December 1, 2011
By Gaston Doiron, M.Eng., P.Eng., and Kelly A. Henry, M.Arch, MBA, LEED AP
The Rotman School of Management at the University of Toronto (U of T) has grown substantially over the last 12 years. Consequently, this innovative business school has outgrown its downtown space, resulting in a $91.8-million expansion project that includes construction of a new 15,004-m2 (161,500-sf) building clad with a curtain wall system incorporating ultra-high-performance concrete (UHPC). (See “Innovative Bridge Decks Field-cast UHPC joints for precast bridges,” by Vic Perry, FCSCE, M.A.Sc., P.Eng., Primo A. Scalzo, M.Sc., P.Eng., PE, and Gary Weiss, P.Eng., MBA, in the September 2007 issue of Construction Canada. Visit www.constructioncanada.net and select “Archives.”) Scheduled for completion in 2012, the building will be more than double the size of the original school.
According to the project architects, Kuwabara Payne McKenna Blumberg (KPMB), the new building design is a direct expression of Rotman’s core mission to promote creativity, innovation, and integrative thinking in 21st century business education. It responds to different scales and conditions presented by the surrounding context––in this case, it mitigates between a historic 19th century residence and the massive scale of the Robarts Library. It carefully maintains views and minimizes shadow impact on one of the city’s architectural treasures––Massey College—to the east. The overall concept advances the school’s long-term sustainability and business strategy, and will meet Leadership in Energy and Environmental Design (LEED) Silver requirements. (See the project description and full credits on KPMB’s website. Visit www.kpmb.com).
The building’s exterior curtain wall includes more than 350 dark grey precast UHPC panels, just 30 mm (1.2 in.) thick. Produced by a local precaster in Brampton, Ont., these unique, ultra-thin panels vary from 0.5 to 1 m (1.6 to 3.3 ft) wide x 3.5 to 5.3 m (11.5 to 17.4 ft) high. In addition to the cladding, over 100 UPHC panels (19 mm [0.75 in.] thick) will be used to create an interior wall––an attractive and functional design feature that complements the building’s exterior.
Early in the conceptual stage, the architects conceived of panels with a specific colour and textural quality desired for the opaque sections of the curtain wall.
KPMB explained the panel colour was derived from the context of the site, specifically from the black slate rooftops of the numerous heritage houses that line St. George Street––the main artery of the university. The panels were to be an evocative element of the building’s design.
The UHPC choice was derived from its ability to achieve these features and meet structural requirements. The east façade was made to facilitate the integration of an opaque panel into a high-performance curtain wall. The esthetics of the precast UHPC was paramount in the decision as it provided the subtle matte finish within the colour range desired. The alternate material choice of a stone panel has severe size limitations, and aluminum panels are limited in finish, therefore unable to provide the same heterogeneous qualities evident on precast and stone materials.
The subtle variations––part of the cast nature of the material––add a natural quality to the panels, and their soft matte finish contrasts with the darker reflective qualities of the adjacent glass. The play of light off the façade changes throughout the day and offers a nice backdrop to Ron Thom’s Massey College.
A façade solution was developed during the early stages to meet design requirements and achieve the architects’ goals. The design criteria was ambitious because the university preferred a long-span façade panel (up to 5.5 m [18 ft]) that was thin and lightweight with a hard, durable exterior wall surface that would endure for a long time––with little to no visible signs of wear from elemental impacts. UHPC provided a very thin, monolithic-plate, slab-type design with a custom coloured and moulded surface aspect that would essentially ‘plug-and-play’ with current curtain wall framing systems, without intermediate jointing.
Through a series of sample runs and panel mockups, the final design took shape. Due to the custom-fabricated panel material, there were several iterations developed to achieve the desired result. It was a new material, so the team needed to understand the limitations and potentials before homing in on the final panel process and appearance. The material’s finish and colour were a challenge due to the extreme detail capable with UHPC, but through a careful approval process with the fabricator, even down to the grit of the sandpaper used on the formwork, the challenges were surmountable.
UHPC offers almost limitless possibilities in terms of colour and texture. It was crucial the panels’ appearance corresponded to the architects’ vision and, once the official sample was approved, the next challenge was to replicate it in a full-sized panel to verify fabrication techniques before production could begin.
Panel size and configuration
At first glance, what may look like the building’s random pattern is far from it. The panel pattern and size on the façade was derived from the following criteria:
The elevations where UHPC cladding is used mostly consists of offices and study rooms. Each office is typically 3 m (9.8 ft) wide and the panel grid is on the metre and half-metre module. Each office has a 1-m (3.2-ft) solid UHPC panel, a 1-m vision glass floor to ceiling panel, a 0.5-m (1.6-ft) panel of solid UHPC, and a 0.5-m glass panel with a full height vent. Reversing the layout of the façade between floors creates a woven, syncopated pattern that unifies the elevation and promotes the natural qualities and capabilities of the UHPC panel. The panels run floor to floor in height, with some locations exceeding 4.5 m (14.8 ft).
The design for an ultra-high-performance concrete project is based on characteristic design values associated with the specific formulation used in each project. The outline for the Rotman building was fairly straightforward. The usable tensile resistance of the UHPC allowed for thinner and more efficient precast panel cross-sections, without the standard use of steel reinforcing as in regular concrete.
Smaller precast cross-sections resulted in lower weights for the assembled curtain wall units which, in turn, allowed for easier handling, transportation, and reduced foundation requirements. The building’s overall weight reduction also decreased the possible seismic loads induced on the structure during a potential earthquake. The Rotman School of Business expansion project used exterior UHPC panels of just 30-mm (1.2-in.) thickness, with no steel reinforcing required. The thin panels were designed to withstand all possible loads within a reasonable deflection criterion.
The precaster and curtain wall manufacturer engineered the panels to be cast with inserts to ensure proper fastening. The panels were then shipped from the precaster to the curtain wall manufacturer, installed in the system, and shipped to the site as completed, assembled sections.
KPMB said the additional weight of the larger UHPC panels (by comparison to stone or aluminum panels), along with the oversized unitized curtain wall panels, created some installation challenges that were easily overcome by the façade contractor. For example, since the UHPC panels had a smooth exterior surface, they were able to continue to use vacuum cup lifters typically used only with glass. This allowed the shop processes to be maintained, helping meet the schedule and keep costs down.
KPMB said there were numerous reasons for using UHPC. Due to a constrained schedule, having a local manufacturer capable of producing and delivering the panels on time was the key to success. Additionally, the ability to bring together two trades––the curtain wall manufacturer/installer and the precaster––to work out details and work toward a common goal was also important.
Collaboration: the key to success
As with most innovative designs, strong collaboration among various key parties is paramount to the success of a new approach. In this case, the precaster was fairly new to UHPC; therefore, processes were developed to ensure a quality consistent finish for every panel. Also, because the curtain wall manufacturer had never worked with UHPC panels (inserted into their curtain wall system), the project managers had to co-ordinate the transfer of knowledge and procedures for everyone involved. This ensured the final product met and exceeded the design and esthetics parameters, which were key requirements by both the architect and owner.
“We really worked collectively to overcome the challenges of this project by fabricating various samples together and then testing them to meet everyone’s satisfaction,” said Peter Calcetas, former director of business development for the UHPC supplier.” The culmination of this collaborative effort was a full-scale, complete prototype mock-up erected on the curtain wall manufacturer’s property. Maximum wind load, pressure, and suction tests were conducted with and without rain. Overall, the system worked very well and surpassed every test.”
Other UHPC façade solutions
While the Rotman project demonstrates how ultra-high-performance concrete can be used in large-format, thin panels, there are several other design directions that may be explored when developing façades with this material. The introduction of curves, perforations, textures, and folds can initiate exciting new design elements never before attempted in façade expression. Not only is UHPC being used to change the surface, but it is also being conceived of as the unitized system itself. No longer is an architect tied to the typical extruded cross-sections of an aluminum curtain wall system––as demonstrated in an award- winning UHPC curtain wall system developed by architect Peter Arbour. (For more information on the Liquid Wall, visit www.theliquidwall.com).
UHPC’s superior characteristics do not stop with its strength and ductility––it is also extremely durable and provides a sustainable structure at the same time. The increased lifecycle of UHPC building façades results in less environmental impact over time compared to an envelope built with conventional materials. Examples of other inspirational UHPC façades include Victoria’s Atrium (For more information on The Atrium, see D. Zakariasen and P. Seibert’s “Precast Solution for Performance Cladding: ultra-high-performance concrete for B.C. building,” in the September 2010 issue of Construction Canada. Visit www.constructioncanada.net and select “Archives.”) with curved, textured elements, and a unique perforated exterior for an architect’s office (Architectes Rocheteau Saillard) in La Baule, France.
Gaston Doiron, M.Eng., P.Eng., is a project manager for Ductal, Lafarge North America’s ultra-high-performance concrete (UHPC). Based in Toronto, he is responsible for the management of Ductal projects throughout eastern North America. Doiron holds a civil engineering degree from Moncton University and a master’s in structural engineering from McMaster University. He can be reached at gaston.doiron@Lafarge-na.com.
Kelly A. Henry, M.Arch, MBA, LEED AP, is the architectural project manager for Ductal UHPC at Lafarge. Based in Calgary, she is responsible for architectural projects across North America. Henry holds a bachelor of science in Microbiology from the University of Florida, and masters of architecture and MBA degrees from the Georgia Institute of Technology. As an architect, she has worked with building information modelling (BIM) technologies, and held an adjunct professor position at Georgia Tech in BIM Theory. Henry can be contacted via e-mail at kelly.henry@Lafarge-na.com.
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