Integrating mass timber in low-rise construction

by arslan_ahmed | September 8, 2023 4:00 pm

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By Jim Taggart

With operating energy reductions now incentivized by Passive House Canada and zero-carbon building programs, alongside incremental improvements mandated in legislation such as the BC Energy Step Code, increased attention is being given to the embodied carbon in buildings. In this regard, mass timber products and systems offer reduced carbon impacts, shorter construction times, and enhanced building performance. In British Columbia, design and construction professionals have embraced technologically advanced mass timber as a new paradigm for low-rise commercial construction.

Mountain Equipment Co-op (MEC) flagship store

In Vancouver, MEC’s new store uses cross-laminated timber (CLT) and glulam as part of a holistic approach to sustainable design. MEC had retained Proscenium Architecture + Interiors of Vancouver and Aedifica Architecture + Design of Montreal for a rebranding exercise to integrate its corporate values with the architectural expression of its stores.

Marking the southeast entrance to Vancouver’s Olympic Village neighbourhood, both MEC and the City of Vancouver wanted this project to be special. Counter to the prevailing trend, it was agreed to down-zone the site, so the store itself would be highly visible, rather than forming the podium of a high-rise structure.

The result is a striking wood and glass structure with 5,730 m² (61,700 sf) of retail and office space, expanding more than three stories, with three levels of parking below grade. Below-ground loading was also required, as the lane at the north side of the building is considered a secondary street and has townhouses facing it. While physically terminating the mid-block lane, the building responds with a glass wall, providing views to the interior atrium, a linear, double-height space around which the store is organized.

The store entrance is on the south side, beneath a projecting CLT canopy that extends the length of the building. The canopy is supported on glulam columns lifted off the sidewalk on concrete plinths as protection against weather. At ground level, the elevations facing the street are predominantly storefront glazing, making this a bright and engaging “gateway” building.

On the upper retail level, the south elevation is split by the glazed atrium, while the west elevation includes a perforated metal mural. Next to the mural, a wide Corten steel scupper guides water from the green roof into a bioswale planter at street level, before discharging it into nearby False Creek.

The interior of the store is bright and airy, extending out to either side of the central daylit atrium. The glulam post-and-beam structure and CLT floors are exposed. An open steel staircase connects the floors, while steel bracing elements contribute to the lateral system. With its carefully routed mechanical and electrical systems, the esthetic is industrial, softened by the warm tones of the CLT display systems and the colourful products showcased.

The third-floor office has a smaller footprint than the retail floors, with setback from the building perimeter creating the opportunity for areas of green and blue roofs. On this level, the wood structure remains exposed, but the building services are run within a raised floor system, finished with concrete tiles to meet the required 45-minute fire resistance rating.

The rectangular site is approximately 62 m (203 ft) in width and 36 m (118 ft) in depth, it is flanked by 2^nd Avenue on the south side and Quebec Street on the west. The underground parking garage is of concrete construction, with its 275-m (10.8-in.) thick roof slab becoming the ground floor of the mass timber building above.

The atrium creates an interconnected floor space requiring sprinklers and a smoke curtain. Rather than using glass curtains, the glulam beams at either end of the atrium were increased in depth to match the longitudinal beams, creating the continuous barrier required by the BC Building Code (BCBC) as a smoke curtain or draft stop. The large size of the atrium necessitated an alternative solution, as did the proprietary firestop system used to seal penetrations through the CLT structure.

The structure

Structural engineers Fast + Epp designed the gravity system as a glulam post-and-beam frame, with CLT floor panels spanning between beams. While the structural grid for the retail floors is rectangular and most often measures 7.5 m (24.6 ft) east to west, the spans vary considerably in the north-south direction. The longest is 8.4 m (27.5 ft), requiring beams that are almost 1 m (3.3 ft) in depth. The glulam columns vary in size, the largest being around the atrium. With the requirement for a 45-minute fire resistance rating, all the glulam elements are oversized to include a charring layer of approximately 30 mm (0.7 in.) on each exposed surface, protecting the integrity of the structural section within.

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The posts are storey-height, with the beams connecting to their vertical faces. This enables the bottom of the upper floor posts to bear directly on the posts below, eliminating cross-grain material from the vertical load path and minimizing the shrinkage effects. The connections between upper and lower posts use embedded steel rods glued in place with epoxy. Post-to-beam connections vary according to location. Where appearance is critical, concealed connectors are used. These steel connectors consist of two steel plates, one screwed into the face of the post and the other into the end of the beam. When brought together, they interlock to complete a concealed connection, protected from fire by the surrounding wood. Where the connections are not visible, more traditional steel-bearing plates have been used.

The floor consists of seven-layer, 245-mm (9.6-in.) thick CLT panels laid in the east-west direction and typically spanning 7.5 m (24.6 ft). The roof panels are also seven-ply as they support the soil and plant material forming the green roof.

The floor and roof panels bear directly on the beams and are secured by long stainless-steel screws. Plywood splines are used to bridge the joints between panels, enabling the floor system to act as a diaphragm and contribute to the lateral system in the building. The 25-mm (1-in.) thick plywood splines are rabbeted into the top layer of the CLT and screwed in place, as no concrete topping is used.

The floor system connects to two steel stair cores in diagonally opposite corners of the building. Other vertical elements contributing to the building’s lateral stability are concealed within partition walls. Buckling restraint braces are in the east-west walls of the stair cores, and where needed within north-south walls.

Where bracing was needed and could not be concealed, exposed steel “omega” bracing is used. These braces run vertically through the building and consist of two sculptural steel elements with a connection between them that provides the necessary ductility. A custom solution of a more traditional eccentrically braced frame, the name was coined by the design team because of the resemblance to the Greek letter “Ω.”

Detailed co-ordination was required between the structural engineer, architect, and mechanical consultant, as there were several locations where sprinkler pipes had to be threaded through glulam beams. Generally, the strategy for integrating building sprinkler systems was to run the main lines along the narrower structural bays at the rear of the building, where the shallower beam depths enabled them to be partially concealed when viewed obliquely from below. Only small diameter branch lines had to be run below the main beams.

The goal

The goal for this project was a flagship store that captured the outdoor spirit and responded to the environmental concerns of the MEC community. As Hugh Cochlin of Proscenium states: “The proximity to cycling paths, the seawall running route and kayaking opportunities on False Creek, the store benefits from the energetic and active environment literally all around it. The architectural expression, with its sloping green roofs, blurring of the indoor-outdoor experience, tall wood structure, and images of the mountains beyond speaks to MEC’s outdoors spirit and acts as a perfect gateway to the neighbourhood.”

Fast + Epp home office

Just a few blocks from the MEC store, the new Fast + Epp home office also uses CLT and glulam in an elegant and economical solution, reflecting the design philosophy of the structural engineering practice.

The four-storey mixed-use building is in an eclectic light-industrial area which has undergone dramatic transformation over the past decade. The 37- x 13.7-m (121- x 45-ft) site is zoned for a floor-space-ratio (FSR) of 3.0, out of which 1.0 must be an industrial use located at street level. The presence of a 1.2 m (4 ft) right-of-way reduced the width of the site, forcing a portion of the industrial use to the second level and making vertical fire separations necessary.

Below grade, the reduced width required the elimination of interior columns in favour of a clear span, post-tensioned slab, to accommodate two rows of parking with a central aisle. This influenced the design of the above-ground structure, where clear spanning glulam beams informed both the subdivision of space and the routing of exposed building services.

These constraints required a rigorous design response, leading to simple, practical, and economical solutions in their use of space and materials. This resonated with Fast + Epp, both client and structural engineer for the project, and with f2a architecture.

To maximize leasable area within the FSR and height limits, floor-to-floor heights were carefully manipulated according to use; level one being 4.8 m (16 ft); levels two and three being 3.6 m (12 ft) and the level four penthouse being 2.6 m (8.6 ft). There
is an interconnected floor space (IFS) between levels three and four. There is a two-hour fire separation between industrial and office occupancies, with one hour required for the other floors and supporting structure.

The IFS forms an atrium, serving as a meeting area and social space for the office. The lower level has a small kitchen, while the upper level accommodates “touch down” workstations—which are common unassigned desks in open plan offices—and, being smaller than the lower floors, it has access to a roof terrace.

Egress stairs, elevators, and a vertical service shaft are in the southeast and northeast corners of the building, adjacent to the lane. These form bookends to the north-south distribution bulkhead, where most of the north-south services run under the transverse glulam beams. East-west electrical and mechanical services run in the spaces between these beams, making them as unobtrusive as possible.

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Incorporating CLT panels

The east wall along the common property line is made of solid CLT panel construction, supplied with cladding clips and girts, external mineral wool insulation, and a vapour barrier membrane. Metal cladding was also applied in the field. A 40 per cent glazing-to-wall ratio, permitting much of the south and west walls to be glazed, was achieved through energy modelling.

Electrochromic glass was used on the west facade, providing protection from solar heat gain and glare without the legal and maintenance challenges of fixed shading, which would have encroached into the city right-of-way. Automatically adjusting tin throughout the day, this glass maximizes natural daylight, maintains views, and controls solar heat gain and unwanted glare. It also reduces the installation and maintenance costs associated with interior blinds. To further increase light and views to the exterior, glulam beams are supported on the west side by slender steel hollow structural sections (HSS) columns.

As noted previously, the superstructure is built on top of the post-tensioned concrete slab that forms the roof of the parking garage. The entire east wall of the building consists of two-storey high, vertically oriented five-ply CLT panels connected at their vertical and horizontal joints using plywood splines screwed to the interior surfaces. Where the joints between the panels are visible, a neater, half-lap connection is used. This wall is connected to the CLT stair cores at the northeast and southeast corners of the building and has glulam pilasters integrated into it at 3-m (10-ft) centres, corresponding to the structural bays. Since CLT is a combustible material, this assembly required the code consultant to write and submit an alternative solution to demonstrate an equivalent level of fire safety to the non-combustible construction mandated by the code.

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Anchored by the stair cores at either end, this composite CLT and glulam wall takes all the shear forces in the north-south direction. In the east-west direction, CLT walls of the stair cores have seismic dampers recessed into them. These dampers use a resilient slip friction joint (RSFJ) and act as a hold-down and energy dissipation system simultaneously. The devices enable the shear wall to rock, then restore itself to its original configuration without the plastic deformation which most seismic resistant systems are designed for.

While CLT shear walls are codified, the damper system is not and had to be covered by an engineering letter of assurance. Additional steel cross bracing is used in the east-west direction north of the building to counteract torsional forces.

The floors consist of 105-mm (4-in.) thick three-ply CLT panels that span 3 m (10 ft) between the main glulam floor beams. The panels are attached to the beams using long stainless-steel screws, set at opposing 45-degree angles. The panels are then fastened together with 25-mm (1-in.) thick plywood splines, nailed in place to create a floor diaphragm. The panels are covered with a 13-mm (0.5-in.) acoustic mat that also acts as a membrane to prevent moisture damage from the 50-mm (2-in.) concrete topping. The roof is also constructed using three-ply CLT, although in this case, the panels are only 89 mm (3.5 in.) thick.

The steel columns supporting the glulam beams at the west facade extend the full height of the building. They are coated in intumescent paint to achieve the two-hour fire resistance required on the ground floor and the one-hour fire resistance required on all but the top floor above.¹ Neither the roof, nor the structure supporting it are required to have a fire resistance rating; therefore, no intumescent paint was used on this level.

Steel chases carrying the cabling for the electrochromic glass are attached to the columns, acting as drag straps tying the horizontal and vertical elements of the structure together.

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The goal

Architect Austin Hawkins of f2a Architecture believes this project represents a new approach to commercial building design. With its combination of panelized prefabrication, greater emphasis on non-toxic materials, and integrated digital technology, it is a step toward what American author John Greer refers to as the “Ecotechnic Future” – one that is simultaneously closer to both technology and nature.

PH1 – 1 Lonsdale Avenue

This is a small restaurant and office infill project in the Lower Lonsdale district of North Vancouver that employed virtual construction and off-site prefabrication to meet access and constructability challenges. The 7.6 m (25 ft) wide corner lot faces Lonsdale Avenue to the east and a service lane to the west, while the north side abuts an adjacent property and the south side faces the new Polygon Gallery and beyond it, Vancouver Harbour.

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Originally an area of waterfront warehouses and marine service facilities, the neighbourhood has been transformed over time to a high density, mixed-use community centred on the Lonsdale Quay Market and Seabus Terminal. The consolidation of land required by the introduction of higher density zoning had left lots like this exceptionally difficult to develop.

As a family-owned property for three generations, the client was waiting for the right opportunity to do something special on the site. The idea of combining Passive House performance with modern mass timber construction was enthusiastically received, despite the many challenges and uncertainties it presented.

A Passive House approach

A waiver of the on-site parking requirement made it possible to design a three-storey building with a ground floor restaurant and two storeys of offices above that would achieve the full 2.53 FSR permitted by the zoning. To achieve a three-storey building, the project made use of exemptions applicable to the extra thick walls used in Passive House construction. However, the 92 per cent site coverage eliminated the possibility of an on-site staging area for materials and equipment typically required for site construction.

Hemsworth Architecture and Equilibrium Consulting structural engineers were confident a mass timber structure could be prefabricated off-site, but as far as they knew, there was no precedent for a prefabricated Passive House compliant structure built without any setback from a shared property line.

Architecturally, the concept was to use the traditional warehouse vocabulary of an exposed heavy timber structure with brick cladding, but in a contemporary way. This strategy has translated into an exposed glulam post-and-beam structure with CLT floors, stairs, and elevator shafts.

The non-loadbearing brick cladding at the southeast corner of the building is “holding back” and was replaced with large areas of glazing, providing restaurant patrons and office workers with an oblique view of the harbour. The remainder of the south facade includes extensive glazing at the ground level, with a staggered pattern of vertical windows on the upper floors.

While the code permitted the three exterior walls facing the streets and lane to be of combustible construction, it required the north wall abutting the adjacent property to be non-combustible. Such walls are typically built block-by-block in concrete masonry, a method incompatible with Passive House performance. A more sophisticated solution was clearly required, one in which the continuous exterior insulation and vapour barrier for Passive House performance could be installed without accessing the outer face of the wall in the field.

Using an integrated design process involving the architect, structural engineer, building envelope consultant, contractor, mass timber fabricator and installer, a prefabricated and pre-insulated wall system was devised. Alternative detailing, assembly, and installation strategies were explored and optimized using virtual construction.

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The chosen wall system comprised full-height CLT panels, with a 75 mm (2.9 in.) wide vertical rabbet cut into the inside face of both edges. The panels were laid flat, and the air/vapour barrier was applied to the outside surface, wrapped around the edges and into the rabbets on the inside face; then strapping and 200 mm (8 in.) of insulation was added.

As part of a carefully orchestrated construction sequence, the completed panels were lowered into place by a crane. The vapour barrier was then completed from the inside by sealing the exposed ends (previously wrapped around the panel edges into the rabbeted surfaces) with a second layer of the barrier material across the panel joints. Adjacent panels were connected structurally using a continuous plywood spline, set into the rabbeted joint and screwed in place. On the exterior, metal cladding was field installed on all parts of the wall not obstructed by the adjacent building. Being combustible construction, this wall system required the code consultant to submit and certify an alternative solution.

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The use of glulam

The foundation is a slab on grade with a 100-mm (3.9-in.) upstand around the perimeter on which the wood structure sits. The primary structure is a hybrid post-and-beam frame. Glulam beams span across the 6.56 m (21 ft) width of the building. The beams are supported on glulam columns on the south side and a combination of glulam and steel HSS columns on the north side. End connections are made using steel hangers and knife plates. The floors and roof consist of five-ply, 175-mm (7-in.) thick CLT panels spanning 6.10 m (20 ft) between beams across most structural bays, with shorter spans at the east and west ends.

Panel-to-beam connections consist of pairs of long stainless-steel screws set at opposing 45-degree angles, while joints between the CLT panels use continuous plywood splines set into the rabbets at the panel edges and screwed in place. This creates a floor diaphragm that contributes to the lateral load-resisting system for the building.

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Lateral forces are transferred to the ground by two circulation cores located in the northeast and northwest corners of the building and constructed using continuous vertical CLT panels. The hold down system for the CLT panels consists of anchors, one end of which is embedded in the concrete slab, the other is epoxy-glued into the CLT.

In the east-west direction, additional lateral resistance is provided by bracing between the columns on the south side of the building. On the ground floor where there are large windows, the bracing is steel tension rods connected at the floor and ceiling level to the glulam structure by steel plates. On the upper floors, the bracing consists of diagonal glulam struts that extend from the base of the glulam posts, at an angle of 30 degrees from vertical, to the underside of the beam above. The location of these braces is carefully co-ordinated with the window pattern on the south elevation.

The integrated design process, factory prefabrication of all mass timber components, and the use of virtual construction, enabled the entire superstructure to be erected in just 10 days. With only hand tools and very few deliveries required, noise and traffic impact on the neighbourhood was greatly reduced.

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The goal

The integrated design, construction process, and the factory prefabrication of all mass timber components reduced the on-site construction time to just eight days for this entire superstructure. This, in turn, greatly reduced the disruption to traffic in a very busy area of the city.

An additional benefit to residents of the adjacent apartments was the fact that construction of the wood elements required only hand tools, which are much quieter than the various machines required for other types of construction. Architect John Hemsworth says, “People are asking for these buildings because they are visually warmer, healthier, and more environmentally friendly.”

Notes

¹ Had glulam columns been used, they would have required a 91-mm (3.5-in.) charring layer on each exposed surface to achieve the two-hour fire resistance rating on the ground floor.

[11]Author

Jim Taggart is a Vancouver-based architectural journalist and educator who has written and lectured extensively on the role of wood in contemporary architecture for more than 25 years. He is the author of the award-winning book, “Toward a Culture of Wood Architecture” (2011) and co-authored with Michael Green, “Tall Wood Buildings: Design, Construction and Performance” (2017). The book was updated in 2020 and a third edition is scheduled for publication in 2024. Taggart was also the 2012 recipient of the Premier of BC’s Wood Champion Award.

Source URL: https://www.constructioncanada.net/integrating-mass-timber-in-low-rise-construction/

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