In what amounts to a harsh indictment of the French process for executing complex public projects, investigators have identified two likely reasons for the partial roof collapse at the 650-meter-long concourse at Paris Charles de Gaulle airport last May 23. Systemic flaws in designing the technically difficult building contributed to deficiencies that led to the early morning failure that caused four deaths.

The government-appointed team identified two key failure mechanisms but cannot say which was the primary trigger. The design process was insufficiently rigorous for so complex a structure, investigators said at a press conference on Feb. 15. No detailed, independent analysis was done to check design models.

The technical investigation, led by former national highways director, Jean Berthier, found "weaknesses at all stages" of design, said team member Jean-Armand Calgaro, a senior government civil engineer and academic. "No one person is really responsible. It is like a chain, and this chain was weak."

But the most serious structural flaws are confined to two special areas of the innovative shell roof at the link to the terminal. For the rest of the concourse, "the safety margin is perhaps a bit limited, but a good rehabilitation can give the right safety margin," says Calgaro.

The $900-million complex, which includes Terminal 2E, its concourse and a link building, was built between April 2000 and January 2003. Aéroports de Paris (AdP) opened it 11 months prior to the collapse.

The 34-m-wide concourse is enclosed by a 30-cm-thick perforated, reinforced concrete vault, rising from second-floor edge beams. Beams rest on neoprene bearings atop two rows of columns, built by Hervé S.A. Horizontal steel ties prevent beams from moving outward.

GTM Construction S.A. built the vault in 10 sections separated by glazed gaps for passenger jetways. Each section is made of 4-m-wide abutting rings lightly connected to each other.

Vault rings were precast near-by in three pieces, for the near-horizontal roof and two...