Civil: Sheppard Subway Don River Bridge

Toronto’s new rapid transit line running east-west under Sheppard Avenue East from Yonge Street to Don Mills Road is scheduled to open later this year. The $933 million subway includes five new stations along the 6.4 kilometre route, but one of the most challenging elements of the project was east of the Leslie Station where the route crosses the Don River.

The vast majority of the subway route is within the Sheppard Avenue road allowance. However, at Leslie Street the line swings approximately 40 metres south to avoid having to pass under several existing bridge structures. The shift in the alignment also permits the subway to pass over the top of the Don River within a new bridge structure instead of tunnelling beneath it. This strategy saves approximately $25 million in capital costs.

The new Sheppard Subway Don River Bridge solves special geometric and unusual loading conditions with a unique design that won a ‘2000 Concrete In Transit Award’ for innovation by the U.S. Portland Cement Association.

The bridge is a 60-m single span, 5.5-m deep, three-cell, pre-stressed concrete box-girder structure. The eastbound and westbound subway tracks are located within the outer cells of the bridge, and its width varies from 16 m to 19 m to suit the flaring alignment of the tracks as they approach Leslie Station.

Watertightness, extreme loads and tight tolerances

Under normal river flow conditions the bridge spans the Don River with over 3.0 m of freeboard. However, under regional storm flow conditions (such as occurred during Hurricane Hazel in the 1960s) the bridge is completely submerged, effectively becoming a weir in the river and subject to extreme hydraulic loading. Stream velocities may exceed 5.5m/sec. The bridge box-structure has to be watertight under these conditions.

The structure’s depth is constrained by several factors. Besides the subway clearance requirements, structural considerations and the need to meet the severe hydraulic constraints of the site, the box structure has to cross under Leslie Street to the west. In addition, the extremely tight tolerances for subway-track construction required a structure with minimum long-term deflection.

Poor ground conditions required extensive use of piled foundations, further complicated by the existence of artesian groundwater pressures at depth. (The subsoil conditions generally consist of fill overlying alluvial deposits and glacial tills separated by interglacial deposits of sands, silts and clay.)

The massive nature of the box-structure adjacent to North York General Hospital and Seneca College Campus, required a unique urban design and landscape treatment in order to soften the bridge’s appearance and integrate it into the adjacent valley environment and bike path system.

Complexity, modelling, joints and bearings

An undistorted 1:30 scale hydraulic model of the Don River Bridge and Don Valley basin was tested by Acres International in their Niagara Falls Hydraulics Laboratory. Hydraulic modelling was also used to determine flow velocities and resulting forces on the bridge structure.

The design of the bridge structure was undertaken in accordance with the Toronto Transit Commission (TTC) Design Manual and the Ontario Highway Bridge Design Code (3rd Edition). Analysis was undertaken using 3-D Finite Element software.

The design of the structure to withstand submerged flood conditions and extreme hydraulic loads was complex. In particular, the effect of flotation and uplift results in significant stress reversals (the bridge is pre-stressed), and the design of the main longitudinal pre-stressing system required a series of near flat tendon profiles to ensure that the combined stresses under all load combinations are within allowable limits. The top slab of the box-girder is transversely post-tensioned.

The extremely high lateral loads are resisted by large lateral guide bearings at each abutment designed to resist forces of over 3,600 kN. A series of multi-directional and uni-directional spherical bearings supporting vertical loads of up to 19,500 kN were also required. The bearings, which are designed to be replaceable, are some of the largest bridge bearings fabricated in Canada.

To ensure the watertightness of the subway structure when submerged, a unique annular expansion joint system was adopted for each cell of the box-girder based on a joint system previously used on aqueducts. The contract specifications required that the seals in the joints be replaceable from the interior of the bridge. In addition to the annular expansion joint system used around the interior of each cell, a further sealed expansion joint assembly was used across the top of the bridge at each end to intercept run-off from the deck surface. This top expansion joint system was customized to suit the landscaping treatment on top of the bridge.

Construction highlights

Pile deflections under superstructure dead loads were estimated and calibrated from the results of pile load testing. The success of this calibration was key to constructing the bottom slab of the bridge to the extremely tight elevation tolerances specified in the contract (13 mm maximum).

The contractor successfully adopted a modified bridge concrete pour sequence for the superstructure in order to separate the top slab pour from the walls. It reduced the size of individual concrete pours and simplified the deck forming and stripping process. Another highlight was the use of an innovative shoring system using extra long regroutable soil anchors to construct the 12.5 m high retaining walls required downstream of the bridge.

Despite difficult foundation conditions, a volatile flood-prone river and a compact site adjacent to one of Toronto’s busiest intersections, the construction of the Don River Bridge was completed on the $15 million budget and almost a year ahead of schedule. Strict environmental controls on in-stream river works were respected, as were noise and access issues relative to the adjacent hospital and college.

Owner: Toronto Transit Commission

Architect: A.J. Diamond, Donald Schmidt & Company