Mass Timber and Wood Framing

New and traditional approaches reduce cost and meet code for mid-rise construction
This course is no longer active
[ Page 7 of 8 ]  previous page Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 Page 7 Page 8 next page
Sponsored by naturally:wood
CLT, LSL, Glulam in a five-Story Academic Building

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.

 

 

[ Page 7 of 8 ]  previous page Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 Page 7 Page 8 next page
Originally published in Architectural Record
Originally published in November 2012

Notice

Academies