May 20, 2016
By Said Elieh
Conserving national history for future generations requires careful protection and maintenance. New construction in these spaces must be sympathetic to the existing landmark structures and settings, and public needs. Sensible construction allows these sites to be better appreciated, studied, and preserved. Additionally, the age and significance of historic locations often require new structures built on or adjacent to them to meet special guidelines, regulations, and codes.
The delicate balance between conserving a historic location for posterity, while enhancing the appeal of the site for contemporary and future visitors, is the focus of this article. It examines the new visitor centre at the Fort York National Historic Site of Canada, and the building materials and techniques employed by Kearns Mancini Architects (Toronto) and Patkau Architects (Vancouver) to enhance the site, while respecting the legacy and tradition of the antique structures within.
Fort York is located in downtown Toronto, nestled near the shoreline of Lake Ontario. Built originally in the late 1700s, it was the site of a major battle during the War of 1812, which lead to the occupation of Fort York and the destruction of York (modern-day Toronto)—at the time, the provincial capital of Upper Canada. The fort functioned as a military post until the 1880s, and was lightly used during WWI and WWII. Since the early 1930s, it has remained relatively undisturbed, despite the significant urban growth around it.
One of the greatest challenges of the project, according to architect Jonathan Kearns, was to create a visitor centre on an archeological site that could not be disturbed, situated in the midst of an industrialized urban setting, and removed from its original historic context. Two centuries of infill and development had extended the shoreline of the lake about 500 m (1640 ft) from its original location beneath the walls of the fort, taking away the impact of the facility as a defense against naval attack.
When building an addition to, or otherwise altering a historic place, The Standards and Guidelines for the Conservation of Historic Places in Canada states,
The construction of an exterior addition in an historic place may seem essential for a proposed new use, but the Guidelines emphasize that such new additions should be avoided, if possible, and considered only after it is determined that those needs cannot be met on another site or by altering secondary, non-character-defining interior spaces. An addition should be designed so that the heritage value of the historic place is not impaired and its character-defining elements are not obscured, damaged or destroyed. The addition should be physically and visually compatible with, subordinate to, and distinguishable from the historic place as stated in Standard 11.
The conservation standard required the proposed Fort York Visitor Centre be new construction, due to the need to preserve the “character-defining” structures of the fort itself. Hemmed in on all sides by glass and steel high-rise apartments, rail lines, and the superstructure of the elevated Gardiner Expressway, the fort grounds afforded a very limited amount of space to build the new visitor centre.
Overcoming the design challenges and respecting the heritage value of the fort and its historic buildings, the new visitor centre (which opened in late 2014) is a low-slung two-storey edifice, tucked away under the Gardiner’s monolithic superstructure.
Experiencing the fort
Reflecting the industrialized character of the neighbourhood today, the entrance to the visitor centre is an artificial escarpment of weathering steel. The vertical faces of the sloped steel panels resemble industrial fencing or fortified walls that feel surprisingly unimposing and natural in contrast with the massive expressway overhead. Inside, the centre houses the lobby featuring permanent and temporary exhibitions, an informal dining area, and offices. Guests move from the lobby exhibits through long hallways that gradually rise toward the fort grounds. The lines that define the entire fort area—the tall legs and long ribbon of the expressway, the rail lines in the distance—all culminate in a literal timeline running the length of the centre’s rising halls, depicting events from the War of 1812.
On the firm’s website, Patkau Architects describe this part of the visitor experience as:
A multi-media immersive ‘time-tunnel’ portraying the dramatic events of the War of 1812/14 rises from the entrance level to emerge on the Commons, connecting the modern city below to the Fort above. The time tunnel is a threshold, a passage in time from present day to the founding of the city. Walking up the gentle switchback slope, visitors are invited to undertake a pilgrimage through the site’s history, arriving at a site virtually unchanged in two centuries. Behind, the building emerges from the ground at the level of Garrison Common as an illuminated wedge, clad in backlit cast glass channels. The roof of this wedge is paved, offering elevated views of the entire Fort and the Toronto skyline beyond.
Once through the ‘time tunnel’ and inside the fort proper, the visitor centre appears completely transformed. Gone are the heavy, weathering escarpments and steep walls of ochre steel. Instead, guests exit the visitor centre through two expansive translucent walls made entirely of channel glass. Illuminating the fort’s history, the graceful glass walls become part of the guest experience, as visitors step onto the fort grounds, while the linearity of the channel glass echoes the centre’s ‘timeline’ theme.
Defining the upper levels and running the entire length of the centre, the channel glass façade rises from the landscape. The cladding was originally imagined as a stainless steel screen wall—a surface to be used for projecting images of battlefield scenes from the fort’s lifetime. The projectable screen idea was not financially feasible, and a new solution was needed to create the desired dynamic, captivating experience for visitors.
To create the cladding for the two upper exterior walls of the centre, the architects selected a channel glass rainscreen configured as a dual-function architectural element—visually highlighting the new structure while addressing more than 90 per cent of the imposed forces of nature.
Channel glass is translucent, U-shaped, machine-rolled (i.e. ‘cast’) glass, available in face widths ranging from 230 to 480 mm (9 to 19 in.). Its unique geometry allows it to withstand imposed forces in ways conventional flat glass cannot, creating virtually seamless glass walls uninterrupted by metal frames. Installed vertically, the glass can span heights up to 7 m (23 ft) and virtually limitless lengths—hundreds of metres or more—a distinct advantage over traditional flat glass. At 7 mm (0.28 in.) thick, individual glass channels are lightweight, facilitating handling and installation. They can be installed by as few as two people, without the need for cranes or other heavy-lifting equipment.
Understanding channel glass
Channel glass rainscreen systems, such as the one cladding the new Fort York Visitor Centre, offer a robust layer of protection to new buildings’ structural walls, preserving structures from the elements and ensuring their longevity. They feature a streamlined linear esthetic, signature to this unique glass type.
A non-porous, non-combustible building material, channel glass can withstand the punishing forces of water, wind, and time. It offers ease of maintenance, durability, and an upscale esthetic. Unlike other rainscreen cladding materials, channel glass can also allow more than 85 per cent of visible light through, maintaining daylight advantages in applications featuring windowed structural walls behind the rainscreen.
At the new Fort York Visitor Centre, about 1000 m2 (11,200 sf) of tempered safety channel glass create a back-lit rainscreen façade, set roughly 0.7 m (2.25 ft) from the building’s exterior walls. The single-glazed, closed-joint, open-wept cladding system (i.e. bottom frame has deep holes to control drainage) provides protection from the elements—wind, rain, snow, and ice—and effectively controls water drainage. The extrusions are aluminum alloy 6063-T5 in standard Class 1 Clear Anodized finish.
The tempered, Safety Glazing Certification Council (SGCC)-certified glass channels are pocket-set into the frame system and captured at the head and sill only, creating a virtually seamless glass façade along the entire length of the building. The light-diffusing ‘orange peel’ channel glass surface texture effectively combines the esthetic benefits of translucency and back-lighting ability. Rigid insulation covers the structural walls behind the glass, providing enhanced thermal performance and forming a uniform, light-washed backdrop for the glass. Lighting fixtures attached to the back of the channel glass systems’ head and sill steel supports illuminate the centre at night, producing an ethereal glow.
The frame system design, eliminating the need for vertical metal supports, allows walls of uninterrupted channel glass, including glass-to-glass corners in a variety of angles, from 20 to 160 degrees. Unique to this proprietary system, visually identical head and sill frame components contribute to the consistent, clean look of the glass façade.
The channel glass walls at the visitor centre’s sloped terrace consist of two parallel single-glazed systems, allowing one side of the glass wall to terminate at the terrace slab, while seamlessly bypassing it on the opposite side. The glass channels above the terrace are gradually stepped—a budget-smart design technique creating the appearance of a continuous slope. The step-cuts are captured inside the head of the frame system, creating an elegant continuous line. The same technique is used to follow the grade at the stepped visitor centre exit, which brings visitors to the historic grounds.
Angle-cuts at the bottom of the glass channels are used at the staircase leading to the centre’s terrace, to create sleek uninterrupted sightlines. Here, the design employs two channel glass profiles: P26/60/7, featuring a 60-mm (2.3-in.) flange depth typical for exterior applications, and P26/41/6, featuring a narrower 41-mm (1.6-in.) flange. (These descriptors simply reference the dimensions of the glass channels. For example, P26/60/7 stands for approximately 26-cm (10-in.) face width, with 60-mm (2.3-in.) flanges and 7-mm (0.28-in.) glass thickness). Glass channels with the narrower flange, ranging in length from roughly 5 to 1 m (17 to 3.5 ft), are used at the interior-facing walls of the staircase to accommodate the limited available space. Mid-point wind clips reinforce the 41-mm flange channels taller than 2.6 m (8.6 ft).
The walls of channel glass approach 6 m (20 ft) continuous height in the tallest sections. The additional strength required is facilitated by tempered channel glass coupled with strategically positioned mid-point wind clips. The wind clips reduce the glass deflection on the taller channels ranging 4 to 6 m (12.5 to 20 ft) in height. The effect of such a large expanse of light-diffusing, luminous channel glass is that of a billowing sail floating above the grounds of the historic fort.
The channel glass used at the new Fort York Visitor Centre is ecologically sound in many ways. It is formed in an ultra-clean-burning oxygen-fuel-fired furnace, which significantly reduces energy consumption and reduces nitrogen oxide emissions by a factor of five when compared to conventional furnaces. The glass features more than 60 per cent recycled material, including approximately 40 per cent post-consumer content. It is also fully recyclable at the end of its lifecycle.
Additionally, the light-diffusing, non-reflective surface texture of channel glass can greatly reduce bird strikes—a common problem with many traditionally glazed surfaces. Essentially, it allows birds to recognize the glass as a solid barrier rather than a clear opening. ( For more on the conservancy and the critical need for avian-safe glass, see “The Invisible Threat: Creating Bird-friendly Buildings,” by Christine Sheppard, PhD, Anne Lewis, FAIA, Bruce Fowle, FAIA, and Guy Maxwell, AIA. The article appeared in the January 2014 issue of Construction Canada. Visit www.constructioncanada.net/the-invisible-threat-creating-bird-friendly-buildings).
Since its opening, the Fort York Visitor Centre has become a focal point for the district. It has helped reinvigorate the area and establish a contemporary cultural landmark in the heart of downtown Toronto. By successfully complementing the historically significant site it occupies and minimizing its own physical footprint, the centre allows a distinguished part of Canada’s rich history to come alive for generations of visitors.
|TECHNICAL DESIGN NOTES FOR FORT YORK’S CHANNEL GLASS RAINSCREEN|
|– Design wind load: 1.27 kPa (27 psf)
– Glass deflection: limited to L/100
– Framing member deflection: limited to L/175 or max 20 mm (0.75 in.)
– Vertical glass span: approximately 6 to 1 m (20 to 3 ft)
– Horizontal glass span: approximately 108.5 m (356 ft)
– Glass layout: single-glazed; vertical walls and glass-to-glass corners
– Glass-to-glass corners radii: 20, 41, 90, 131, and 160 degrees
– Frame profile dimensions (approximately): depth = 82.5 mm (3.25 in.); head and sill height = 58.5 mm (2.3 in.); stock length = 6.5 m (21 ft)—longer spans were created by splicing extrusions
– Glass profile dimensions (approximately): face width = 260 mm
(10.3 in.); height range = 6 to 1 m (20 to 3 ft); two various depths (flanges) = 60 mm (2.4 in.) and 41 mm (1.6 in.); thickness = 7 mm
– Glass weight: 22 kg/m2 (4.5 lb/sf)
– Glass safety: factory-tempered, Safety Glazing Certification Council (SGCC)-certified safety channel glass
– Glass sealant: standard silicone (the framing system does not require the use of structural silicone for typical installations).
Said Elieh is the director of technical design for Bendheim Wall Systems, and the manager for its decorative glass rainscreens division. He has more than 15 years of construction industry experience, and holds a master’s of architecture degree. Elieh can be reached via e-mail at firstname.lastname@example.org.
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