...sides. GTM erected the ring sections on mobile falsework and structurally tied them together.
|COMPLEX Most serious flaws were a result of inadequate three-dimensional modeling of complex structural scheme. (Photo courtesy of Eiffel/Laubeu)|
To stiffen the shallow vault, curved steel girders embrace its two sides. The tensile girders are held away from the vault by regularly spaced, 10-cm-dia steel struts on 20 cm-dia-plates embedded in roughly 10-cm-deep shell recesses.
At the 20-m-long collapse area, on one side of the link building, lower sections of three alternate shells were omitted for footbridge access to the concourse. Steel connections transferred loads from the shortened shells to the full ones on either side.
Two failure modes have emerged, said investigator Denis Aubry, a civil engineering professor at lEcole Central de Paris. At the footbridge opening, on the north side, several external struts punctured the shell. Retrospective calculations showed some struts would have been overstressed. Immediately opposite, on the south side, the shell edge beam fractured, falling off its bearing to the ground. Either mechanism could have been the prime trigger, said Aubry.
Concrete creep and fatigue caused by cyclical loading accelerated the failure, Aubry said. The north and south sides of the concourse experience marked temperature variations in changing sunlight.
No one realized exactly how complex the building was, so all of the procedures, even administrative ones, were not suited, said Calgaro. "GTM was only in charge of the shellnot the piers, not the bearings and, in fact, this was one major problem," he adds.
"Nobody made a full...three-dimensional model, including the shell, the piers, the bearings and the foundations. That is the reason [to] say the process was not good," said Calgaro. Click here to view chart
Paul Andreu, who retired in 2003 as AdPs chief architect, led the design. In the normal French way, detail design was done by the contractors. That work was checked both by AdP and Bureau Veritas, a Paris-based international certification agency.
The investigative team confined its work to analyzing the failure. A separate probe by the justice department, due to report within two or three months, will apportion individual blame, if any.
Established last May, Berthiers investigative team also includes Anne Froment-Meurice, a building contracts specialist from the Court of Auditors.
To maintain independence, the investigators drew necessary technical support only from the governments central civil engineering laboratory and bridge design organization SETRA. "The most brilliant civil engineers in France are involved as experts for one company or another," explained Calgaro.
At the governments request, the investigators also traced the projects procurement process to test the adequacy of official procedures. "In all the processes there were weaknesses," said Calgaro.
As a building, the concourse went through less rigorous structural analysis than would similarly complicated pieces of civil engineering, said Calgaro.
"In France, we have a distinction between civil engineering works...and buildings," he said. "But there is no distinction between complex...and common buildings." While complicated bridges would undergo peer review, no such procedure was applied here, he added.
Though procured as a building, the concourse was in fact "very, very deformable [and] complex," said Calgaro. "The model to study this structure should have been more complex perhaps than the models used." "The responsibility of Paul Andreu is perhaps to think that this structure was not so difficult to control, in particular the control of deformations, or flex-ibility," Calgaro explained. He also hits at increasingly tight budgets. Tough price bidding is "false economy," he said. Though it is necessary to give architects freedom, it has to be recognized that if structural behavior is complex, there must be enough money and "very, very clever" designers.