success, leading to many other contracts for the product and eventually to the long-running family business in Trenton, New Jersey.
His wire-rope experience gave him the confidence to propose his second feat: a wire-rope suspension aqueduct to carry the Mainline canal up in the air across the Allegheny River into "downtown" Pittsburgh. Such a thing had never before been tried. The city fathers were reluctant partners in this venture, but Roebling was a persuasive man, and they gave in at last. The aqueduct was some 1,100 feet long, about four feet deep and eight feet wide, which meant that it would hold about 2,000 tons of water. The community breathed a collective sigh of relief in 1845 when the water flowed in and the structure stood firm. Roebling's career path was set steeply up from that moment on. To build this structure successfully, Roebling had to develop at least preliminary solutions to all the typical suspension bridge problems: first, the wire-ropes themselves, then the suspension systems and tower supports, and finally the anchorage for the cables.
His next project followed this one almost immediately: a major wire-rope suspension bridge across the Monongahela River to replace one which had burned. Pittsburgh was and still is a compact place, and this bridge was less than a mile across town from the aqueduct, on the Monongahela River. Finished in a remarkably short nine months, including the winter season, Roebling intuitively solved one of suspension bridges' perennial problems in the design for this very early one: stiffness of the deck. Roebling went on to design and build the Niagara Falls and Cincinnati suspension bridges. He died tragically from lockjaw before the Brooklyn Bridge was even fairly started, but his capable son, Washington, carried out his father's ambitious plans.
This photo shows all three of the chain-suspension bridges known as the Three Sisters across the Allegheny River.
Pittsburgh is often known as the "City of Bridges" because of its unique topography and the many bridge-building solutions to the problem of keeping all its parts well connected across its three major rivers. During its century or more as the world's steel capital, the rivers were used primarily for transportation and waste disposal and not much else. As steel production has declined, water quality has enormously improved. In fact, the Pennsylvania State Bass Fishing competition was held in Pittsburgh in October 2001.
With these changes, the community has refocused its attention on the aesthetics of the rivers, and public works have sprouted all up and down the river banks to bring people back in touch with the water. Two new sports stadiums are on the North Side, with fabulous views of the skyline, within easy walking distance of downtown. People-oriented parks now line the shores, and anchoring one of these is a large new convention center. There is a fascinating historical connection between the convention center and Roebling's pioneering engineering work.
Rafael Vinoly, the Argentine-born, New York-based architect who designed the new center, has created a tensioned-cable roof for this huge building, spanning 10th Street and virtually on the river bank. Its roof has a clear span over 100 yards long and is a dramatic addition to the waterfront. But the most interesting thing of all, from an engineering point of view, is that this dramatic roof is based on updated solutions to the very same engineering problems Roebling solved over 150 years ago just a few hundred yards from this very spot.
Pittsburgh's 9th Street Bridge, with the David L. Lawrence Convention Center in the background
In the present version of the suspension system, the roof is built with a series of 16 steel and concrete "masts" from which the cable system hangs. The boat-building allusion continues: The side of the building nearest the river is known as the bow and the opposite end the stern. The main cable bundles are created onsite by stringing a seven-strand twisted steel cable from bow to stern over the head of the mast and back again, over and over until 169 strands are in place. These are then clamped in fittings in the shape of a hexagon and their ends anchored in a huge cast weldment, in turn anchored to bow or stern. A four-inch damping cable is strung across the wide open space which is the building's exhibition floor, and tensioning cables are strung from each upper cable bundle down to the damping cable. Then the delicate part: The entire system is tensioned by sets of jacks to pull it into alignment, actually rotating the bow of the system some 15 inches. What is created is a truss-like web, with some members in compression and some in tension, now a firm support on which to place the steel and glass roof and end-walls. The sweep of the roof line looks for all the world like half a suspension bridge, a fitting connection with its historical roots.
The whole plan is a magnificent venture, entirely made possible by the skilled iron workers and operating engineers who deal with the inevitable situations that no one could envision on the drawing boards, those times when the parts just don't seem to fit together the way they should!
These are the members of Iron Workers Local #3 and Operating Engineers Local #66, the men and women who put up Heinz Stadium for the Steelers and PNC Park for the Pirates. This has been a time of big public works for Pittsburgh, and it's soon to be followed by another, the extension of Pittsburgh's subway system under the Allegheny into the North Side.
Donald L. Gibbon was originally trained as a geologist and learned most of what he knows of bridge building in the Corps of Engineers. He is a Senior Materials Analyst for Matco, Inc., and as such does failure analyses on all sorts of engineering materials, from concrete to steel to electronic circuit boards. He is also a professional photographer and gives walking tours of Pittsburgh (see www.pittsburghwalking.com). He can be reached at email@example.com.
A modified version of this article appeared in Western Pennsylvania History, Spring 2002 issue.