The de la Concorde Overpass over Autoroute 19 in Laval, Quebec, collapsed on September 30, 2006, killing five and injuring six. The Government of Quebec convened a commission to investigate.
The bridge was an unusual side-by-side prestressed box girder configuration designed and built between 1968 and 1971. The ends of the girders rested on cast-in-place concrete cantilevers extending out from the abutments. The bridge was an unusual structure, difficult to inspect, and no more bridges of this type were built in Canada after 1972.
The cantilever supports also doubled as expansion joints and had very complex load transfer mechanisms. The joints were difficult to seal and maintain and allowed water and de-icing chemicals to collect at the cantilever supports. Substantial repair work undertaken in 1992 might have caused some damage that later led to the bridge collapse.
The cantilever supports relied on a complex reinforcement detail to transfer bearing forces into the top bars through stirrups. However, the tops stirrups were placed slightly below the top reinforcement, leaving a horizontal plane of weakness.
The investigating commission cited as primary physical causes the improper detailing of reinforcement (the top bars were not anchored), improper installation of reinforcement, and low-quality concrete. The concrete specification was confusing and appeared to allow the use of weaker and less durable concrete. At the time of the collapse, the concrete strength was a bit higher than the specified 27.6 Mpa (4,000 psi), but a higher strength would have been expected after 36 years. The air content and de-icer scaling resistance of the concrete were also poor. The commission also cited the contributing physical causes of lack of shear reinforcement in the thick cantilever slabs, lack of waterproofing of the cantilevered concrete, and possible damage caused during the 1992 repair work.
Just prior to the collapse, puddles of water and chunks of falling concrete were observed. Some drivers also noticed bumps at the expansion joints when crossing the overpass. The concrete cantilever peeled off just below the top layer of reinforcement, and the prestressed box girders fell onto Autoroute 19 and two passing cars. Three cars and a motorcycle fell with the overpass.
The final report of the investigating commission was published (Commission 2007) and is available online in English at http://www.cevc.gouv.qc.ca/UserFiles/File/Rapport/report_eng.pdf
