Determined Causes
The commissioners agreed that the overpass collapsed due to shear failure in the southeast abutment. This was due to a horizontal plane fracture that had slowly grown over the years. The fracture allowed the part of the abutment below it to break away from the part above, causing the collapse.
The three main causes of the fracture and subsequent collapse were:
- During design, the steel reinforcement was concentrated in one layer, causing a weak plane. This did not contravene the code provisions of the time.
- During construction, the reinforcement was not put into the proper locations, as defined in the design, exacerbating the design weakness. The contractors and inspecting engineers were blamed by the commission for this cause.
- A low quality concrete was used in the abutments, causing poor freeze-thaw behavior. This was blamed on poor communication at all levels, including that of the government-defined requirements at the time
Three other contributing causes were identified by the committee, but not agreed to by all the experts.
- All thick reinforced concrete slabs should have shear reinforcing, and this is a deficiency in the existing bridge design code.
- Proper waterproofing was never installed, even during the bridge repairs done in 1992.
- Extensive concrete removal and rebar exposure caused during the 1992 repairs caused weakening of the structure.
The final report names four individuals who were responsible for unprofessional work on the overpass:
- Marcel Dubois, engineer with Desjardins Sauriol et Associés (DSA), for improper supervision during construction
- Claude Robert, president of Acier d'Armature de Montreal (AAM), for poor quality control caused by passing off responsibilities
- Tiona Sanogo, engineer, for the damage caused during the 1992 repairs
- Christian Mercier, engineer, for an incomplete inspection of the overpass in 2004
Read more about this topic: De La Concorde Overpass Collapse
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