Canadian Consulting Engineer

Street Front Transformation

December 1, 2009
By Crispin Howes, P. Eng. Halcrow Yolles

The Art Gallery of Ontario (AGO) in downtown Toronto reopened its doors at the end of 2008, having undergone a radical transformation. Most visitors' first encounter with the transformed institution i...

The Art Gallery of Ontario (AGO) in downtown Toronto reopened its doors at the end of 2008, having undergone a radical transformation. Most visitors’ first encounter with the transformed institution involves the new glass and timber faade that extends 600 ft. (183 m) along Dundas Street West and rises 70 ft. (21 m) above street level. Architecturally, this north faade is intended to unify a building that has undergone six separate additions and renovations in the last century.

The form of the faade has four distinct zones. The central zone with a gently varying curvature encloses a 400 ft. long sculpture court known as the Galleria Italia. Abutting this central portion are two exterior zones with an aggressively warped curvature, which are referred to as “wings.” Finally, a “skirt” extends the full 600 ft. of the building. The skirt helps to conceal a stainless steel-lined gutter that runs above the pedestrian level.

Required: maximum transparency and radiating elements

“Maximize transparency” and “utilize a series of radiating elements” were two clear architectural goals from the start.

From a structural perspective, to maximize transparency involves optimizing the framing members using the minimum volume of material. The challenge is to optimize the members while dealing with forces from multiple loads, including wind and gravity.

The remedy was to separate the facade into two specialized structural systems. The first specialized glulam framing system is referred to as the “mullion grid” and it provides direct support for the glazing panels. The weight of the glazing is transferred directly to a second floor steel platform through straight vertical mullions. The wind load on the face of the glazing is resisted by a system consisting of curved horizontal glulam mullions.

The second system, referred to as the “primary layer,” is separated from the “mullion grid” by a 3-in. (75 mm) gap that allows for geometrical variations. The curved radial arches within this layer span from the second floor platform to the roof line of the existing buildings and are the main elements for resisting overall wind loads on the faade. The slope of each radial arch varies in proportion to its distance from the main entrance.

Out-of-plane bending in the radial arches due to the slope is controlled with rows of straight horizontal elements, which are referred to as “louvre headers” since they also support a series of sun shades. The rows of louvre headers are stabilized by curved structural steel box sections at each end of the main zones of the faade.

The solution included nearly 1,800 pieces of Douglas Fir glulam pieces and involved evaluating over 200 load case scenarios.

Both of the glulam framing systems are supported on a structural steel platform that also provides support for the sculpture court on the second floor. The steel platform is a new addition that is supported along the inner edge by the existing reinforced concrete buildings, and along the outer edge by a series of concrete encased structural steel columns spaced up to 60 ft (18 m) apart.

Structural Steel Platform And Sliding Bearing Connections

A challenge was how to connect the unified faade to this long span steel framing that is prone to expansion and contraction during changes of temperature. A fixed connection would have pulled the faade apart as the beam lengthened or shortened. To remedy this problem, Halcrow Yolles specified a sliding bearing connection made of two plates: one steel, one teflon. These plates slide against one another, allowing the faade to maintain its structural integrity as the steel framing below reacts to environmental temperature changes.

The Dundas faade structure was achievable only through close collaboration between the structural and architectural teams. Building Information Modelling was also a vital part of the design process.

Owner: Art Gallery of Ontario. Structural engineer: Halcrow Yolles (Hugo Blasutto, P. Eng., Eric Gordon, P. Eng., Kari Valli, P. Eng., Chris Hendrata, Crispin Howes, P. Eng., Raef Ghali). Architect: Gehry International. Wind, snow, ice consultants: RWDI


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