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Collapse
Since the soldiers were marching together, they caused the bridge to vibrate and twist from side to side, dislodging an anchoring cable from its concrete mooring. Though a thunderstorm occurred during the collapse, engineering reports at the time indicated that the collapse was due to the soldiers instead of the storm. 226 soldiers died in the river below the bridge. The failure was attributed to a combination of dynamic load and corrosion of the anchors for the main cables.
Dynamic Failure of Bridges
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 cause by winds to the Menai Suspension Bridge in 1825, 1836 and 1839.[1] 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. 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.
Torsional oscillations caused the catastrophic failure of Galloping Gertie (the orignal Tacoma Narrows bridge), in 1940, when winds blowing at about 40 mph were enough to induce resonance. The torsional waves induced by aerodynamic flutter of the deck, grew in amplitude until the deck failed, and it fell into the waters below.
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.[1]
Results
The disaster led France to abandon suspension bridges until 1870. There had been similar failures elsewhere, such as that at Yarmouth in England in May 1845, when 79 people were killed by the fall of the main deck. The fall of the Angers bridge raised serious questions over the integrity of suspension bridges, and some engineers (like John A. Roebling) used reinforced decks in future structures such as the Brooklyn bridge. Louis Vicat reported in 1853 on the problems with the anchorages, and all other similar bridges in France had to be carefully inspected.[1]
A new bridge was built on the same site in 1960, using reinforced concrete beams.
References
Further reading
- R.E.D. Bishop, Vibration, Cambridge University Press (1965) for a discussion of the engineering problems of unwanted vibrations.
- J.P. Den Hartog, Mechanical Vibrations, Dover Publications (1985)
External links
- Pont de la Basse-Chaîne in the Structurae database
- "The Angers Bridge catastrophe" English translation of article originally in La Revue du Musée des arts et métiers
- Angers bridge collapse at Bridgemeister
- suspension bridge design
- Point of Collapse, Village Voice, August 27 - September 2, 2003, by Robert Julavits for problem on the Brooklyn bridge
- lecture on steel and iron in structures
