No fancy frills

"The difference between the old and the new? There's no comparison. It's two different worlds," sums up Fadi Moubayed, Transport Ministry projects manager for the Laval-Mille-Îles Region.

Three major elements differentiate the new structures from the old. The first of these is the addition of a third source of support, a six-column beam at the centre of the Papineau-Leblanc Autoroute. This is a rejection of the thinking of the 1960s when it was believed that the elimination of a middle support columns would also eliminate visual obstruction for motorists attempting to negotiate this curving part of the autoroute.

"We analyzed the depth distance (used in the new structure with middle support). There is no problem. The visibility standards (of the new structure) are at least two or three times superior to the norm," Mr. Moubayed explains.

The second major change relates to the support for the bridge's concrete roadbed, with builders opting for simple steel beams. The roadbed which collapsed rested on box girders of prestressed concrete. Prestressing is a mechanical process which allows the concrete to react better to pressure during use.

In many ways, steel beams are a simpler option, Mr. Moubayed explains, especially with respect to time factors and manufacturing procedures. Prestressed concrete beams on the other hand "serve best in difficult cases such as in a curved structure," the engineer explains. In such cases which are more complex, these prestressed concrete beams react much better to tension, compression and torsion. But for the present needs at Concorde and de Blois which are simpler, the steel beams are the best solution.

And lastly, the new bridge has no overhang extending laterally on either side. It was this part of the bridge hanging over the autoroute and on which rested the roadbed that broke up and collapsed last September sending the bridge crashing down onto the highway below.

The chosen procedure eliminates the problem of maintenance and inspection of the expansion joints. The problems faced by inspectors when examining the old overpasses have been cited on numerous occasions since the Johnson Commission began to hold public hearings into the collapse of the Concorde bridge.

The examining of the roadbed concrete and bearing pads proved virtually impossible in the old concept. In order to determine the damage caused by a leaking joint, it would have been necessary to raise the superstructure, a complicated procedure which would have necessitated a prolonged shutdown of traffic.

In the new structure, the inspection of joints can be done under the structure and from the abutment, with the use of a platform. "From that vantage point, we can see everything," says Fadi Moubayed.

Downward slopes on the bridge's superstructure will also allow for much easier drainage of rainwater, he mentions. "We're doing everything to respect the standards of the day," he concludes.