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In a span of two and half years, more than 300 groups of visitors came to tour, learn from and exchange ideas about the mass timber educational building going up on the George Brown College campus in Toronto. “Early on, we understood that George Brown really wanted to showcase this project more so like a living lab,” says Brian Fowler, senior project manager for PCL Construction, general contractor for Limberlost Place.

Visitors traveled from as far as Japan and Dubai, and as close as the campus itself. “It could have been city officials, it could be professors from other institutions, it could be our competitors or other general contractors who wanted to see what we’re doing. We were open and willing to discuss what was working for us, what wasn’t working for us,” says Fowler. “It told the story right from the beginning.”

There are many stories to tell about Limberlost: how the largest mass timber building of its kind in Ontario evolved, overcoming design, engineering, code and construction challenges along the way; and how teamwork became the way to inform and expand what was possible on many of those fronts.

Open spaces on many floors provide students options for interacting or relaxing.
Photo by DoubleSpace Photography

And there is the story the building itself tells, both as a standalone and as a model to be copied. “We wanted to generate inspiration for others to actually see the value, and the opportunity to transfer the [structural] system, once we came up with that elegant system, to understand how we carved into it a journey of the life of the building,” says Carol Phillips, partner at Moriyama Teshima Architects, which designed Limberlost in partnership with Acton Ostry Architects.

The net-zero carbon structure opens to a three-story-tall light-filled atrium, one of many open spaces throughout, and is topped with a sloped roof. It houses the college's School of Architectural Technology, Brookfield Sustainability Institute, an Indigenous student center and a childcare center.

Structural engineer Fast + Epp devised the building’s composite timber-steel-concrete slab band system and steel core. “We wanted to try something different with using a steel brace frame core.... It’s like a prefabricated kit coming alongside the prefabricated timber, and it was a raging success,” says Robert Jackson, partner at the structural engineering design firm.

Inspired by cast-in-place concrete parking structures, Fast + Epp applied the concept to a “re-imagination of that in hybrid mass timber,” for the slab bands, says Jackson. The system uses 8 ft by 30 ft cross-laminated timber panels joined with concrete placed over kerf plates and reinforcing steel. The composite slab band panel then connects to glulam wallums—or “wall-like columns.” The use of shallow slab bands allowed the design team to fit 10 stories within a 38-meter height limit. Other advantages included cost savings on the envelope and minimized mechanical system obstructions. The wide spaces enabled by the system achieved the architectural goal of allowing for easy future changes to classroom size using partitioning.

PCL joined early in the design phase, and during preconstruction consulted with mass timber supplier Nordic, the structural steel supplier, and the mechanical and electrical trades, whose input helped with project budget and sequencing—including the decision to prefabricate the slab bands off-site to save on time, cost and availability of shoring that would have been required on site.

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Team dealt with tight tolerances, tricky sequences for feature staircase (top left), bridge (top right) and glulam columns (bottom).
Photos courtesy PCL

‘LEGO Instructions’

Using a 4D schedule, PCL ran several scenarios before deciding on erecting two stories of prefabricated steel at a time, following with the timber. Because each piece needed to fit in a specific order, “we created a visual set of plans that we would call LEGO instructions,” says Mike Love, senior superintendent at PCL, which determined the pieces that needed to be trucked on site and combined with columns for each day.

Poor soil conditions and high water table led the team to build a raft slab with concrete caissons reaching 12 m to bedrock. Decentralized mechanical systems led to usable space in the peaked roof.
Drawing by Moriyama Teshima Architects

PCL worked with its Denver office, which had previous experience on mass timber projects, and had its crew spend a week training with the local carpenters union to make sure the massive pieces, with the composite slabs each weighing 17,000 lb, were handled safely and efficiently. “Before we even got on site, we knew how these things were going to be rigged,” says Fowler.

A pedestrian mass timber truss bridge that would connect an existing building to Limberlost’s fifth floor had also originally been planned for prefabrication at the facility, but when it was determined it would not clear the path to site, the team decided to “just go to basics,” says Fowler. 

Using a kit of parts, team members erected the bridge alongside the spot from which it was then lifted and set into the building. “It nestles into the new building rather than hangs off the old building,” says Fast + Epp’s Jackson. This required the crew to “rotate it and then drift it inside the building a few feet, then continue rotating as we straightened it out … dropped it into place and pulled it back outside the building. We talked about this for a year and a half prior, and then in one day, as we all kind of stood there, watching, holding our breath, it got dropped into place pretty quick,” says Love.

When design for the project began, Toronto’s building code only allowed for mass timber structures of six stories, requiring the design team to obtain alternative compliance. Needing to prove that the system was at least as safe as a concrete or steel building, the design team increased the fire rating to two hours from the necessary one hour. “Rather than working by the prescriptive letter of the code, we proved the intent of the code was met, and in fact, exceeded [it] to assure that this is a very, very safe building,” says Phillips.

Key to preventing delays was an iterative process the architects engaged in early with the City of Toronto so officials could review and comment, with the feedback then reflected in changes made to subsequent designs.

The city code has now increased to eight stories for assembly occupancy buildings, shorter than Limberlost and with less allowance for exposed wood than its 50%. “It still remains a bit of a demonstration piece, but it nevertheless certainly impacted the changes to the code,” says Phillips.

Three-story glulam columns showcase the atrium, central staircase and seating area. Daylighting studies and models led to window placements that balanced outside views with energy performance. High-to-the-slab windows make it possible to achieve deep light penetration.
Photos by Doublespace Photography

Use of nature extends beyond mass timber, which was sustainably sourced from the structure’s namesake, Ontario’s Limberlost Forest. Phillips adds, “We really looked at our site with our engineers, our sustainability, our mechanical, electrical and structural engineers, and said: ‘What are the parameters that make this work … in the most sustainable way?’” The sloped roof is designed to get maximum value out of solar panels positioned there, providing about 20% of the building’s energy. Heating and cooling are provided by Enwave’s Deep Water Lake system, with Lake Ontario water running in a looped system.

The ventilation system runs separately, and two solar chimneys create a stack effect to help the building achieve completely natural ventilation about 30% of the year. When that happens, “it’s seamless,” says Phillips. “You can even feel the same kind of breezes that you feel outside.”

Seamless may also be an apt way to describe the project’s collaborative ethos. It was a “great representation of true teamwork by multiple people to get this building across the line,” says Jackson. “That’s not to anyone’s … specific credit but to the team that pulled it off.”