By Gregory MonePosted 08.29.2007 at 5:58 pm0 Comments
The nearly 6,000-foot-long Tacoma Narrows Bridge, known as Galloping Gertie, opened up on July 1, 1940, and collapsed just four months later. Winds reached 42 miles per hour on that fateful day, which proved too intense for the structure. There were a number of causes, but the basic problem was that engineers hadn't yet learned to account for wind loads in their designs. During the planning phase, the engineers reduced the proposed depth of the concrete and steel girders beneath the roadway from 25 to 8 feet. This loosened the stiffness of the road, and made it much more susceptible to wind. In fact, before the collapse, local residents had noticed that less intense gusts could cause the bridge to move. But those movements involved longitudinal waves – one end of the bridge rose, the other fell, in a less dramatic fashion than what we see in one of the early scenes in this clip.
Prior to the collapse, though, the wind induced torsional movement. In other words, the road started to twist. While the center line stayed stable, one side of the roadbed rose and the other dropped. When this twisting motion peaked, the sidewalk on one side was 28 feet higher than the opposite one.
Eventually, this twisting motion proved too much for the structure. The cables started to snap, and chunks of the bridge fell into the water below. Finally, the entire center collapsed. With this mass gone, the sections on either end sagged dramatically, dropping more than 40 feet. Nowadays wind-tunnel testing is fairly standard for bridge designs. When engineers drew up the plans for Gertie’s replacement, which has been standing for more than 50 years, you can bet they spent a lot more time factoring in the breeze.—Gregory Mone
The recent bridge collapse in Minnesota happened in large part, people think, because the bridge was of a non-redundant design. Which is to say that if certain elements of the bridge failed, there was no backup, and the whole bridge would fall. Which it did.
We are told by the authorities that modern bridges are not built this way anymore, that they always have built-in redundancy. But redundancy is expensive: You kind of have to build everything twice. What if there were a better way?
Stephen Wolfram, with whom I co-founded the company that makes the Mathematica software, decided to take a look at the problem using the technique of experimental structure generation. You can read about it in his blog post.
The idea is to use simple programs to generate vast numbers of possible bridge designs—say, different possible layouts of struts—and then run a simulation on each one to see how well it performs. Some will be obviously stupid, some will be the same as current designs, but if you're lucky, maybe some of them will be better. They might find a way to spread the effects of a failure in one place out over the whole structure, for example.
This type of exploratory search for designs is the ultimate form of thinking outside the box. The structure-generating program intentionally doesn't know anything about good bridge design: It's going to come up with things that make no sense whatsoever. But experience in other fields shows that sometimes the completely ridiculous idea is the one that turns out to work. —Theodore Gray
