The Toronto City Hall, completed in 1965, was one of 50 projects honoured to celebrate the 50th anniversary of the Precast/Prestressed Concrete Institute. The organization selected the 50 most significant projects using precast concrete components with selections coming from nearly 400 candidates nominated by PCI member companies.

The building's design resulted from an international competition in which designers were given no budgetary limit. After Finnish architect Viljo Revell was named the winner, however, budget restraints were imposed, requiring much of the design to be reworked. The facility contains four main sections: the Public Square, a circular Town Council Chamber and the two Office Towers, one on either side of the council chamber, standing 27 and 20 stories tall. These are fronted by three precast concrete arches spanning a large pool.

The curving towers feature architectural precast panels faced with Italian marble. These panels acted as stay-in-place exterior formwork for a cast-in-place reinforced concrete frame.

The council chamber consists of precast concrete struts supporting a reinforced concrete dome roof with a prestressed ring beam. The central portion of the chamber was constructed of an inverted concrete cone reinforced by two prestressed ring beams. The cone is supported by a cylindrical reinforced concrete shaft that reaches to the shale foundation.

Although the notion of using precast concrete as formwork has largely died out today, this building remains one of the most dramatic government buildings in North America.
 
The facilities of the existing Toronto City Hall were insufficient to meet the needs of the rapidly expanding city. To remedy this deficiency, the Toronto City Council recommended that a world-wide competition be held for the design of a new Civic Square and City Hall. At a meeting of the City Council held on September 3, 1957, the terms of the competition were approved. Five architects were appointed as judges. A total of 520 competitors from 42 countries submitted designs to this competition. Viljo Revell's greatest breakthrough, with the help of his assistants, was the winning of the architectural competition for the design of the City Hall of Toronto. Unfortunately he never saw the finished work; he died in Helsinki, Finland, in 1964 at the young age of 54. The buildings were officially opened on September 13, 1965.

DESIGN TEAM
In October 1958, Viljo Revell entered into a partnership with John B. Parkin Associates, Toronto, and this combined organization proceeded to carry on all the architectural and engineering work which was necessary to produce the final contract drawings and Specifications.

CIVIC SQUARE
The site is located at the north-west corner of Bay and Queen Streets in downtown Toronto, just west of the old City Hall. The Civic Square, located in the south portion of the site, forms a forecourt to the City Hall. At the south end of the square is a reflecting pool which is used as a skating rink in winter. Surrounding the Civic Square is an elevated walkway, which is connected to the south-east corner of the podium.

The floor of the Square is composed of precast concrete paving slabs that are supported on the roof of the underground parking garage. The paving slabs are constructed of precast concrete having a specified 28-day cylinder strength of 5000 psi. Each of the 6 1/2 x 6 1/2 ft. slabs is grouted in position on precast concrete pedestals. A 3/8-in. gap between the slabs permits rain to drain to the roof surface below, thus preventing the accumulation of surface water.

PODIUM
The podium is two stories in height above the street level and contains a basement and a subbasement below. The government offices for both the City and the Municipality of Metropolitan Toronto are located on the second floor of the podium. The first floor is the public access area with entrances from both the street and the Civic Square. The first floor contains a library, a registry office and a land titles office. The basement has a parking area for parking 75 cars, and a number of miscellaneous offices. The subbasement houses mechanical equipment.

COUNCIL CHAMBER
The mayor and the councilors sit in the council room in a slightly sunken area in the center of the chamber. Around them and at a higher elevation is a semicircular seating gallery which accommodates the public gallery. The second floor level consists of the gallery slab, behind the public seating gallery, and the members' lounge at the south of the structure. The structure of the council chamber consists essentially of three portions. The first portion is a reinforced concrete dome roof with a prestressed ring beam, supported on inclined precast concrete struts. These struts are pin connected at each end, are inclined at an angle of 30 deg. to the horizontal, and are arranged in a "zig-zag" pattern around the circumference of the ring beam. This arrangement provides torsional resistance to withstand unsymmetrical loading. The second portion is the main body of the structure and is an inverted cone, inclined at an angle of 30 deg. to the horizontal. The cone is reinforced by two prestressed ring beams. The prestressing force in the ring beams is provided by means of Freyssinet 12/500 post-tensioning cables, each of which has a nominal steel area of 1.73 sq in. The cone is supported by the third portion of the structure; a cylindrical reinforced concrete shaft which passes down through the podium to a foundation resting on shale.

OFFICE TOWERS
The office towers are two crescent-shaped buildings which partially envelope the council chamber. Each of the towers consist of a curved reinforced concrete exterior wall, and an interior line of columns. The length of the east tower, measured along the center line of the exterior wall, is approximately 325 ft, and the corresponding length of the west tower is approximately 225 ft. The typical floor slab is a continuous one-way reinforced concrete slab, 6 in. in thickness. This slab is supported by radial reinforced concrete beams which span from the exterior wall to the interior columns and cantilever 15 ft 7 in. beyond the exterior face of the columns. The west tower, at the left, contains 20 floors and rises to a height of 260 ft 6 in. above the first floor street level. The east tower contains 27 floors and extends, 326 ft 6 in. above the street. Each of the towers have two upper mechanical room floors. The various offices required by the City are located on the other levels.

For design purposes, the towers were considered to be vertical cylindrical shells reinforced by a series of transverse diaphragms (the floor slabs) and also reinforced by longitudinal columns or buttresses. The presence of these reinforcing elements transforms the classical cylindrical shell into an orthotropic shell structure subjected to the action of vertical (gravity) forces and horizontal (wind) forces. The magnitude and distribution of the wind loads were determined by wind tunnel tests on a scale model of the structure, that were carried out by the Institute of Aerophysics of the University of Toronto.

CONSTRUCTION
The office towers were constructed using tower cranes, two of which were installed in. the east tower and one in the west tower. Construction of both towers was carried out simultaneously and progressed at an average rate of one floor in 3 weeks.

Perhaps the most unusual aspect of the construction was the procedure adopted for erecting the back walls. The tower walls are clad with precast concrete panels, with a 2 in. air space being left between the precast panels and the exterior face of the .walls, The precast panels themselves have a facing of marble, and hence serve both to enhance the beauty of the towers, and to provide insulation for the exterior walls, thus minimizing the temperature differential between the walls and the interior columns.

The precast panels were used as formwork for the walls, permitting all work to be carried out inside the building. Each panel was hoisted into position by the tower crane, and positioned over dowels projecting from the lower panel. It was then braced on the inside by adjustable shores, after which vertical steel slip forms were installed behind the panel to form a 2 in. air space between it and the wall. A space was left between the slip forms, into which anchor bolts were placed in threaded inserts provided on the inside of the precast panel. The spaces between the forms resulted in vertical concrete ribs, two per panel, connecting the wall and panel. Following installation of the anchor bolts, the reinforcing steel and interior formwork were placed, and concreting was carried out in two lifts per story height. Compared with the alternative of constructing the walls first, and then erecting the precast facing panels, the procedure used had the advantages of safety, speed, and cost savings. It also produced a positive anchorage system for the precast panels.

TORONTO CITY HALL FACTS AND FIGURES
CONSTRUCTION STARTED: November 7, 1961
OPENING DAY: September 13, 1965
COST OF COMPLEX: $31,000,000

AREAS:
Total Site - 5 hectares (12.75 acres)
Building - 1.4 hectares (3.50 acres)
Gross Floor Area - 75,890 meters (816,900 square feet)
Reflecting Pool - 55 meters x 30 meters (182 feet x 98 feet)

HALL OF MEMORY:
Central Column Diameter - 6 meters (20 feet)
Concrete Thickness - 0.7 meters (27 inches)

COUNCIL CHAMBER:
Diameter - 47 meters (155 feet)
Height (floor to ceiling) - 12 meters (40 feet)
Weight - 3,633 tonnes (4,000 tons)
Seating Capacity - 300

TOWERS:
East - 27 floors, 99.5 meters high (326.5 feet)
West - 20 floors, 79.4 meters high (260.5 feet)

STRUCTURAL SYSTEM:
Reinforced concrete slab, column and wall, with cantilevered floor and shell construction for Council Chamber

MATERIALS:
Precast concrete cladding, stainless steel curtain wall, terrazzo and carpet floors, stainless steel trims and elevator cabs, wood doors and rails, aluminum suspended ceiling.
69,600 cubic meters of reinforced concrete (91,000 cubic yards)
8,165 tonnes of steel (9,000 tons)
8,733 square meters of plate glass (94,000 square feet)
161 kilometers of piping (100 miles)
304,800 meters of electrical wiring (1,000,000 feet)

ACKNOWLEDGMENTS
The architects and engineers for the project were Viljo KeveIl-John B. Parkin Associates, Toronto, Invaluable assistance was obtained from Severud-Elstad-Krueger-Associates, New York, consulting structural engineers. The general contractor was Anglin-Norcross (Ontario) Ltd. The subcontractor for the office tower precast concrete work was Beer Precast Concrete Limited.

Photo Credits: Panda fonds, Canadian Architectural Archives, University of Calgary Library

Reference: "Toronto City Hall and Civic Square" by Hedley E. Roy - Journal of the American Concrete Institute, December 1965