Angers Bridge Collapse
In 1850, in the midst of a thunderstorm, part of the Batallion of the 11th light infantry regiment totalling 500 men was crossing the suspension bridge in Angers, which had been in service for twelve years.
The cables suddenly broke and sent the roadway of the bridge hurtling into the river along with all who were on it, bringing a total of 220 victims. Emotions ran high and an enquiry was immediately demanded. Republican newspapers, rather than asking why the cable broke, wanted to know why the soldiers had used this bridge rather than the big stone bridge in the town centre. For their part, both government and legal authorities were mainly concerned with defending the army and saw the only possible cause for the catastrophe as being the faulty construction of the bridge.
An army troupe crosses the bridge though not in step as ordered by the lieutnant colonel in charge. However, strong winds cause the bridge to oscillate which is exacerbated by the soldiers trying to balance on the tilting bridge. The bridge collapses due to overloading and oxidated cables, killing 226 soldiers.
The Angers bridge was not the first suspension bridge to collapse. Previous failures included Dryburgh Abbey Bridge in 1818, The Royal Suspension Chain Pier in Brighton in 1836, and damage caused by winds to the Menai Suspension Bridge in 1825, 1836 and 1839. Subsequent spectacular suspension bridge collapses caused by wind include the failure of the Tacoma Narrows Bridge in 1940. However, the Angers bridge failure caused by far the greatest number of casualties.
As a result of the bridge's collapse, marchers were advised to break step when crossing bridges, a reiteration of much earlier advice given to Roman soldiers, and to the UK Military after the collapse of the Broughton Suspension Bridge in 1831. Ironically, crowds of people that don't intend to march often start walking in step to match the swaying of the bridge and maintain their balance. The problem is caused by transfer of momentum from the pedestrians to the bridge structure, causing resonance. The same problem occurred in 2000 at the recently completed Millennium Bridge (London), when over 600 people walked across the bridge, causing lateral vibrations. The problem was corrected by installing large dampers to absorb the vibrations and change the natural frequency of the structure.
In addition to dynamic effects, the cable anchorages at Angers were found to be highly vulnerable, with the wire strands having separated from their cement surrounds. This allowed water to penetrate, leading to corrosion of the wires.