The Millennium Bridge

Published on: November 1, 2001

Whilst many nations were lost in the gloom-and-doom predictions surrounding the arrival of Y2K, the UK governement focused its attention on a rather ambitious building programme to mark the end of the 1900s. There were many interesting projects sponsored by the Millennium Commission, with works as diverse as the Tate Modern, the Rochdale Canal Restoration, and The Earth Centre receiving funds. However, the bulk of the successful projects were ignored in the press, which chose instead to focus on the "failures" associated with the Commission, most notably, the Millennium Dome and the Millenium Bridge in London.

I'm going to continue that trend and discuss the Millennium Bridge project, not because I consider it a failure, but because it turns out to present a rather interesting architectural problem. You might recall from the news coverage that the futuristic footbridge did not operate according to plan when opened in June 2000 (I'm considering it a 20th century project because 1) it was designed during the 1900s, and 2) the 20th century didn't technically start until January 1, 2001). You might particularly remember the interviews with the panicked people who felt as if they were about to be thrown into the Thames due to the swaying and vibrating motion of the bridge when fully loaded with foot travelers. How could a carefully engineered bridge misbehave so badly?

The culprit in this matter is "synchronous lateral excitation," and its not so hard to understand as it might sound. If you paid careful attention in your physiology classes in college, you'll remember that when a person walks, the foot gives a sideways push with each step. This sideways force works to propel the body in a certain direction. When people walk together in a group, they start out walking rather randomly, with each person stepping at a different rate. After a bit, however, paces start to equalize. You might have even deliberately switched your pace so you were walking step-in-step with your friends. Suddenly, all of you are pushing outward at the same time.

On solid ground, this might not make much difference. But on a bridge, it can lead to big events. It's not just that the bridge will sway a little bit more as a group walks across with the same pace. As each person feels the first bit of sway, he or she compensates and tries to walk with the sway. This causes more outward force than before, however, and the sway grows even more emphatic. The interesting thing about this effect is that it only happens when an optimum amount of outward force is reached. According to the designers of the bridge, Arup, only after a critical number is reached will the sway be felt: 500 people could walk on a bridge and no sway would be felt. Add ten more, and suddenly the problem appears and throws everyone into the water. That's what makes it so disconcerting and difficult to test--at low numbers, no sway occurs. At high numbers, it hard to believe anyone could miss it during the design process.

How did the consulting engineers miss this problem? Afterward, the press trotted out the disastrous results of the Pont Solferino in Paris as a very obvious warning sign. This French bridge was closed down in 1999 due to the immense amount of sway caused by foot traffic. In their defense, Arup engineers suggest that the only conclusion to be reached from the Pont Solferino was that that particular design was flawed, and that the swaying effect wouldn't be endemic to all bridges. If I was going to criticize the engineers for anything, it would be this conclusion. After all, if engineers routinely account for vertical vibration (such as that caused by soldiers marching in lock step over a bridge), wouldn't it make sense to account for lateral motion as well?

The good news from all this is that we architecture students have conclusive evidence of the need to consider synchronous lateral excitation when designing foot bridges. And although it ultimately will cost more money, more time and more resources, the engineers believe they can eventually solve the problem. Speaking from an academic viewpoint rather than an economic one, that's pretty exciting. Arup is installing dampers to stifle the effects of both lateral and vertical excitation. In searching for a solution for the lateral sway, one researcher thought he detected vertical motion as well. Arup says no vertical displacment has yet been detected, but if it is like lateral excitation, all the bridge needs is to reach the critical number of passengers to end up like the infamous "Galloping Gerdie," the bridge that collapsed into the Tacoma Narrows due to wind forces. Thus, they're trying to engineer for it now, just in case.

Designers and engineers might take some time to look at the problems associated with the Millennium Bridge. Arup itself has published extensive documentation on the original design as well as the modifications, and also has a nice news archive for browsing.