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Structural engineers have identified several factors that could have led to the collapse of the terminal building at Charles de Gaulle airport in Paris.

Four people were killed last Sunday when a 30 m section of the curved concrete roof at Terminal 2E collapsed, burying people under tonnes of twisted steel and glass. Further cracks appeared in the terminal, designed by architect Paul Andreu, on Monday.

Experts have been reluctant to speculate on the causes, but in an interview with ºÚ¶´ÉçÇø this week, Ian Stephenson of structural engineer Sinclair Knight Merz said the design "lacked robustness". He said a number of factors could have led to the failure of the roof.

Stephenson said the problem could have originated between the curved precast concrete sections that form the tubular roof and the supporting steelwork truss (see "How the roof was constructed" opposite). He commented: "There could have been a failure in the connection between the steelwork and the precast concrete units."

A faulty connection could have led to the precast concrete wall sections collapsing, Stephenson said.

He said: "If the steelwork connections had failed, then the roof's precast concrete sections would have gone into tension – a mode for which they were never designed."

The idea of "tension" can best be characterised by comparing these concrete walls to a biscuit. If someone attempts to bend the biscuit, the danger is that it will shatter into crumbs.

The curved concrete roof sections are supported by concrete beams on either side. These beams run along the length of the building and are in turn supported on a row of columns. During the construction of the roof, cracks were reported in the tops of some of the supporting columns because there was insufficient reinforcement. This was rectified by wrapping additional reinforcement around the column tops.

Stephenson said that another possible reason for the collapse was that the problem with the reinforcement of the columns had not been tackled.

He said: "The failure of the roof's base may not have been properly addressed. This could have led to the failure of the bearing between the tops of the columns and the longitudinal beam."

Stephenson said that an alternative reason could be that the walkway linking the terminal to the aircraft could have been hit by a vehicle.

He said that, because the terminal was such a complex piece of engineering, a number of factors could have led to the accident.

Another expert reiterated that it was unclear why the roof had collapsed. "It could have been caused by design or construction failures," he said.