All suspension bridges are prone to vibration and swaying caused by moving traffic and wind. These vibration are not a problem as long as they are not in resonance with the bridge’s natural frequency. However, if these vibrations fall in harmony, like the forced steps of marching soldiers in unison, the swaying of the bridge can intensify. If the frequency of their steps closely matches the bridge's natural frequency, the soldiers' rhythmic marching will amplify the vibrations on the bridge causing structural failure.
Broughton Suspension Bridge.
One of the first bridges to become victim of mechanical resonance is the Broughton Suspension Bridge across the River Irwell between Broughton and Pendleton in Greater Manchester, England. The bridge was built in 1826 by John FitzGerald to link his estates and coal mines at Pendleton with the Adelphi and Lower Broughton districts, and was the only means of communication between the townships of Broughton and Pendleton. One of Europe’s first suspension bridges, the Broughton Suspension Bridge was a source of great local pride as suspension bridges were then considered the “new wonder of the age”.
On 12 April 1831, a regiment of soldiers were crossing over the bridge, marching four abreast, when it began to vibrate with their footsteps. The soldiers found the vibration amusing and some of them started to whistle a marching tune to “humour it by the manner in which they stepped”, causing the bridge to vibrate even more. The head of the column had almost reached the Pendleton side when they heard “a sound resembling an irregular discharge of firearms”. Immediately, one of the iron columns supporting the suspension chains on the Broughton side of the river fell towards the bridge, carrying with it a large stone from the pier to which it had been bolted. The corner of the bridge, no longer supported, then fell 16 or 18 feet into the river, throwing about forty of the soldiers into the water or against the chains. The river was low and the water only about two feet, so there were no fatalities but some of them did suffer broken bones.
As a result of the collapse of the bridge, the British Army issued an order to "break step" when soldiers were crossing a bridge.
A sign on Albert Bridge, London, asks soldiers to break step when marching over. This bridge is also prone to vibration when large numbers of people walk over it. Photo: Iridescent/Wikimedia
Another example of marching soldiers taking a bridge down took place less than twenty years later in Angers, France. On 16 April 1850, a battalion of French soldiers was marching across the 335-foot Angers Bridge over the Maine River. A powerful thunderstorm was raging and the wind was making the bridge oscillate. As the soldiers attempted to cross, their bodies acted like sails, catching even more wind. Survivors reported feeling disoriented, as they struggled to maintain their balance and prevent themselves from falling. Although the soldiers had been instructed to break step and space themselves farther apart than usual, their efforts to match the swaying motion of the bridge may have inadvertently caused them to march with the same cadence, resulting in resonance. There were close to 500 soldiers on the bridge when the upstream anchoring cable on the right bank broke in its concrete mooring with a noise like “a badly done volley from a firing squad”. A second later, the adjacent downstream cable broke and the deck collapsed throwing soldiers into the river. A total of 226 people died.
Angers Bridge prior to collapse in 1850.
The collapse of Angers Bridge>
The failure of the bridge was attributed to rusting anchorage cable, but the storm and the marching soldiers did have a not so insignificant hand in the collapse. The disaster led France to lose hope in suspension bridges, and their construction was suspended for twenty years.
Another reminder of the vulnerability of suspension bridges was seen in 2000, during the opening of the London Thames Millennium Bridge. As crowds packed the bridge, their footfalls made the bridge vibrate slightly. Many pedestrians fell spontaneously in step with the vibrations, increasing them. Although engineers insisted the Millennium Bridge was never in danger of collapse, the bridge was closed for about a year while construction crews installed energy-dissipating dampers to minimize the vibration caused by pedestrians.
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