New Bytown Bridges

Along the ceremonial route of Canada’s capital are two bridges connecting the residences of the Prime Minister and Governor General to the Houses of Parliament. The Bytown Bridges carry Sussex Drive over two branches of the Rideau River in Ottawa’s central core, and as such represent a vital link within the city’s transportation network.

A crossing has existed at the site since 1846. In 1954, two three-span structures had been constructed, and these represented one of the first uses of precast prestressed concrete girders in Canada.

Both bridges had been rehabilitated over the following decades, but by 2001 their superstructures had deteriorated to the point where they had to be replaced.

The high-profile location of the project, in the midst of landmark residences, embassies, government offices and tourist attractions, complicated the design and construction phases. However, Delcan Corporation in collaboration with the city of Ottawa and its project manager Abdol Nouraeyan came up with an aesthetically pleasing and innovative bridge design. The concept incorporates a hybrid precast/cast-in-place semi-continuous span, with seismic isolation bearings.

The project had to undergo both a federal and a provincial class environmental assessment within a very limited timeframe as certain parts of the design and construction had to be fast tracked to accommodate a scheduled visit of Queen Elizabeth II to Rideau Hall. The requirement was to ensure that all construction activities in the vicinity of the Governor General’s residence were completed before Her Majesty’s visit. Besides the two bridges, the project included the full reconstruction of a significant stretch of Sussex Drive. The three-year construction project was completed on schedule in August 2005, with only a 15-day extension (due to a 1/100 year probability storm event that hit the area in September 2004). It was essentially on budget at approximately $12 million.

Seismic factors governed the design

The new bridge design had to meet high seismic requirements since it is located in Ontario’s highest seismic zone (Zone 3), which governed the structural design for most elements. Not only was the seismic nature of the site greatly responsible for the need to replace the substructure, it also played a key role in the structural design of the new river piers. The very large width-to-height ratios for the wall piers that were imposed by the site conditions meant the bridge had a very stiff substructure with very little ductility. Since the ductility of the substructure could not be relied upon to help dampen the seismic loads, a conventional “brute force” approach would have been very difficult. It would have needed a wide and thick footing embedded 1.0 metre into the bedrock and anchored with the help of 82 rock anchors, each embedded to a depth of 8.1 metres. The lack of feasibility in this approach, as well as the almost prohibitive cost, forced the engineers to investigate alternatives.

Their solution was to use seismic isolation bearings. This type of bearing works by partly isolating the large mass components of the bridge from ground motions induced during an earthquake. The specialized devices transfer the energy of a moving mass (kinetic energy) into heat through friction and spring (potential) energy. The seismic forces are thereby dissipated through controlled friction and the resulting dampened reactions minimize the impact on the structural members.

The resulting pier design with isolation bearings was a shaft of only 1.6 metres wide, no footing, a 0.6-m rock embedment and 30 rock anchors. Each rock embedment was only 3.0 metres. The design achieved savings in materials and associated labour that far surpassed the cost of the specialized bearings.

Isolation bearing features

The type of isolation bearing selected for this project consists of a disc type highload multirotational bearing coupled with polyurethane displacement control springs. These springs are referred to as Mass Energy Regulators (MERs). The bearings were developed in the early 1990s based on research conducted at the National Center for Earthquake Engineering Research at the State University of New York at Buffalo. The centre’s research showed a cost effective isolation system with the following advantages:

* the bearings significantly reduce seismic forces transferred to the bridge substructure;

* the designer can direct seismic loads to the elements most capable of resisting them;

* the bearings can accommodate multi-directional non-seismic movement such as in horizontal curved bridges;

* the system requires small movement expansion joints.

To promote their longevity and long-term performance, the stainless steel sliding surface of the bearings is facing down, and the critical bearing elements are enclosed by a protective barrier.

The engineers put together a rigorous testing program for the bearings. The tests included all those specified in section 17.2.2 of the AASHTO Guide Specifications for Seismic Isolation Design, as well as a more unique cold temperature testing to ascertain the bearings’ performance under sustained extreme cold temperatures. Ottawa’s yearly minimum daily mean temperature is –34C.

Although the use of seismic isolation bearings turned out to be the most cost-effective solution, the cost of each individual device was still considerable. Consequently, their use was limited to the piers only, and more conventional elastomeric bearings were used at the abutments. The elastomeric bearings were oversized in thickness to reduce their individual stiffness and therefore minimize their effect on the overall dampening of the bridges.

Hybrid precast/cast-in-place superstructure

A superstructure of precast box girders was selected, partly because the system was easily erected and required no falsework in the river. The girders also prevent birds from perching and roosting, which was one of several specific design requirements.

Since each of the bridges was to consist of three simple spans of 12 precast, prestressed box girders, a conventional design would have required 24 isolation bearings for each of the two piers per bridge, for a grand total of 96. To reduce the quantity of these costly bearings, an innovative hybrid precast/cast-in-place design was developed. The idea was to have only one isolation bearing for each girder line at every pier, which effectively cut in half the total number of required isolation bearings and reduced the required width of the pier bearing seat. The design was accomplished by having the isolation bearings centrally located on the piers under an extra wide continuous diaphragm. The diaphragm was poured monolithically with the deck and the girders were rigidly connected to it. During construction, the girders were erected onto temporary wood blockings at the piers until the diaphragms were poured over the previously installed isolation bearings.

Once the concrete in the diaphragms and deck had reached its desired strength, the temporary supports were removed and the girders were then supported at the piers through their connection with the diaphragms and deck. The design of the critical girder connections to the pier diaphragms was done with numerous embedded dowels at the girder ends and sides. The pier diaphragms were also sized to match the full width of the piers to increase the interface area with the girders along each side. They were poured monolithically with the deck to benefit from the composite action with the girders.

Crest and fascia

To help emphasize the crossing of the Rideau River through a very flat terrain, the designers introduced artificial crests, or humps, into the bridges. The differential height of the crests is 0.6 metres over the length of the crossing, a height deemed sufficient to be visually noticeable without compromising the rideab
ility of the roadway. Also, as pedestrians and motorists move onto the bridges at the north and south ends, they pass thresholds marked by balusters at the abutments. The railings are an open design to allow views onto the waterway.

Another architectural feature is the cast-in-place concrete fascias, which are mounted on the exterior of the precast girders to hide them and provide a smooth and continuous arch.

Client: City of Ottawa

Prime consultant: Delcan Corporation, Ottawa (Bruce Friesen, P.Eng., Sylvain Montminy, P.Eng., Hugh Hawk, P.Eng., Ken Smith, CET, Dave Hearnden, P.Eng., Ron Fournier, Kelly Roberts, Alain Gregoire, P.Eng.)

Geotechnical subconsultant: Golder Associates

Architect: Barry Padolsky Architect

Landscape architect: Corush Sunderland Wright

Contractor: R.W. Tomlinson