Mass Timber and Wood Framing

The University of British Columbia Earth Sciences Building is a five-story wood structure that uses CLT, glulam, and LSL. Architect: Perkins+Will Canada Architects Co.

Photo by K. K. Law

(Left) Semi-balloon framing is more common in buildings five stories and higher; (Right) it is often used as an alternative to platform framing where fire requirements or shrinkage may be a challenge.

Photo courtesy of US WoodWorks

Designers used balloon framing to prevent shrinkage on this Vancouver Island development. Photo courtesy of Misra Architect Ltd.

Located on the southern tip of Vancouver Island, the six-story Skyline Condominiums enjoy an idyllic location and postcard views.

Designers used balloon framing to reduce the potential for shrinkage. “Normally, the wood member shrinks approximately 3/8 of an inch per floor for 12 percent moisture in the wood. To avoid that, we used plywood, joist hangers for the joists, and balloon framing,” says Pradip Misra, principal, Misra Architect Ltd. Because wood shrinks to a greater degree across the grain than parallel to grain, it's the horizontal members, such as the sills and joists than can be problematic, rather than the posts and studs. “The joists are hung from the side of the bearing walls. This eliminates the floor shrinkage in the total building shrinkage,” says Steve Hoel, P.Eng., Struct.Eng., principal, JSH Engineering Ltd.

Skyline's location is prone to humidity, seismic activity, and wind. This resulted in more than a few design challenges. Buildings in this area are required by code to withstand the most severe earthquake loads in Western Canada, and there is always a tendency to build with either steel or concrete. However, the design team and the owners went with wood. Wood-frame structures, which have numerous nailed joints, have more load paths and are inherently more ductile than those with a rigid connection—which make them more flexible and allows them to dissipate energy when subjected to the sudden loads of an earthquake. “The mass of the building also contributes,” says Sukh Johal, technical advisor for WoodWORKS!BC. “Wood buildings are lightweight, but perform well structurally.” Since forces in an earthquake are proportional to the weight of a structure, wood-frame buildings that are properly designed and constructed perform exceptionally well. Misra says that mechanical options were also a factor in seismic design, notably “hold-downs,” known as continuous tie rod systems or 1-inch diameter steel bars that extend from the concrete slab to the upper floors. “This is a self-adjusting system to keep all floors intact in the event of seismic forces.”

Wood also offered advantages in terms of cost. Developers, who opted for a six-story structure in response to the increased height allowed in B.C., compared the costs of concrete, steel, and wood in an extensive study. “A concrete-and-steel superstructure ran 20 to 25 percent more than wood, and a structured steel superstructure 12 to 15 percent more than wood,” says Biki Kang of Kang & Gill Construction Ltd. “Wood was found to be the most economical choice.”

Photo courtesy of Tien Sher Group of Companies

The Quattro 3 condominium opened in September 2012, the first multi-family six-story building in Metro Vancouver, built from wood. The project used platform framing to increase affordability and occupant comfort. Featuring 164 units from studios to two-bedrooms and one floor of commercial space, Quattro 3 is part of a larger planned community that will include up to 1,900 homes.

While experts say that mid-rise wood frame construction is too new to B.C. to make a definitive statement about the cost competitiveness of wood, what can be said is that based on projects under construction in 2011 the above-grade cost of wood mid-rise construction will reduce costs by ten percent vs. steel or concrete construction. “There's always a difference between concrete and wood in terms of affordability,” says Charan Sethi, president and CEO of the Tien Sher Group of Companies, developers of Quattro 3. “In the Surrey market, concrete units sell for $50 to $75 per square foot more than comparable wood units.” Sethi feels the mid-rise wood building benefits from lower per-unit land costs and construction costs that can then be passed along to the consumer. “We're always looking for ways of bringing affordability to the working class market,” Sethi says.

Sustainability, locally sourced materials, and ease of construction were other factors that drew Sethi to wood. Sethi says, “Construction was completed by our experienced framing crew and on-site staff. The process was efficient.”

Occupant comfort was a prime concern. Noise transmission is a performance challenge in any multi-family building and in wood construction there are several solutions to mitigate sound transmission, including greater mass, increased stiffness, sound-absorbing insulation, resilient metal channels, and structural discontinuity between noise and occupant. At Quattro, 1.5-inch lightweight concrete topping between floors was used for additional soundproofing and robust insulated double 2-inch-x-6-inch framed party wall construction was employed to minimize vibration and sound transfer between homes.

Fire experts maintain that six-story wood-frame buildings are as safe as any other form of buildings. “For a sprinklered occupied building, there is absolutely no safety issue,” Sethi says. According to Len Garis, Surrey's fire chief and president of the Fire Chiefs' Association of B.C., “the fire safety of people living in six-story wood-frame buildings should be equal to the safety in any other building.” Garis was quoted as saying, “These buildings are fully sprinklered and our research suggests to us that in a sprinklered building, a fire is likely to never leave the room of origin, has historically never left the floor of origin in a building. Where the sprinkler system needed to operate to put the fire out, it put the fire out.”⁹

Quattro 3 makes an ideal introduction to mid-rise wood buildings in Metro Vancouver not only in terms of affordability, but in occupant comfort and safety as well.

Photo by Matt Todd Photography, courtesy of WoodWorks

The Marselle is a hybrid wood and concrete structure consisting of five stories of wood-platform framing plus a wood mezzanine and a six-story concrete podium, of which, two stories are above ground. The combination of “five and one-half over two” made Marselle the tallest modern wood-frame structure in Seattle.

Marselle's main distinction lies in the way wood framing was used to maximize the value of the finished space. Two key code requirements allowed that to happen.

The City of Seattle's 2003 Building Code (SBC), under which the Marselle was built, was based on the 2003 International Building Code (IBC). Under the 2003 IBC, mezzanines were limited to 33 percent of the floor area beneath. However, SBC allowed five stories of wood construction over two stories of Type I construction plus 50 percent for a mezzanine. Mezzanine levels are not considered floors. Under the SBC, Type V-A construction allowed a fully sprinklered 70-foot maximum height for five stories while Type III-A allowed a fully sprinklered 85-foot maximum height for five stories. Zoning regulations allowed a maximum height of 85 feet. Therefore, the design team took advantage of Type III-A construction by adding a mezzanine level on top of the allowed five-story structure, to fill in the building envelope to the 85-foot maximum zoning height allowed.

The other criteria concerned construction type. According to architect Michael Shreve with PB Architects, “under the 2003 SBC, section 504.2, the allowable height for buildings of Type III-A construction in Group R-2 could be increased to five floors of wood-frame construction above two stories of Type I concrete construction and an 85-foot maximum height. The two floors of concrete construction were required to be Type I construction with a three-hour horizontal fire rating separating the wood construction from the concrete construction.”

“Once built, the top level units would have water views of Seattle's Lake Union,” explained structural engineer Panos Trochalakis, P.E., S.E., principal with Yu & Trochalakis, PLLC. “For obvious reasons, the developer wanted to take advantage of that.” The extra half-story mezzanine added about $250,000 to the construction cost of the building, but the architect and builder estimated that the added height and space increased the value of the complex by $1 million and afforded revenue-generating views on the top floors.

Attention was devoted to maximizing the strength and durability of the wood members, with architects designing for potential shrinkage. As is typical with wood-framed podium structures, Marselle's contractors used a continuous rod tie-down system with a shrinkage compensation device to limit deflection and avoid wall separation due to wood shrinkage under wind and seismic forces. They also included an expansion joint in the cladding; were careful to specify proper panel spacing in the exterior wall sheathing; and included a provision in the mechanical system to allow for movement.

The use of wood also had a positive impact on schedule and budget factors, with pre-panelized walls enabling a speed of construction that made a big difference. The panels, which ranged from 4 to 20 feet long, were pre-assembled by a local framer in an off-site warehouse, and then quickly and easily lifted into place at the job site by crane. “If you can install the walls on a building the size of Marselle in less than two weeks per floor, then you lower construction costs significantly,” Trochalakis says. “Lower construction costs were the number one reason to use wood. Wood-frame construction is virtually impossible to beat in terms of cost.”¹⁰

Photo courtesy of UBC

Billed as the most innovative high-performance building in North America, the Centre for Interactive Research on Sustainability (CIRS) at the University of British Columbia is expected to be the university's first LEED® Platinum building, and is on track to receive Living Building Challenge recognition. The building was envisioned as a new baseline in sustainable high-performance buildings, and incorporated regenerative design—an emerging alternative to current green design practices that seeks not merely to reduce harmful environmental impacts, but to actually improve both the natural environment and the lives of their human inhabitants. Mass timber played an important part in achieving these goals. CIRS features a simple structural design, comprised of a combination of glulam members, dimensional lumber, plywood, and a minimal amount of concrete. The moment frame structure was designed to create an open, column-free floor plate for flexibility of use and interior arrangements, and to allow for large openings in the walls that maximize daylight and views¹¹. The members' size and solidity make the structure non-combustible heavy timber construction under British Columbia Building Code—a designation that simplifies the building design.

In order to reinforce external charring in case of fire, designers sized all members slightly larger than required. A building-wide sprinkler system and on-site water supply and water pump were also incorporated to enable the heavy timber wood structure to meet the requirements for non-combustible construction required by code.

Designers opted to size the structure to support a live load of 100 pounds per square foot (4.7 kilonewtons/square meter), double the requirements of the British Columbia Building Code, to achieve an increased capacity that allows for additional equipment and future changes in use. Structural members were larger where larger loads required more support, notably in the auditorium where beams and columns were sized to handle both a long span over the seating and the load of the green roof. “From a structural point of view, the modern engineered materials...have increased the strength of wood so that they have a much greater structural capacity,” says Paul Fast, managing partner, Fast+Epp.

Construction of ESB, five-story wood structure at UBC using CLT. Photo by K. K. Law, naturally:wood

The University of British Columbia's Earth Science Building (ESB) at the Point Grey Campus in Vancouver is composed of two five-story wings connected by an atrium. The ESB's new five-story north wing will house the academic research, lecture, and office spaces and, unlike its neighboring concrete laboratory south wing, uses wood as the primary structural material. Designed for LEED® Gold status, the north wing is supported by glulam columns and beams; floors consist of LSL and concrete; and the roof and canopies are built with CLT. According to the University, it is North America's largest panelized wood project and its largest application of CLT to date.¹²

Composite floor system. The floors in the north wing are constructed predominantly of a composite system that was first adopted in Europe and is well regarded for its engineered technology and ability to achieve long spans. The floor system consists of 3.5-inch-thick laminated strand lumber panels topped with foamed board insulation and four inches of reinforced concrete poured in situ. Hybrid floors in Europe are often a prefabricated element; however, this being one of the first hybrid floor applications in North America, it was fabricated on site for this project.

The composite connectors are secured into slots cut into the face of the wood panels. These plates extend past the layer of the insulation to support the reinforcing bars for the concrete topping. The insulation provides an acoustic break and also acts as a barrier to prevent moisture infiltration while the concrete sets. This assembly is over 50 percent lighter than a solid concrete floor, allowing for larger spans which increase the 'loose fit' opportunities for designers. The ends of the 22-foot-long wood-composite floor panels are supported by steel beams above the first floor of the ESB project, and on glulam beams to create the ceilings for floors two through four.

The University plans to implement the floor system at its Okanagan Campus Fitness and Wellness Centre in Kelowna. ¹³

Vibration. Designers chose a wood-concrete composite floor system in part to provide a high level of vibration performance. With a sandwich of 3.5 inches of LSL, one inch of foam board insulation and four inches of reinforced concrete, the wood-concrete composite system's acoustic performance offers excellent sound absorption.

During a fire, the exterior layers of wood will char creating a layer of insulation that prevents the interior from burning, thereby maintaining structural integrity. Crosssection taken from 3-ply CLT panel protected by two layers of ½-inch gypsum board and exposed to the standard fire exposure (CAN/ULC S101) for 1 hour and 15 minutes. Photo courtesy of FPInnovations

Fire resistance/building code. As the British Columbia Building Code prescribed fire safety requirements did not allow an entire heavy timber or wood-frame construction, an engineering assessment was documented in a peer-reviewed Building Code Alternative Solution. The assessment examined how the wood construction might perform in both the pre-flashover and post-flashover fire environments. Flashover is the point in a developing fire when all combustibles within a room ignite nearly simultaneously and begin to burn. This phase of fire's growth is critical since environmental conditions are not survivable and the heat impact on walls and ceilings becomes severe. To stop fire before it reaches this phase, designers specified an automatic sprinkler system, and a fire retardant coating to alter the surface burning characteristics of the woods interior finishes, a technique used to slow or completely eliminate fire from growing across the wood's surface. Further, analysis showed the risk of fire growth and spread would inherently be limited by the building's many separate, individual offices and rooms, creating many compartments each capable of containing a small fire. The wood structure was also engineered to account for the charring of the wood members and the gradual loss of cross-sectional area. Sufficient uncharred fiber remains so the wood elements continue to resist the expected gravity loads during the fires duration.

Steel beams to wood columns. Wood performs favorably in compression circumstances when forces act in parallel to the grain, as opposed to when the grain is perpendicular to the load path. On the ESB's second level, long steel beam spans connected to wood required a butt connection in which the top flange of a steel beam is notched into a glulam column and pinned for structural support.

Wood beams to wood columns. Throughout the project, the structurally engineered timber systems used quick, safe and efficient connectors. Approved for load bearing timber construction, these connectors are designed for easy assembly of even the most complicated nodes. Like the traditional dovetail connection, these connectors consist of two parts that lock into place to create a form-fitting connection. With each connecting system designed, forces are transferred perpendicular to the installation direction for tension and compression purposes. The hardware can be factory pre-installed in the beams and columns, ultimately reducing on-site assembly time. Connectors are completely concealed, and would be protected during a fire by the insulating char layer that would form on the encapsulating wood members.

Heavy timber shear bracing. As part of the Seismic Force Resisting System (SFRS), at the end walls of each story, diagonal glulam heavy timber braces were integrated to provide controlled resistance to the shear loads across the structure. Since the glulam members themselves do not have the ability to dissipate energy, the system relies on the predictable ductility of the connections, achieved by using multiple steel knife plates with a large number of tight-fitting, small-diameter pins. The slots in the glulam where the knife-plates are inserted were designed with longitudinal slack, which leaves room for controlled movement under shear loading that allows the tight-fit pin connection to yield in tension and compression. The precision required for these sophisticated connections was enabled by prefabricating both wood and steel components with computer numerically controlled (CNC) machinery.

Full-story transfer truss connections. Throughout the second story, steel diagonal braces and steel beams, together with the glulam columns and glulam beams with an integrated concrete top plate, create a type of truss structure. The first story above grade consists of a building-wide assembly space, with the ceiling structure of the second floor supporting the entire building above it. To carry this load, full-story steel glulam hybrid transfer trusses were designed to essentially convert the entire second floor structure into a 'roof truss' capable of carrying the load of the remaining floors across the entire span of the structure below.

'Free-floating' staircase. Located within the impressive atrium linking the academic and laboratory wings is a five-story free-floating cantilevered solid timber staircase. The clean and elegant lines of the massive timber demonstrate the innovative aesthetic and structural capabilities of wood in construction. Based on a German wood-steel composite, the rigid composite of the wood with glued-in steel connectors pushes the envelope of load bearing capabilities of wood construction. Adopted from projects in Germany, this system uses similar steel plates but for the slots that are cut into the ends of the glulam slabs and glued into place with special adhesives.

A five-story ‘free-floating’ cantilevered staircase is built entirely of solid timber. Architect: Perkins+Will Canada Architects Co. Photo by K. K. Law.

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