Most people have seen the video of Washington's Tacoma Narrows Bridge in a physics class. It shows the bridge swaying wildly in the wind with its deck rolling like an ocean wave while a brave soul scampers around on it. Soon the deck breaks apart and then collapses. In this famous case, engineers determined that the aerodynamics of the bridge caused it to reach a resonant frequency when hit with wind from a certain angle.
If that leaves you wanting more of an explanation, Herb Rothman would be the man to ask. As a structural and transportation engineer at Weidlinger Associates, an engineering firm in Manhattan, he has cultivated a specialty in suspension bridge engineering. He knows people who designed famous bridges, and he's worked with several notable structures both in designing new ones and rehabilitating old ones. With its size and location among so many waterways, New York City has proven a hot bed for his craft. "In the last 10 or 15 years, most of the work we've been doing is rehabilitating New York City bridges," he says.
Raised in Queens, Rothman grew up under the influence of his father and uncle, both architects. "I think I was bred to be an architect, but I decided I didn't have the talent," he reflects. A natural second choice was civil engineering because his father and uncle were also both engineers.
After getting a B.S. in civil engineering from Rensselaer Polytechnic Institute, Rothman joined the engineering firm of Ammann & Whitney in 1945. An interest in transportation engineering emerged, with an emphasis on structural design and bridges. He had risen to become the firm's chief bridge engineer when he left in 1977 to join Weidlinger. Having designed a host of bridges, tunnels, urban roadways, and transit projects, he brought transportation engineering to Weidlinger.
Weidlinger Associates specializes in structural and civil engineering in designing structures such as airports, museums, highrises, government buildings, and sports facilities as well as the infrastructure for rapid transit systems. Unique for an engineering firm, the company also has an applied science division staffed with scientists and Ph.D.s that conduct research such as wind tunnel testing on bridges and the effects of wind, earthquakes, blast, and vibration. With Rothman leading the way, Weidlinger has become renowned for preserving and reconstructing landmark suspension bridges. Now 77, he has retired from managing the firm as chairman but remains a principal. "Now I'm just concentrating on the bridge work," he quips.
Why does Rothman likes working on bridges so much? "When you're all done with it, you've got something big to see for what you did. It's not just a bunch of equations or lines. It's pretty obvious you did something worthwhile." Also, having grown up with architects, he learned that with any other kind of structural work, such as buildings, the architect is usually the boss. Not the case with bridges.
In notching famous bridges on his resume, Rothman got an early start by partnering with O.H. Ammann in designing New York's George Washington and Verrazano-Narrows bridges. This leads to talk of his favorite bridges. "The one you get the most discussion about is the Verrazano-Narrows because it was the longest bridge in the world for quite awhile," he says. Another favorite, the Passaic River Bridge on the New Jersey Turnpike was the longest plate girder bridge in the U.S. when built. He also served as principal designer for the Walt Whitman Bridge over the Delaware River in Philadelphia.
Recently, a new one has emerged as his pet: the San Francisco Oakland Bay Bridge. The mammoth crossover actually consists of two tandem suspension bridges, a tunnel that comes out on an island, then a long cantilevered bridge, and finally a long viaduct to Oakland. Rothman was chief engineer for a project to replace the east span, the cantilevered section. "The long cantilever wouldn't stand up under a big earthquake, and CalTrans decided it would make more sense to start over than reinforce it," he explains. For resisting seismic activity, he came up with a self-anchored suspension bridge, meaning the ends of the cables are connected to the bridge deck, rather than anchored in massive concrete on land. It ranks as the world's longest self-anchored bridge.
But most of Rothman's recent work has consisted of rehabilitation, work that once saw him climbing cables on bridges to inspect them -- "the most frightening part is looking down and seeing a truck thundering by," he muses. His project load consists of East River bridges and the Tappan Zee in New York and the Ben Franklin and Walt Whitman bridges in Philadelphia. "Most of our work is in the mid-Atlantic area with a little bit in California."
A notable long-term project has Rothman working on New York's Bronx-Whitestone Bridge, designed about the same time as the Tacoma Narrows Bridge. The engineers of the two checked each other's designs, and they were similar, except the Bronx-Whitestone is wider. After the Tacoma Narrows fiasco, officials knew the right wind, albeit rare, could bring down the Bronx-Whitestone in similar fashion.
Undertaken in 1985, revamping the Bronx-Whitestone Bridge involved installing a 94-ton tuned mass damper, an energy-absorbing system that improves aerodynamic stability. This required wind tunnel tests and field instrumentation. According to Rothman, "The wind puts energy into the bridge that drives it up and down and makes it oscillate. The tuned mass damper is like a shock absorber that throws the energy away and keeps the bridge under control. How successful it is I can't tell you. We're still waiting for the 100-year storm." Reconstruction on the bridge also includes redecking to lighten it because of cable deterioration, repairs to cables, and stiffening of girders and other members.
According to Rothman, they plan to remove the tuned mass damper and put non-structural streamlining on the bridge that makes it more wind resistant. They're determining the optimum shape through wind tunnel testing. "If we shape the bridge correctly, the instability that threatened to ruin the Whitestone Bridge and ruined the Tacoma Narrows Bridge, has been eliminated."
Rothman says he often applies lessons learned from rehabilitating bridges to designing new ones. "You find out what didn't work. Before all this rehabilitation started, the maintenance people who had the problems of repairing the things over a long period had virtually no contact with the people who designed them. So whatever mistakes were made, we kept right on making. That's not true anymore." He tries to design bridges so areas are accessible. "We know that if it's hard to get to, it won't be gotten to. Things like this become very important to us. We also know what didn't last very long."
But as much as Rothman likes rehabing old bridges, he says he'd like to see a change. "We've been doing so much rehabilitating, I'd like to get back to doing new bridges. New bridges are easier. Rehabilitating is difficult because first of all, you have to maintain traffic on most of them, so you've got to rebuild it while keeping traffic on it." And he adds, "With the old bridges, your main function is to not change the appearance. With new ones, you have a lot of control over it. It's a lot more fun." Since he plans to keep going "until they throw me out," he'll probably get his wish.