By: Allen Zeyher Managing Editor
Balancing the 20th century at midspan was the 1950s, a decade that was especially inventive as far as bridge building.
There must have been something in the water. Or maybe it had something to do with the Soviet menace of the Cold War.
Not only was the 1950s the start of serious commercial use of prestressed concrete in U.S. bridges—the Walnut Lane Bridge over Lincoln Drive in Philadelphia, a girder bridge, was the first in 1950—it also was the dawn of cable-stay bridges in the U.S.
“Before the ’50s, they did not build cable-stay bridges, because they did not have very good high-strength cables,” Dr. Man-Chung Tang, chairman of T.Y. Lin International, San Francisco, told Roads & Bridges. “In 1952, the Germans started to use the lock-coil strand in cables, which is very high strength. They made the cable-stay bridge possible.”
The first cable-stayed bridge was built in the mid-50s, according to Tang, along with the first segmental bridge.
A lock-coil strand has a trapezoidal wire in its outer layer. When the strand is pulled, the wires lock together to seal out water.
“Today we have much better cable,” continued Tang. “We don’t use the lock-coil anymore, but that’s a further development afterwards. We now use seven-wire strands. It is a bit more sophisticated, so it offers better protection than the lock coil. It’s also less expensive.”
Going way back
Just a couple of years after the Wright brothers made their first airplane flight, engineers and construction workers were building the Victoria Falls Bridge to span the gorge below Victoria Falls. The bridge was commissioned by Cecil Rhodes—the same Rhodes who was born in Britain, became a mining magnate by exploiting the natural resources of South Africa, colonized what became the state of Rhodesia (which later became Zambia and Zimbabwe) and provided money to create the scholarship to the University of Oxford.
The Victoria Falls Bridge is now a favorite spot for bungee jumping, according to the American Society of Civil Engineers (ASCE).
A year later, in 1906, Roads & Bridges—under a different name—began publishing. As part of our series of articles looking at the history that has taken place over the life of Roads & Bridges magazine, this month we focus on innovations in bridge design.
Bridge designers of 1906 were shifting from using stone to construct their arch bridges, which were very popular at the time, to using steel-reinforced concrete. The last great stone arch bridge built in the U.S. was the Rockville Bridge, according to the ASCE, connecting Harrisburg, Pa., with the west shore of the Susquehanna River. The bridge has 48 spans, each 70 ft long, and carries three rail lines for Norfolk Southern and the Amtrak Keystone Corridor. Pennsylvanians celebrated the bridge’s 100th anniversary in 2002. The Rockville Bridge has survived over 100 years, including Hurricane Agnes of 1972 when the Susquehanna reached 15 ft above flood stage, and is still in use.
The problem limiting the construction of arch bridges today is economics.
Arch bridges “require the minimum material, but require the most labor,” said Tang. “So when you have cheaper labor, you can build arch bridges more efficiently. In the last 50 years, with the labor cost in the U.S., obviously, the arch is not the most popular thing. In addition, the equipment has been developed to such a point that now we can build arches with less labor, too. So now, therefore, the arch is coming back.”
Prestressing of concrete tops the list of innovations in bridge engineering for Raymond McCabe, the senior vice president and national director of bridges and tunnels for HNTB Corp.
“It greatly helped with the durability of concrete bridges,” McCabe told Roads & Bridges. “It increased the spans that we could achieve with the same weight. It allowed for segmental construction to come into the fold and provided for use of precast construction.”
With prestressing, in the form of pretensioning or post-tensioning, concrete bridge members could be stronger for the same size, or they could be smaller and give the same strength with less weight and less cost. It allowed bridge designers to create longer spans with the same depth of structure.
A major milestone in the development of prestressed segmental bridges was the work of German engineer Ulrich Finsterwalder, who in 1952 pioneered building segmental concrete bridges one cast-in-place segment at a time, according to Tang. By casting a segment in a form, stressing it, then moving the form down the line to build the next segment, he could build a bridge as long as he wanted. With a second form, he could build from both ends.
Beholder’s eye
Finsterwalder’s Bendorf Bridge over the Rhine River at Koblenz, Germany, is an example of how prestressed concrete allowed for shallower girders. “The resulting girder has the appearance of a very shallow arch, elegant in profile,” according to Encyclopædia Britannica.
The elegance of slim girders and piers is an aesthetic principle well acknowledged by bridge engineers.
Aesthetics is an important—and somewhat elusive—factor in bridge design.
“You can have a bridge and show it to 100 people, or two bridges, and 50 will like one and 50 will like the other,” said HNTB’s McCabe. “I still believe that if the form is correct and people can visualize how forces flow and can understand the function of it that those continue to make the most attractive bridges.”
“I think in the future communities will demand more bridge aesthetics,” Linda Figg, president and CEO of Figg Engineering Group, told Roads & Bridges. “We see that happening over the past 10 years pretty significantly, but I think even in the future this is going to be a dominant part of bridge design.
“The most important aspect of aesthetics in bridge design is the efficiency of the design,” Figg continued. “By being efficient in the shapes and the sizes and the approach to the alignment, there is an inherent aesthetic quality that is derived. But we must also think about the culture and the site of the community and how that community is evolving in terms of its relationship to the development in the area and its relationship to preserving nature. As those things continue to evolve, people want their infrastructure to tie closely to the spirit of their community.”
Cable-stay bridges, especially, gain an aesthetic appearance because of the cable-stay cradle, which allows more slender pylons.
“This means a reduced construction cost,” said Figg, “and it also creates a more streamlined aesthetic appearance.”
Up and away
In the future, Figg said, one way bridge design will develop is directly above highways.
“With the increasing expense and decreasing availability of right-of-way, bridge designers will have to build more long ribbons of bridges down the middle of an existing highway, with a single line of bridge piers in the median or on the shoulders—sort of an elevated highway more than a bridge per se.”
As examples, she cites the Lee Roy Selmon Crosstown Expressway in Tampa, the San Antonio Y in downtown San Antonio and the JFK AirTrain down the middle of the Van Wyck Expressway in New York.
The piers of these elevated highways are slim. In some cases, they are only 6 ft wide, a slenderness made possible by high-strength concrete.
“We can make very high-strength concrete today,” said Tang. “The only problem with that is that the higher strength it is, the less we know about it.”
There is less theoretical understanding and less practical experience with cutting-edge materials such as super-strength concrete or high-performance steel or fiber-reinforced polymer.
The inability to mass-produce steel delayed its supplanting of iron. Before the invention of better milling processes, steel was very expensive, and iron could only be made in small pieces, according to Tang. Iron structures were made of many small pieces riveted together—a costly process. Once steel milling was more advanced, it became less expensive.
The term “high strength” might give the impression that it is a distinct class of object significantly different from all earlier concrete. In reality, the dividing line between regular strength and high strength in concrete or steel or between high strength and high performance or high performance and ultrahigh performance is somewhat arbitrary.
Steel has developed along a similar curve as concrete, resulting in similarly higher strength, longer spans, lower cost and the fading out of steel trusses in favor of better-fabricated and more economical steel girders.
Each bridge type—arch, girder, suspension and cable-stay—has a range of applications where it is most economical. China has cheap labor, so arch bridges are competitive. River navigation channels require wide-open space, where cable stays for medium-long spans and suspensions for really long spans rule. For shorter spans, segmental girder bridges are dominant.
“The context of the site is the first element that drives the structure type,” said Figg.
A plethora of variations on those basic structure types allow designers freedom to explore options.
Bridges can use low-sagging cables, called stress-ribbon bridges, said McCabe, or flat cable stays, also called extradosed bridges, or self-anchored suspension bridges, such as the San Francisco-Oakland Bay Bridge.
The limits are set to a large extent by economics rather than by technology: “In my opinion, bridge technology has developed up to a point, you basically can build any bridge you want as long as you have the money,” said Tang. “Some people think there is a limitation on the span length. I don’t think so. We are by far not reaching the allowable span limit yet. We still have a lot of room for development.”
Figg added: “Technology in terms of our computer analysis has helped us to accomplish designs much quicker and to gain the kind of efficiency that results in today’s modern, elegant structures. That is a major change in our industry. It’s the ability to analyze something and to accomplish it with multiple iterations in such a short time that you’re able to be more creative and go beyond the boundaries of design that we had 50 years ago.”
