Broughton Suspension Bridge Collapses

April 12, 1831:

Broughton Suspension Bridge was a suspended-deck suspension bridge built in 1826 to span the River Irwell between Broughton and Pendleton, now in Salford, Greater Manchester, England. It was one of the first suspension bridges constructed in Europe. On 12 April 1831 the bridge collapsed, reportedly owing to a mechanical resonance induced by troops marching over the bridge in step. A bolt in one of the stay-chains snapped, causing the bridge to collapse at one end, throwing about forty of the men into the river. As a result of the incident the British Military issued an order that troops should "break step" when crossing a bridge.
The bridge's construction has been attributed to Samuel Brown, but this has been questioned some sources have suggested that it may have been built by Thomas Cheek Hewes, a Manchester millwright and textile machinery manufacturer.
The bridge was rebuilt, but was replaced in 1924 by a Pratt truss pedestrian bridge, which is still in use.
On 12 April 1831 the 60th Rifle Corps carried out an exercise on Kersal Moor under the command of Lieutenant P. S. Fitzgerald, the son of John Fitzgerald. As a detachment of 74 men returned to barracks in Salford by way of the bridge the soldiers, who were marching four abreast, felt it begin to vibrate in time with their footsteps. Finding the vibration a pleasant sensation some of them started to whistle a marching tune, and they began 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 fireams". 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 sixteen or eighteen feet into the river, throwing about forty of the soldiers into the water or against the chains. As the water was only about two feet deep at that point none of the men were killed, but twenty were injured, including six who suffered severe injuries including: broken arms and legs, severe bruising, and contusions to the head.
An investigation found that a bolt in one of the stay-chains had snapped at the point where it was attached to the masonry of the ground anchor. There was criticism of the construction method used, as the attachment to the ground-anchor relied on one bolt rather than two, and the bolt itself was found to have been badly forged. A number of other bolts were also bent but had not broken. It emerged that three years previously the distinguished Manchester engineer, Eaton Hodgkinson, had expressed some doubt about the strength of the stay-chains, as compared with that of the suspension chains. He had said that they should be rigorously tested, but they were not. It also came to light that some time before the accident one of the cross bolts had started to bend and crack, although it was believed to have been replaced by the time of the accident. The conclusion was that, although the vibration caused by the marching had precipitated the bolt's failure, it would have happened sooner or later anyway.

Well, resonance and poor design and construction caused the collapse.  It is an interesting story, though.