It should be no surprise that Ancient and Medieval bridges, even the stone ones, have struggled to stand the test of time. Many bridges have failed or swept away before the advent of the expertise of industrial or civil engineers. But there have been bridges that in the 19th and 20th Century that failed, and some quite badly. John Rennie’s Old London bridge was beginning to sink before it was transported away to America.
It wasn’t the only bridge that he built in London that didn’t last. Rennie designed the first Waterloo Bridge (combined railway and pedestrian) that crossed the River Thames. Made from heavy granite, it had nine arches, each one a span of 120 feet. The bridge was the tragic scene of suicides, the accidental death of an American daredevil in 1841, painted by Monet and John Constable, and used by Michael Faraday in 1832 to make experiments concerning salt water and magnetic fields. Such a great history, yet unfortunately it had a major structural challenge.
Rennie’s Waterloo bridge survived from 1817 to the 1920s when it developed a serious problem with its piers. The removal of the Old London Bridge had changed the flow of the river and the sediment had damaged the foundations. The bridge was closed and heavier elements had to be removed. In the 1930s the bridge was demolished and some of the granite stones presented to various parts of the British Commonwealth. The replacement budge was fitted with jacks so that it can be lifted and levelled if the piers are attacked by subsidence again. Lesson learnt.
The most recent bridge escapade is that of London’s Millennium Bridge. A pedestrian footbridge built across the River Thames just south of St. Paul’s Cathedral and opened in 2000. Two days later it closed for another two years. The bridge quickly earnt the name the “wobbly bridge”. The phenomenon that caused the problem is synchronous lateral excitation. The swaying motion of people walking across the bridge causes sideways oscillations and further caused the people to sway in step (more like stagger), increasing the oscillations, and the overall effect.
On the day of its opening up to 2000 people walked across at the same time. The designers hadn’t anticipated that everyone would simultaneously lurch to one side in order to stay upright. Lightweight bridges are known to suffer from the phenomenon, especially suspension bridges. There is a sign on London’s Albert Bridge dated to 1873 warning soldiers not to march in step across it, instead they are required to break step while crossing.
This is a real threat of danger as was proven by the collapse of the Broughton Suspension Bridge in 1831 due a troop of soldiers marching across causing damaging vibrations and similarly in France in 1850 when troops marching over the Angers Bridge suspension bridge caused it to collapse killing two hundred men.
“Breaking step” isn’t a feasible solution for the publicly accessible wobbly bridge, and even after limiting the number of people crossing, which caused major queues, the bridge was deemed unsafe and closed. A solution was found using dampeners but the whole delay was deeply embarrassing. The “Wibbly Wobbly” name stuck even after the bridge was stiffened and reopened. Now at least an army could march across it.