Canadian Consulting Engineer
Air Canada Centre, TorontoEngineering
Yolles Partnership Inc., TorontoCategory: Buildings and StructuresWhen the Air Canada Centre opened in February it had an exciting history even before any sports events had been played out in the new ...
Yolles Partnership Inc., Toronto
Category: Buildings and Structures
When the Air Canada Centre opened in February it had an exciting history even before any sports events had been played out in the new arena. Though the centre was originally planned as a facility for the Toronto Raptors basketball team, just months before the opening date the Toronto Maple Leafs hockey team bought the arena and the designers had to make $30 million in changes so that the facility could accommodate both teams — that with a massive project that was already on a fast construction track.
Even before the ownership change, however, the project team had to face complications from the restricted urban site between an expressway and a railway line, and the need to incorporate facades of a historical Post Office Building. The many complexities of this project, however, must have been a stimulus rather than a hindrance to the structural engineers since Yolles Partnership Inc. won an award of excellence for their part in the project, which included designing a unique roof.
The Air Canada Centre consists of a 21,000-seat, 74,000 m2 arena and an adjacent 15-storey, 14,000 m2 office tower. The arena’s primary uses are for professional basketball and hockey, but it can also be used for other events such as concerts and conventions.
The arena has a reinforced concrete substructure and superstructure, supporting a long span structural steel truss roof. The office tower is a reinforced concrete, flat slab structure which is physically separated from the arena with an expansion joint above grade.
The arena has three levels below grade: two for parking and the main event level containing the basketball court/ hockey rink, change rooms, private boxes and service areas such as kitchens, shipping and receiving. At grade is the lower concourse which provides the major public access as well as concessions and washrooms. Above grade are two main floors containing private boxes, the upper concourse, and mezzanine and balcony seating.
A general contractor was selected during the design process (rather than the traditional way after design work is complete). Thus, the consulting team presented alternative design solutions to the contractor who was able to assess them for cost effectiveness, feasibility, and their impact on the project schedule before construction started.
The unusual geometry of the roof was governed by two primary architectural criteria. First, the height was limited by local bylaws, which resulted in a 28 metre clear height to the underside of the roof structure — 2.5 metres lower than any similar facility. Second, the building program required that a media gondola and private boxes be installed within the roof structure. To optimize the floor space, the bottom chord of the north-south trusses had to stop well short of the vertical support. The configuration of these trusses resulted in the “gull winged” shape of the truss (see figure page 34). As well, the roof had to support two mechanical rooms.
After considering a wide variety of solutions and geometries, the team decided on a two-way kingpost truss system spanning 102 metres and 110 metres in the north-south and east-west directions respectively. The east/west lateral loads are resisted by a system of chevron diaphragm bracing connecting the three middle north-south trusses at the truss top chords. Two bays of diaphragm bracing connecting the pairs of east-west trusses resist north-south lateral loads. This diaphragm bracing is also used to resist seismic, wind and stability forces.
The trusses support continuous wide flange purlins which support the 76 mm deep steel deck, but also transfer the lateral loads and bracing forces to the diaphragm system. All primary trusses are supported on unidirectional sliding bearings at the top of the concrete frame. These bearings allow movement parallel to the truss spans to prevent the development of large lateral thrusts due to gravity. The fixity of the bearings perpendicular to the truss spans allows the transfer of horizontal shears to the concrete frame.
The fastest and most economical approach to construct the roof was to erect the north-south trusses first as self-supporting elements. The east-west truss sections between the north-south trusses were installed but not activated. Thus the dead load of the structure (about 30 per cent of the total load) was locked into the one-way system. The remaining loads were supported by the two-way system after all the bolted splices of the east-west trusses were finished.
For this unique approach Yolles needed software which could superimpose the member forces from two different stiffness models: a one-way model for dead loads and a two-way model for all remaining loads. The firm retained software developers to devise a custom package to combine member forces from two different design models in their analysis package. The automated tool can analyze and design a structure through several iterations until a solution converges. About 10 iterations were required.
The mechanical rooms located within the east and west ends of the roof had to be constructed as soon as possible. Since the weight of the rooms and their equipment could not be supported economically by the one-way system, temporary shoring towers were constructed at the extreme east and west ends of the roof to support their weight.
Urban value and historic conservation
The building sits on what was a virtual wasteland in the downtown core. The Air Canada Centre is a step towards redeveloping and revitalizing the area. Moreover, linked to Union Station, it has direct access to the city’s hub for subway and suburban trains.
The site is severely constricted on a two kilometre strip of property which is bound on the north by a rail line and to the south by the elevated Gardiner Expressway. These factors called for careful planning in both the design and construction of the project, and resulted in an unusual geometry in plan.
One of the biggest challenges was the requirement of the Toronto Historical Board that facades of the 1950s Post Office building be retained on the south and east sides. The existing masonry walls were very fragile. In addition the bracing had to be on the inside of the structure away from a road. A series of vertical trusses was set up to support the original columns of the old facade, while the foundation of the new building was set inside the bracing. The new upper floor levels were cantilevered out to the back of the facade.
The final complexity was the purchase of the facility by the Toronto Maple Leafs at a time when the structure was virtually complete. The changes introduced by the new owner turned the project into a renovation and yet the team had to meet the same opening deadline. A level of boxes was hung from the roof, which had been originally designed with the required capacity. Two levels of private boxes were added to the east end of the building and space for expanded dressing rooms was “mined” below a slab on-grade structure at the north end.
Since the centre opened it has held well over 80 events. The owners are well satisfied with a project that was completed on time and on the budget ($180 million total), despite the extraordinary circumstances.
In a project that had many twists and turns, the structural engineers for Toronto’s new hockey and basketball arena managed to devise an innovative roof system. The two-way kingpost steel truss spans over 110 metres and has a low profile to avoid blocking downtown views.