London's Millennium Bridge recently reopened, minus the sway. PaddyLeahy describes how to make a bridge stay put.
London's £18 million sterling Millennium Bridge was officially opened for the second time about two weeks ago, a very quiet affair given the embarrassment following its first ribbon-cutting ceremony.
It opened to much fanfare on June 10th, 2000, but closed only two days later. Heavy pedestrian traffic made the striking span across the Thames pitch and sway, so much so that it was shut down on safety grounds.
Part of the problem relates to modern bridge design which aspires to low weight and high sophistication. It can lead to unexpected problems. Attempts to build light and relatively flexible structures, particularly long-span suspension bridges, often open the way to unwanted sway and vibration.
One bridge that paid the ultimate price for this was the Tacoma Narrows Bridge, south of Seattle in the US. Built in 1940, the suspension bridge had a span of more than half a mile. Shortly after opening it displayed an alarming tendency to sway in moderate winds, and was quickly nicknamed "Galloping Gertie".
Not long after, a wind of only 42 miles per hour caused a rhythmic swaying in the bridge that matched its fundamental frequency. The wind speed was just right to amplify the motion and the bridge quickly shook itself to pieces.
Its last hours were captured on film and the record of its movements were of immense benefit to engineers later studying the cause of the failure. This greatly improved our understanding of the dynamic response of long-span suspension bridges to strong winds. A new bridge was built across the Tacoma narrows, and it still stands, carrying 90,000 vehicles a day.
Cork's well-known "Shaky" Bridge - officially known as Daly's bridge - is a suspension bridge across the north channel of the River Lee, connecting Sunday's Well with the Mardyke and Western Road.
A pedestrian bridge with a 49 metre span, it was built in 1926 and has given enjoyment to children who jump on it, making it shake. The small deflections were never a threat to the bridge and after three-quarters of a century it is still one of Cork's best-loved bridges.
London's Millennium Bridge was less fortunate. With its low-slung supporting cables the bridge was hailed a triumph of modern design and is certainly very elegant.
It soon became famous for all the wrong reasons. It wobbled, with a considerable lateral swaying movement, much to the embarrassment of Ove Arup, the structural engineering firm responsible for its design. The company's engineers quickly identified the problem, a phenomenon they termed "lock in".
Foot traffic caused a small movement in the bridge, and once started, pedestrians unconsciously tended to walk in step with the swaying motion. The collective "marching" caused a rhythmic force that amplified the motion, making the bridge sway even more.
This would not have happened in normal circumstances, but with the large number of people on the bridge on the day of the opening, the effect was dramatic.
Following extensive studies, Ove Arup's engineers came up with a proposal to eliminate the problem by installing hydraulic dampers. The remedial work, costing an estimated £5 million sterling, was completed and on January 30th, in a large-scale test, 2,000 volunteers marched across the bridge. Engineers monitored 24 movement sensors which proved that the sway had been eliminated. The bridge came back into service on February 22nd.