Commission reports on causes for Laval overpass collapse
October 22, 2007
By Canadian Consulting Engineer
The Johnson Commission looking into the cause of the collapse of the de la Concorde overpass in Laval has identifie...
The Johnson Commission looking into the cause of the collapse of the de la Concorde overpass in Laval has identified the physical causes of the tragedy and deplored the Quebec Transport Ministry’s management of its infrastructure. It notes that even though the Ministry (MTQ) staff were aware that the Concorde overpass was an unusual structural design that posed particular problems because it could not be inspected properly, the staff failed to pay it the necessary attention. The summary of the report says: “Based on the inspection reports submitted and the testimonies heard by the commission … it is clear that for over 30 years, the [Ministry] staff was aware of the peculiarities of the de la Concorde overpass, a structure built according to an unusual concept that posed serious inspection problems … Yet for the entire period of time during which the MTQ was responsible for the overpass, the structure was never subjected to an inspection and maintenance program that took into account its particular characteristics, notably, the critical beam seats at the end of the cantilevers.” Repairs during 1992 were done, but the specified protective membrane was not installed and the condition of the structure was not re-evaluated.
As for assigning blame, the commission said that “no single entity or individual can be assigned the responsibility of the collapse.” However, it did find that some of the companies involved in the construction project over 40 years ago, including the engineers, Desjardins Sauriol & Associes, and contractor Inter State Paving Inc., played an “inadequate role,” in particular over their site and supervision duties. However, the commission also noted that the bridge design “did not contravene any critical provisions of CSA Standard S6-1966,” the standard of the time.
The commission said it could identify “with a high degree of certainty” the chain of physical causes of the bridge collapse. The overpass consisted of prestressed concrete box girders that crossed over Autoroute 19 below with a single span. The box girders were side by side, resulting in a uniform surface underneath the central span, which rested on a beam seat that was continuous over the entire width of the bridge. The box girders at the central part of the deck rested on beam seats located at the end of the cantilevers, directly under the expansion joints. The expansion joints could not be inspected without lifting the deck, yet because they were highly exposed, they were critical zones.
The report says “To sum up, the de la Concorde overpass was a unique, vulnerable structure. In fact, because they are impossible to inspect, continuous beam seats along spans have not been built for the last 30 years and would not be allowed under current codes.”
The experts consulted by the commission agreed that the overpass collapsed as a result of shear failure of the south-east cantilever. It was the deterioration of concrete, not the rebar, that caused the collapse, and this was due to the development and growth of a crack in a zone of weakness located under the upper rebars starting from the beam seat area.
The experts reached consensus that: (1) there was improper rebar detailing during design; (2) there was improper rebar installation at the time of construction; (3) low quality concrete was used for the abutments.
They also found several contributing physical causes. There was a lack of shear reinforcement in the thick slab; the surface of the thick slab was not watertight; additional damage was caused by work done in 1992.
The 220-page report is available in French and English at www.cevc.gouv.qc.ca