Wednesday, October 09, 2002

I love bridges

One of the nice things about Brunel's state of the art in 1840 suspension bridge in Bristol is that there are a couple of quite spectacular more modern
bridges quite close nearby. You only have to drive a few kilometers out of town to come to the Severn Estuary, and the two crossings from England into Wales. The recently constructed Second Severn Crossing is rather boring: two long causeways at each end and a single cable stayed span of around 456m in the centre.

Perfectly nice engineering, but cookie cutter stuff. There are lots of other spans just like it. (And the whole system including the approaches is very similar to the older Sunshine Skyway bridge in Florida. The whole trend towards cable stayed bridges, hugely influential as it is, is more an achievement of materials science than civil engineering. (I am enormously interested in materials science, but I am trying to write about bridges).

The older (1966) First Severn Crossing, is much more interesting, however. It is a particularly beautiful bridge, without question, but it is also historically quite memorable. There is a nice picture of it here.

Throughout the 18th and early 19th centuries, suspension bridge design advanced, and as it did so, engineers became more and more confident of their abilities to build bridges with steadily more flexible and thinner decks. If you look at bridges built over the years, the decks suddenly get thinner.

Eventually, a suspension bridge was built over the Tacoma Narrows, just ouside Seattle in Washington State. It had a very thin deck. Many structures have so called resonant frequencies . If a structure is vibrated at one of these frequencies, the structure starts vibrating in such a way that the oscillation rapidly grows. Once the oscillation starts, the wind continues to amplify the oscillation. It so happened that one of the resonant frequencies of the deck of the Tacoma Narrows Bridge was similar to the frequencies of certain gusts of wind that occurred in the area. Wind blowed onto the bridge, and the bridge shook itself to pieces. (There is some quite famous film footage of this event. If you have ever studied physics or engineering, this has almost certainly been shown to you at some point). Although the bridge could easily withstand the forces exerted on it by traffic running over it, it could not withstand the wind.

The response of engineers to this disaster was to go back to building bridges with reinforced trussed decks. The first few big bridges built after this event have very thick decks, and as years go on, you can see them get thinner as engineers get more confident again.

However, the Severn Crossing was the first bridge built after the Tacoma Narrows disaster with a very thin deck. The engineers who designed it had satisfied themselves that they fully understood what wind could do to the bridge, and they designed an aerodynamic deck that would prevent the wind from causing this to oscillate. The bridge is still there, so one assumes that they did fully understand the problem. Most large bridges built since have similar aerodynamic decks. The big exceptions are bridges built in Japan, which still have large trusses. The Japanese claim that they are being doubly safe, although some people simply suspect that the Japanese construction industry are just looking for ways to build things that cost as much money as possible.

Overbuilding construction projects in Japan. Who would ever think such a thing.

The other sort of oscillation that can cause a bridge to oscillate at its resonant frequencies is human steps. Vehicular traffic is generally no problem, but steps are regular. If a lot of people are crossing a bridge with similar periods between steps, this can cause the bridge to shake alarmingly. (This is why soldiers normally break step when marching across bridges).

As has been happening in most cities on a river, the waterfront in London has in recent years been redeveloped, and various new facilities have in particular been built on the (formerly industrial) south bank of the river Thames. Two of the nicer new developments are the Tate Modern art gallery and Shakespeare's Globe Theatre, which are just opposite the river from St Paul's Cathedral in the City of London. (The Tate Modern gallery is a converted power station, and a remarkable work or architecture, although the art itself is not very interesting. The Tate Britain gallery further upstream and on the other side of the river at Millbank has a much greater collection. If nothing else, however, the creation of Tate Modern did free up more space at Millbank for the Tate's magnificent collection of British art. And I have written about the Globe elsewhere).

The authorities decided that a new pedestrian bridge connecting these facilities to the City was desirable, and a thin, elegant footbridge, known as the Millenium Bridge, was constructed. (Remarkably, this was the first new bridge crossing of the Thames in London for something like 100 years. Existing bridges had been replaced by new bridges, and a number of tunnels had been built, however). The bridge was opened in 2000, and a huge number of people walked over the bridge on its first day. However, the bridge shook alarmingly, and engineers were so worried that the bridge was immediately closed again. It seems that the engineers had not taken resonance into account properly. (They should have, however, as similar problems had occurred with the Auckland Harbour Road Bridge in New Zealand in 1975. It seems though that they did not properly read the literature). The engineers spent two years designing bracing structures that were then attached to the bridge to prevent this problem in the future. The bridge was re-opened, and a lot of people have walked over it since.

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