120 years on, scientists discover why the Tay Bridge collapsed

A "digital microscope" that lets scientists examine photographs at high resolutions has finally proved what caused the Tay Bridge disaster of 120 years ago in which 75 people died.

A "digital microscope" that lets scientists examine photographs at high resolutions has finally proved what caused the Tay Bridge disaster of 120 years ago in which 75 people died.

Though engineers have suspected for years that the rail disaster was caused by using the wrong kind of iron for the design, they have differed on where it was being used: some said the casings of the 18-month-old bridge were at fault, others that it collapsed through metal fatigue, and others that the "lugs" by which the columns of the bridge were cross-braced together had failed.

Now, techniques developed by Dr Peter Lewis at the Open University and Dr Guy Jones at Cambridge University have been used to investigate digitised photographs taken after the catastrophe, when the bridge once hailed as a "marvel of engineering" fell apart under a passenger train during a storm on 28 December 1879.

Their findings show the cause was the choice of cast iron, which is brittle, for the lugs. The increasing stresses and strains imposed on the lugs by the design of the bridge led them to fail. "It must have been progressive," said Dr Lewis, a senior lecturer in material forensics at the Open University, Milton Keynes. "As each lug failed that would put extra load on the next one, and the whole lot would just unzip."

The Tay Bridge cost £300,000 to build, and used 4,000 tons of cast iron, 10 million bricks and 15,000 casks of cement. During its construction 20 workmen died. But at two and a half miles long, it was reckoned the longest in the world; Queen Victoria crossed the bridge and knighted its designer, Thomas Bouch.

Then, in 70mph gales, the Edinburgh express tried to crawl across. The highest girders collapsed, plunging the engine and all six carriages into the freezing water. An inquiry blamed Bouch for a "badlydesigned, badly constructed and badly maintained" project. He died six months after the findings.

The newest examination does not vindicate him either. Bouch changed the design of the lugs from previous cast-iron bridges, for the worse: the bracing bars between the columns were strong enough, but the holes in the lugs weakened the structure - and proved the weak link.

The discovery came through looking at high-quality digitised versions of 50 photographs taken after the accident for a Board of Trade inquiry. Dr Lewis said: "They were taken using very long exposure times and very fine-grained silver emulsion. As a result, there's a huge amount of information captured in each one. The pictures show hundreds of shattered lugs on the piers of the bridge.

"We have been able to confirm that [the lugs] definitely did fracture," he said. "We have combined that with modern knowledge about the qualities of cast iron to explain what happened." In particular, designers now know that holes in any structure concentrate stresses to the extent that they can make an otherwise robust design fail abruptly - as happened on the Tay Bridge.