Structures: New Facade, New LIfe
December 1, 2012
By By David De Rose, P.Eng.,Halsall Associates
[See digital edition for images]
[See digital edition for images]
From a distance you might not notice anything has changed. But take another look at Toronto’s First Canadian Place, particularly on a sunny day, and you will see that the tower gleams with a subtle new lustre.
At 72 storeys, First Canadian Place on King Street West in the downtown core is Canada’s tallest building. Its new facade completed earlier this year was designed to modernize yet preserve the original iconic design of architect Edward Durell Stone, while at the same time bringing the aging office tower up to the highest contemporary standards for safety and performance. The elegant simplicity of the building’s new exterior belies a project of complexity and innovation.
First Canadian Place was built in 1975. Clad with white Carrara marble panels, it created a unique and striking landmark in Toronto’s financial district. As early as the 1980s, however, the marble had begun to show signs of losing strength and weathering. A maintenance program was implemented to manage safety through to the end of the cladding’s service life.
In 2005, the owners engaged Halsall to evaluate the maintenance program and develop options for addressing the cladding’s needs over the long term. The owners decided to completely reclad the building. There were several reasons: to renew the building’s grandeur; to ensure the building’s market competitiveness; to avoid the disruption and cost from the ongoing management program; and to reduce risk.
After reviewing several options, the owners chose to replace the approximately 45,000 original marble panels with 5,600 large glass panels. These consist of 8 ft. x 10 ft bright-white, fritted-glass laminated panels at the primary elevation. At the building’s inverted corners 8 ft. x 5 ft. bronze glass panels enhance the verticality of the elevation. Halsall was part of the design team selected to deliver the project.
The project has wider implications. As the “modern” buildings of the 1970s and 1980s enter middle-age, they are increasingly appreciated for their aesthetics. However, in many cases their cladding systems are reaching the end of their service life. The recladding of First Canadian Place offers a striking example of how the need for cladding renewal and the desire to maintain the original architectural beauty of these iconic buildings can be successfully resolved.
Increased wind loads
The first challenge in designing the new cladding was to predict the increased wind loads to which it would be subject. Thirty years after the original design, the advancement of wind engineering and modeling technology, along with changes to the building density around First Canadian Place, led to a dramatic increase in what experts stated the design wind load should be.
To arrive at the appropriate design wind loads, scale model tests were performed. Halsall then worked closely with world-leading wind experts at the Boundary Layer Wind Tunnel Laboratory (BLWTL) at the University of Western Ontario as well as RWDI consulting engineers in Guelph to extensively re-evaluate and peer review the most recent wind studies. Ultimately Halsall was able to achieve agreement amongst the experts on the appropriate peak 50-year return cladding loads. Although still an increase over the original design, they were 20% less than conventionally predicted peak design wind loads.
The increased wind loads applied over the full height of the tower necessitated careful analysis for air/wind pressures in the cavities behind the new cladding. Using wind tunnel results to model cavity pressures in a cladding system is highly complex, so in order to verify the results of the analysis, Halsall designed, installed and monitored a wind pressure instrumentation system for the existing cladding. This system comprised over 100 sensors that measured wind pressures at six zones on the building at a rate of three times a second for over a year.
The results were analyzed in cooperation with BLWTL and the findings showed that significant cavity pressures could develop across the new cladding where unimpeded air flow within the cavity is allowed to occur; this is particularly critical at building corners where cavity pressures can enhance total suction loads on the cladding elements. This knowledge necessitated the design of compartment closures and vent areas in the cladding system in order to control pressures in the cavity and on the proposed cladding elements.
Custom triple-laminated glass
The architect’s initial design concepts proposed using insulated glass units in the new cladding panels. However, there were concerns with the long-term durability or, most importantly, post-breakage behaviour of such units. As a result, a custom solution was designed with triple-laminated, heat-treated glass.
Triple-laminated glass was chosen for several reasons. It is durable and allows for redundancy in the case of any impact or breakage due to overload. It satisfied the architect’s aesthetic, which called for a bright white appearance and a dynamic frit pattern to be incorporated into the glass design. And it permitted the in-shop fabrication and assembly of unitized spandrel panels, which gave better quality control and repeatability during the field installation.
Each glass assembly is 21 mm thick and consists of three layers of heat-strengthened low-iron glass. Iron content is an important determinant of glass clarity. With its higher iron levels, regular float glass has a noticeable green tint at thicknesses exceeding 6 mm. Since the cladding on this building required the glass to be 21-mm thick for structural purposes, a very low iron content glass was selected to provide a clear appearance.
The assembly also includes:
• a white ceramic frit pattern on glass surface #2
• a clear interlayer adhesive between surfaces # 2 and #3 (clear polyvinyl butyral [PVB])
• a white interlayer adhesive between surfaces #4 and #5 (white PVB interlayer). The interlayer was to make the overall assembly whiter and to capture shadows from the frit pattern on surface #2. The result is a dynamic pattern as the sun moves across the sky.
Because such assemblies are not covered by current design standards and building codes, Halsall had to perform an exhaustive structural analysis and modeling of the triple laminate glazing under different conditions. The final design was validated through peer review by leading industry authorities in laminated glass performance.
At the higher wind load zones, the span of the glass was reduced by incorporating a mullion, structurally glazed to the back of the laminated glass. To our knowledge, this application of triple-laminated glass panels is the largest of its kind in North America.
Glare effect modeling
Replacing the marble with white glass meant that the cladding surface would be changing from low to relatively higher reflectivity. This could potentially impact neighbouring buildings, public spaces and traffic, including on an expressway to the south of the building. Halsall therefore prepared a computer simulation model to quantify the impact of glare from reflected light. We found that First Canadian Place’s new fritted-glass facade produced 50-60% less glare than neighbouring buildings clad with reflective surfaces. Moreover, it yielded no values in excess of the “glare threshold” i.e. the level of glare that causes visual discomfort to onlookers (which the simulation defined as locations with 7x the luminance of the overall finished surface).
There is currently no code or standard applicable to glare from buildings. It is conceivable, however, that the increasing use of reflective materials, particularly glass, in urban design might precipitate future regulation and that this project’s glare analysis will prove prescient.
Construction while occupied
This is one of Canada’s preeminent commercial buildings. It h
ouses the head office of the Bank of Montreal as well as several major law firms and corporations. It provides work space for almost 9,000 people. The recladding project was planned to proceed 24 hours a day, five days a week, while the building remained fully occupied.
To assess the impact of construction on tenants and the labour crew, Halsall coordinated extensive remedial work trials in the pre-construction stage. These included exterior and interior acoustic testing during various grinding and welding activities. We conducted tenant view studies, air sampling to check for lead, VOC, and odour levels. We studied material cure times and did performance studies (including behaviour in cold weather), trials to achieve reliable welds, and testing to check the impact of welding operations on the interior fireproofing.
From design to execution, the recladding project relied on exhaustive analysis, technical innovation and extensive collaboration between the design team and other industry leaders in research, manufacturing, fabrication and glazing. The result is a state-of-the-art cladding system that fulfils the building’s owners’ requirements for both safety and aesthetics. cce
David De Rose, M.A.Sc., P.Eng., BSSO, is national building restoration practice leader at Halsall Associates, Toronto. E-mail firstname.lastname@example.org
The recladding of First Canadian Place recently won the 2012 Ontario Building Envelope Council’s Award of Distinction for Design.
Project name: Recladding First Canadian Place, Toronto
Owners: Brookfield Properties, Canadian Pension Plan Investment Board, Alberta Investment Management, Manulife
Façade engineers: Halsall Associates (David De Rose, P.Eng., Hamid Vossoughi, P.Eng., John Kosednar, P.Eng.)
Base-building structural engineers: Halcrow Yolles
Architects: Moed de Armas & Shannon (design), Bregman+Hamann
Construction manager: Ellis Don
Other key players: BLWTL/RWDI (wind consultants),
Antamax (glazing mock-up contractor)