Robert Stephenson. Source: Engineering Timelines.
"Numerous new bridges were needed when the railway line from London to Holyhead in Wales was built in the 1840s. The project’s chief engineer, Robert Stephenson, chose a cast iron girder design to cross the river Dee just outside Chester, and the bridge was finished in November 1846. About six months later, on 24 May 1847, a local train was crossing the final span when one of the girders failed suddenly, sending most of the train crashing into the river below. Five lives were lost. The accident created a national furore, and Stephenson came close to being accused of manslaughter for the design (Lewis and Gagg)."
According to matdl.org,
"Because cast iron is weaker in tension than in compression, the tension flanges of girders were larger than compression flanges by a ratio of 16:3, following the ratio of material strengths. The Dee Bridge had a relatively long simple span for the day of roughly 29 m (95 ft). The girder cross section is shown. The bridge was also designed with a relatively low factor of safety of 1.5.
The bridge was completed in September 1846. In May 1847, 125 mm (5 in) of ballast was added on top of the wooden decking, in part to reduce the risk of fire from the sparks of the locomotives. When the first train crossed the bridge after the installation of the ballast, the bridge sank beneath it. The locomotive reached safety, but five cars fell into the river. Five people were killed and 18 were injured.
The Dee Bridge Girders. Source: matdl.org.
'The most likely cause of failure was a torsional buckling instability to which the bridge girders were predisposed by the compressive loads introduced by the eccentric diagonal tie rods on the girder' (Petroski, 1994, p. 90). In effect, the top flange of the girder became a column under the compressive force and buckled out of plane, causing the collapse."
The following is a London newspaper report from the week of the Dee Bridge disaster:
Frightful Accident on the Shrewsbury and Chester Railway
Transcript from the Illustrated London News 29 May 1847
A very alarming and fatal accident took place on Monday on this railway, by which several persons were killed and many received serious injuries.
The train which leaves the Chester Station at half-past six o'clock had just arrived at the new iron bridge which crosses the river Dee, at the extermity of the race-course, when the furthest portion of the three iron arches or spans composing the bridge gave way with a tremendous crash, carrying the whole of the train (with the exception of the engine and tender, which reached the other side in safety) into the river below. Nine persons were taken out in a dead or dying state, and several others mutilated and injured in various ways. The stoker of the engine was thrown from his place upon the tender, and killed upon the spot.
The iron bridge consisted of three spans, each span 100 feet in width. Each span is composed of massive iron girders, supported by stone of the most firm and durable construction. There are four of these girders in each span, one on each side of the up and down lines of rails. Strong wooden beams were fixed across the girders, and along these the lines were laid. The girders themselves were formed of two pieces of iron, firmly riveted in the centre, and seemed well adapted to sustain an immense weight.
This complicated device was used by Robert Stephenson to make a bridge beam that utilised the tensile strength of expensive wrought iron with cheap cast iron which was only reliably strong in compression. Source: Corus in Construction.
The train consisted of one first-class carriage, two second class carriages, and a luggage-van; but it is stated that there were not more than two dozen passengers. The train was proceeding as usual along the line, had already crossed two of the arches, and was in the act of crossing the third, when, without one moment's warning, all the carriages were precipitated into the river, a depth of about 30 feet; the engine and tender, which had crossed the bridge, pursuing their course along the line. The sudden shock and concussion rendered almost all the persons in the carriages totally insensible of their situation. One man, indeed, named Proud, recovered himself almost immediately; he found himself in a carriage turned upside down in the river, and, being fully sensible of the horrors of his situation, he exerted himself to the utmost, and succeeded in getting through the carriage window, whence he precipitated himself into the river, and swam ashore. The crash was heard at a great distance, and assistance was promptly on the spot, Mr. Jones, the house surgeon of the Infirmary, being very active in rendering every aid to the unfortunate sufferers. In a brief space of time four dead bodies were taken out of the river, and twelve or thirteen of the passengers, who were more or less wounded, were extricated from their perilous situation, and conveyed to the Infirmary.
Section of failed bridge from the official report. Source: Disaster on the Dee: Robert Stephenson's Nemesis of 1847 (Paperback).
The account of Clayton, the engine-driver, is as follows:- When passing over the third span from Chester, he felt the rails sinking beneath him, and he instantly put on the steam, and then felt the carriages severed, while the engine and tender cleared the bridge, and reached the abutments on the Wrexham or south bank of the river in safety; but the jerk or wrench arising from this severance threw the tender off the rails, inclining it sideways towards the stone parapet. The tender was finally thrown somewhat on its side, and about three feet off the rails, on the east side; this shock severing it from the engine, the iron bar or hook connecting them being snapped in two. The stoker, whose name is Anderson, was by this shock thrown off the tender upon the rails, and the screw-jack from the tender falling on him, killed him on the spot. The engine continued its course along the line, and, fortunately, Clayton, the engine-driver, escaped without hurt.
As soon as the agitation consequent upon such a dreadful occurrence had subsided, attention was directed to the fallen arch; but, strange to say, only one of the girders, that on the outside, had given way, while the other remained perfectly firm and entire. Of course, the weight of the carriages bore down the rails and the horizontal beams, which, with the girder, now broken into several pieces, feel into the river. It also tore with it a portion of the stone-work in which it was fixed on the Welsh side of the river. Very fortunately, however, nothing seemed to have fallen upon the carriages, and though they were crushed one against the other, they did not appear so completely smashed as would have been the case had the arch been built of stone.
Illustrated London News etching of Dee bridge disaster, 1847. Source: Wikimedia Commons.
Among the persons killed are:
John Matthews, a coachman on one of the Welsh mail coaches between Chester and some part of Wales. He was a passenger to Ruabon or Wrexham, in the second-class carriage.
Knyvett, also a coach driver, and a passenger to Wrexham.
George Roberts, guard of the train, who met an instantaneous death, having been precipitated from the top of the carrige on to the bank of the river, amid the falling ruins.
The stoker, a young man.
The guilty locomotive from the Dee accident. Source: Disaster on the Dee: Robert Stephenson's Nemesis of 1847 (Paperback).
Thirteen persons are known to have been injured:
Mr. and Miss Town, of Wrexham (brother and sister). The injuries which Mr. Town has received are of a very serious character. He has sustained a severe concussion of the brain.
Mrs. Evison, a middle-aged lady, from the neighbourhood of Ruabon and Wrexham. Her injuries are very severe, consisting of a fracture of the hip-bone, the nature or extent of which has not been accurately ascertained.
Mr. Isaac Jones, of Wrexham, said to be a tailor and draper, has had his skull severely fractured, and lies now in an exceedingly critical state.
Mr. John Jones, from the neighbourhood of Wrexham, a severe contusion about the head, which is not, however, reckoned imminently dangerous.
Mrs. Elizabeth Jones (wife of the above) has had her thigh fractured.
Ann Evans, servant to Captain Hoskins, who resides near Ruabon. This young woman has suffered to a greater degree than, perhaps, any other of the unfortunate individuals who were injured. Her thigh is fractured, and she has likewise sustained a serious fracture of the collar-bone, and a number of internal injuries.
Mr. David Evans, of Wrexham, or the immediate neighbourhood, had his thigh fractured.
Mrs. Evans, his wife, received a number of bruises, none of which are of a serious character.
A boy named Stevens, the son of one of the station keepers on the line, and himself employed on the line, was dreadfully injured.
Mr. John Bruce Ford, of Manchester, received a cut on the head, and other injuries, none of which are considered at all serious.
A married female, name unknown, severe concussion of the brain.
A boy or man, connected with some of the offices, named McGregor, had his skull fractured, but was, nevertheless, quite sensible, and conversed with the house surgeons.
Happily, several persons escaped without the least injury.
The engineer was so horrified at the occurrence, that he went on his engine in an almost insensible state for a distance of about two miles.
One of the coachmen who was killed had £150 in his pocket.
The Inquest on three of the bodies lying at the workhouse was commenced on Tuesday. The only witnesses examined were those who could identify the bodies.
Clayton, the engine-driver, who had so narrow an escape himself, identified the body of the stoker; and the bodies of the two coach-drivers having been identified by their friends, the inquest was then adjourned to the chester Infirmary, where the body of Roberts, the guard, is lying, having been conveyed thither, though it seems the house-surgeon, who saw the men carrying it on the Roodee, examined the head, and found it so frightfully fractured, that life was then extinct.
Clayton having identified the body as that of Roberts, the Coroner intimated to the Jury that he should not examine any further witnesses that day, and declared the inquest adjourned to Friday (yesterday) morning.
We have annexed a sketch of the scene of the catastrophe [see above], taken from the great Bridge over the Dee, by Mr. A. W. Hunter of Liverpool, who was a passenger in the next train after that to which the accident occurred.
Peter R. Lewis and Colin Gagg have written a very good, comprehensive study of the Dee Bridge catastrophe entitled, Aesthetics versus function: the fall of the Dee bridge, 1847.