De la Concorde overpass collapse
On 30th September 2006 the central ‘drop-in’ span of the de la Concorde highway over-bridge in Laval, Canada, suddenly collapsed onto the road below causing 5 fatalities and 6 injuries. This shocked both the general public and the professional communities. A Commission of Inquiry was quickly established and reported in October 2007.
The Commission concluded that the collapse was due to a number of factors, none of which was sufficient in itself to cause failure, but which came together on that day. They reflected both technical shortcomings, and human failings: the latter amounted to negligence in respect of the construction, and serious lapses in the management of the structure throughout its life.
The de la Concorde overpass structure was built in the 1960s as part of Canada’s post-war infrastructure expansion. The structure was innovative for its time in the manner in which the central span was designed- allowing a single span over the roadway. The in-situ side spans had raking columns. A short cantilever, ending in a half-joint, picked- up the central ‘drop-in span’ of precast prestressed concrete boxes placed side by side. The structure failed at the half joint.
The Commission gave early warning to the authorities of shortcomings they had discovered in the design standards for thick slabs without shear reinforcement in the presence of concrete deterioration. As a consequence, the maintenance inspection standards, and associated quality controls for highway bridges, have been reviewed throughout Canada.
This type of half-joint is no longer an accepted detail: it is vulnerable to deterioration and very difficult, if not impossible, to inspect and maintain. The joint relies on accurate reinforcement placement and good quality concrete. Neither was achieved in this case.
The Commission of Inquiry concluded that the fate of the structure was found to be determined by:
- Poor design (although in accordance with contemporary codes).
- Failure on the part of the designers to look beyond the design code.
- Poor quality control and supervision during construction.
- Deterioration over its life as a consequence of poor quality materials, salt contaminated water and freeze-thaw cycles.
- Poor maintenance and repair procedures.
- Inaccessibility of critical elements.
- Failure by the maintaining authorities to act decisively.
Although the UK’s procedures differ from those of Canada, the conclusions drawn by the Commission should be carefully considered, viz:
- Failure of parties to discharge their contractual responsibilities in respect of quality control and supervision during the construction.
- Failure of the maintaining authority to relate the maintenance regime to the needs of the structure and its risk profile; inspections were not allocated sufficient time and lacked proper consideration.
- No one shortcoming was dominant in the cause of the collapse: it resulted from an accumulation of shortcomings over time.
The Commission recommended various measures designed to ensure engaged consultants and contractors are competent, have appropriate staff and identify sub-contracted work. It was also recommended that the performance of engaged organisations be recorded and that appointments take account of these wider issues. However, the inference is that once satisfied, cost determines the successful appointee.
The need to undertake an assessment of highway structure proposals from a holistic perspective stands out from this incident. Good design does not just stem from application of design codes. It requires consideration of aspects outwith many codes: buildability, required supervision levels, ease of maintenance and whole-life costings.
Forward-looking authorities are using these guides to manage infrastructure effectively with regard to relative risks. However, the standard at which it is applied does appear to vary (along with appropriate funding). Some authorities are no longer able to provide dedicated bridge engineers to manage infrastructure, nor do they engage consultants to undertake the management function. Occasional engagement of consultants is sometimes based on lowest cost rather than a mix of cost and quality.
The competency of those undertaking inspections is a subject which is attracting the attention of some of the major clients. There is a strong argument for a recognised training regime and NVQ or similar qualification.
The pressures on maintenance budgets will lead to a temptation to delay any remedial work if the safety of the structure can be justified (sweating the asset); although more sophisticated analysis can sometimes support this approach, it requires careful thought by those with suitable competency, and an appreciation of the long-term consequences.
This topic paper was issued by SCOSS in 2008. You can view the original here.
 Related articles on Designing Buildings Wiki
- Cracking and building movement.
- Defects correction period.
- Defects in construction.
- Elements of structure in buildings.
- FC Twente stadium roof collapse.
- Fit for purpose.
- Latent defects.
- Patent defects.
- Ronan Point.
- Structural engineer.
- Structures at the end of their design life.
- Structure definition.
- Torre Windsor office building fire.
Featured articles and news
High quality and high density homes - is it what we need or is it storing up trouble?
Government announces its intention to strengthen planning rules to protect music venues and neighbours.
National Audit Office reports that there is little evidence that PFI offers better value than other forms of contracting.
What is liquidation and how does it apply to contractors in the construction industry?
Scrutiny is placed on Carillion's controversial 2013 decision to extend subcontractor payment terms to 120 days.
RSHP unveil their involvement in a boundary crossing which will provide a new entry point into Hong Kong.
With PFI currently under the spotlight due to Carillion, this introductory article explains what they are.
Estimates suggest that up to 30,000 small firms could be at risk of non-payment as a result of Carillion's collapse.
Sir Oliver Letwin to lead an independent review into the delays in the delivery of housing.
As Carillion collapses, read our article explaining insolvency in the construction industry.
43,000 jobs at risk as Carillion declares insolvency.