How did it come to this?

The French lost more than a building when Terminal 2E partially collapsed at Charles de Gaulle airport on 23 May. Four people lost their lives, many more left harrowed by the experience, and the nation suffered a blow to its collective pride in its reputation for innovative and bold engineering. Everybody had questions, and everybody wanted answers.

Three weeks ago, on 6 July, an initial attempt at an answer duly came, in the form of an interim report published by the French transport ministry. Early in the piece though it may be, its failure to answer many key questions has led to raised eyebrows among structural engineers.

Made from conclusions based on observations from outside the building and detailed photographs, the report contains two pages of text and an explanatory diagram. Dismissing an early theory blaming failure in the foundations, it suggests that the apparent cause of the collapse was the failure of the connection between the inner curved-concrete shell of the terminal and its outer steel-reinforcement structure (see “The rise and fall of Terminal 2E”, page 58). The report contends that steel struts connecting this steel outrigger to the concrete tube punched through weakened concrete on the north side of the terminal. Without the support provided by the steel outriggers the concrete was then structurally on its own and failed in bending, in the same way that a plank of wood snaps in half when overloaded. The report says that the concrete shell “folded like a wallet” in the vicinity of the failed connection, the roof crashed down and the structure fell off its raised concrete foundation on the south side.

However, the report fails to conclude whether this suggested reason for the collapse was indeed its ultimate cause or whether it was just a consequence of another more fundamental mode of failure. This has led to much conjecture among structural engineers.

Henry Bardsley, a leading engineer at Paris-based firm RFR, which worked on other terminals at Charles de Gaulle, broadly supports the theory put forward in the report. His backing is given additional credibility as Bardsley says he had privileged access to detailed photographs of the damage. He agrees that the struts punched through the concrete, a failure known as “punching shear”. “The steel rib failed in its function and put the full load on to the concrete. The concrete was not designed for those loads so it failed in bending,” he says.

However, Bardsley disagrees with the report’s suggestion that weak concrete could have been to blame. “Concrete just doesn’t degrade; it only fails if you overload it.”

Indeed, it is this aspect of the report that many structural engineers find fault with. Charles Goodchild, principal structural engineer at the Concrete Centre, says: “It just doesn’t sound right. Concrete as a structural material gets stronger with age.”

Other leading structural engineers agree that deteriorated concrete is unlikely to have caused the collapse. The report says differential contraction and expansion of concrete and steel could have weakened it where it connects to the steel strut. “It just doesn’t happen like that,” says John Roberts, director of engineer Babtie and chairman of the Institution of Structural Engineers group investigating the collapse of the World Trade Centre. “It’s possible the steel was varying much more in temperature than the concrete and weakened it, but it seems very unlikely after a year.”

So, if deteriorated concrete wasn’t to blame, what was? Bardsley places the blame for the failure of the connection on poor reinforcement design in the concrete where it meets the connecting strut. This is because the precast section that failed had a large cut-out in it to accommodate a link bridge allowing passengers access to the terminal. “The behaviour of the concrete module was substantially modified to accommodate the link bridge, so the failure occurred in an atypical place,” he explains. “There’s been an oversight in the way the reinforcement was detailed for this special case.”

He reckons the special module accommodating the walkway was a modified version of the standard modules used elsewhere in the building. These standard units were thoroughly checked and prototyped before the terminal was built. “I don’t think the special case was checked as thoroughly as the standard elements, which often E

E happens,” he explains. “This structure shows it is not easy to adapt a building for a special case. When you have a standard precast element you should not adapt it but specially design it for its purpose.” Bardsley thinks the rest of the terminal should be retained and repaired because the failure occurred in an atypical, localised place.

Chris Wise, director of structural engineer Expedition Engineering, agrees that weakened concrete seems an unlikely cause, but diverges from Bardsley’s theory. “The chances of a strut failing are very small, so it’s much more likely the arch changed its shape and then failed at one point, followed by another as the deformations increased,” he explains.

Wise says the concrete shell relies mainly on its geometric form for its robustness. And because it is a “sensitive, thin structure”, just a slight change to its shape could have led to the collapse. This is because the forces or thrust lines transferred through structural materials must be safely contained within them. While ancient domes and arches were designed with thick shells to give a generous margin of error when calculating the position of the thrust lines, Terminal 2E’s concrete shell is so thin that the forces have nowhere to go if its shape shifts slightly. “It is a mark of

21st century confidence that these things boil down to a minimal thickness,” Wise says.

Geometric distortion of the terminal’s shape could have been caused by a combination of several factors. Wise says concrete will definitely “creep” or permanently move if it is highly stressed. This is especially significant near large openings where the compressive stresses are high. Furthermore, the outer steel reinforcement is not as rigid

as it could be because the connecting steel struts lack triangular bracing, thus allowing movement.

Finally, out-of-tolerance construction could have contributed to the geometric distortion. “It’s possible it got 50 mm out when it was built,” Wise explains. “That would reduce the factor of safety by about a third, if it is lightly reinforced.” Reports of cracks prior to the disaster lend some credence to Wise’s theory, as they could be evidence that the building had moved slightly before reaching the critical point of failure.

Until more investigative work is done it is impossible to give a definitive reason for the collapse. Bardsley and Wise have suggested equally convincing alternative explanations that could answer the crucial question – was the failure of the connection between the steel and the concrete the cause, or a result, of the building’s failure?

The other important question is whether the rest of the terminal needs to be demolished. Bardsley thinks not. Wise says the whole building would have to be carefully surveyed to see how far out of tolerance it was – if geometric distortion was the cause. Babtie’s Roberts says it would be hard to justify keeping the structure unless a definitive reason for the collapse is found – which might be difficult.

The French are probably hoping the finger of blame can be pointed at localised failure due to the large openings. Otherwise not only would they have to bear the *750m (£498m) cost of a new terminal, but also suffer the final ignominy of seeing a once celebrated building demolished.