Canadian Consulting Engineer

Feature

Restoring the Cummings Bridge

The Cummings Bridge in downtown Ottawa was one of the first multi-span, open spandrel, arch bridges in Canada to be built with reinforced concrete. Finished in 1921 and designed by municipal engineers...






The Cummings Bridge in downtown Ottawa was one of the first multi-span, open spandrel, arch bridges in Canada to be built with reinforced concrete. Finished in 1921 and designed by municipal engineers, it spans 213 metres (700 feet) across the Rideau River, about two kilometres east of Parliament Hill.

The picturesque, designated historic structure had deteriorated to such a point by 1996 that there was a question about whether it could even be saved. Temporary shoring and columns had been holding it up since the 1980s, the concrete was scaling and spalling, and the reinforcing steel was rusting. It had also undergone many changes over its life. In 1967, for example, the original concrete balustrades had been switched for a modern steel railing, and standard aluminum cobra-head street lights stood where originally there were charming concrete lampposts and globes.

After many consultations with local heritage groups and public agencies, the Region of Ottawa-Carleton decided it would completely restore the structure to its former glory, even though this meant demolishing and recreating almost everything apart from the arches and the pier and abutment bases. The entire deck, the spandrel walls, and the piers and abutments from the top of the arches had to be reconstructed. By the end of the day the bridge was to look almost as it did in 1921, but it would be two metres wider to accommodate four vehicle lanes, expanded sidewalks and a bicycle lane. At the western end, the three-span approach structure, which was badly deteriorated, frequented by vagrants and strewn with garbage, would be filled in using a unique retaining wall system.

Reworking history

With Delcan’s Ottawa office as prime consultant and bridge engineer, the project team set about recreating the bridge with its distinguishing triangular skewed lookouts, pier cutwaters and decorative recessed panels. They decided to reconstruct the spandrel walls to be similar to the originals, but change the basic structural system. Instead of having longitudinal beams, the replacement deck spans longitudinally between cross walls. This approach simplified the construction and made it possible to have a lighter deck. The overall width of the deck was increased from 16.15 metres to 18.21 metres by cantilevering off the sides, and adding brackets similar to the original construction. The cantilevered sections were built from above rather than disturbing the riverbed by constructing scaffolding from below.

As is common in structures of this type, there were two transverse expansion joints in the deck at each pier, one at each face. The team wanted to eliminate these concrete-to-concrete, open joints in the new design, but to use a continuous deck would have placed undesirable stresses on the structure. Consequently they kept the original articulation, but left the reconstructed hollow pier shafts open at the top, cantilevered the deck slabs over the pier walls, and so were able to reduce the joints to a single one at the centre of each pier. The new deck slabs are supported on elastomeric bearings on the pier walls, and the expansion joints are sealed.

Railings and lighting

A big question was how to recreate the original open balustrade and yet meet contemporary road design codes. The team designed a precast balustrade that is similar to the ornate original, but that is heavier to meet the design loads. Though it doesn’t strictly conform to the code, the balustrade design was accepted since no serious accidents have been recorded on the bridge in the past. The balustrade was precast in three metre panels and anchored into the rebar reinforcing in the cantilevered deck.

Similarly, the question was how to recreate the original low level globe lamposts and yet meet current illumination standards for safety. The original bridge had pairs of lamps at each pier on either side of the lookouts. To add light, the project designers added an extra pair of lamps at the centre of each span. At the ends of the bridge they created a gateway effect with a cluster of lights mounted on the flared wing walls.

West approach

The three span western approach to the bridge was in such bad condition it had to be removed, and the Region of Ottawa-Carleton wanted it replaced with earth fill. However, there was little room to manoeuvre heavy equipment in a site bounded by private property and a steep, wooded hillside. The designers therefore rejected building a gravity wall or conventional cantilever wall, and felt that a conventional reinforced mechanically stabilized earth (MSE) system with its precast panels was aesthetically unsuitable.

Instead, they developed a unique retaining wall system known as Terratrel. Designed by Reinforced Earth Company as a low-cost wall that is usually temporary, it has welded wire fabric facing panels. A geotextile on the inside face of the wire mesh stops backfill falling through the mesh. A 300-mm thick, cast-in-place, reinforced concrete facing was attached to the mesh, and detailed with recessed panels similar to those on the piers.

As work continued over the two years of the $6 million project, it was staged so that traffic continued to flow. By the time the bridge was officially re-opened in October 1998, it stood as a prime example of how a heritage structure can be restored to its original beauty, and yet still function well for society today.–BLCCE

Client: Regional Municipality of Ottawa-Carleton

Prime consultant/engineering/environmental/ landscape: Delcan Corporation, Ottawa. Project team leader: Bruce Friesen, P.Eng.

Architect: Barry Padolsky Architect

Geotechnical: Golder Associates

Lighting: Gabriel Design

General contractor: Entreprise Bon Conseil

Precast concrete: Beton Bolduc

DELCAN CORPORATION