Employing wood in commercial buildings

by nithya_caleb | January 9, 2020 11:19 am

by Jim Taggart, FRAIC

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In 2009, the British Columbia Building Code (BCBC) was amended to permit residential buildings of up to six storeys to be constructed in wood. Since then, through a continuous process of consultation and research, the potential for expanding these provisions to other building occupancies has been under consideration at the national level.

Changes introduced in the 2015 edition of the National Building Code of Canada (NBC)—when it is adopted by the provinces—will expand these provisions to office-type buildings, but also permit mixed-type occupancies on the first two storeys. As a result, wood buildings may now include office, residential, mercantile, assembly, low-hazard, or storage/garage-type uses. As of 2018, such buildings have been permissible under BCBC.

This article examines two wood buildings, both with primary retail, commercial occupancies, but employ different mass wood products to achieve varied effects. A more in-depth look at these projects can be found in a new case study[2] released by the Canadian Wood Council/Wood WORKS! BC.

Askew’s Uptown Supermarket

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Located in Salmon Arm, B.C., a city of about 20,000 residents midway between Vancouver and Calgary, Askew’s Uptown Supermarket is the first phase in a new mixed-use development that is destined to become the commercial heart of a fast-growing residential neighbourhood.

The high side of the sloping site borders a frontage road paralleling the Trans-Canada Highway. This slope presented challenges in establishing suitable grades for buildings with large floor plates and for engineering conventional approaches to vehicular access. In response, Allen+Maurer Architects (who also created the master plan for the site) inverted the usual suburban supermarket arrangement. Instead of a box surrounded by parking, they pushed the 3000-m² (32,291-sf) building to the edge of the site, tucking it into the slope. As future phases are completed, they will frame a village square at the centre of the site.

The supermarket’s design responds to priorities of the client, the needs and aspirations of the community, and the materials and construction expertise available in the region. The forest products industry has been a traditional mainstay of the local economy, although it has recently experienced a decline. With family roots in the forest industry, architect Chris Allen saw this project as an opportunity to fight back. Similarly, as a service provider to that industry for several generations, Askew’s Foods wanted to make its project a statement of community support by maximizing the use of local wood products and labour.

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The resulting building is both simple and elegant—a radial plan generated from the centrepoint of the future town square, slender concrete and steel tree columns for maximum flexibility, a fully glazed front façade, and a vast ‘floating’ wood roof was constructed using nail-laminated box beam elements prefabricated from locally sourced wood.

With a design snow load of 0.5 psi (3.5 kPa), a traditional wood roof for a building of this scale would have consisted of glue-laminated (glulam) beams and purlins with heavy timber decking above. Such a roof would have required considerable depth, and the lattice of heavy timber members would not have achieved the floating effect desired by the architects. Additionally, for consistency of appearance, the tapering structural bays would have required glulam beams of equal depth, despite variations in span from 5 to 10 m (16 to 31 ft).

Instead, for economy, efficiency, and improved esthetics, structural engineers Fast + Epp designed the 3000-m² roof of the building as a series of 1200-mm (47-in.) wide nail-laminated timber (NLT) box panels, supported on a primary structure of steel beams. These beams rest on tubular steel branches that spring diagonally from cylindrical concrete columns as noted above. Lateral resistance in the vertical plane is provided by steel cross-bracing along the south wall, long shear walls on the east and west sides, and two short sections of shear wall on the north side of the building.

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The NLT box beams have a consistent depth and continuous soffit treatment, but with variable joist spacing according to the length of span. The joists are regular 2×12 sections, used singly or doubled up for panels spanning up to 7 m (24 ft)—the longest available section. For the 10-m spans, 7 and 2-m (8-ft) sections were overlapped longitudinally and nailed together face-to-face in pairs to form beams. All the panels were finished with one layer of 13-mm (¹/₂-in.) plywood sheathing on top, while the spaces between the joists were filled with alternating 2×4 and 2×6 members at the bottom.

A 300-mm (12-in.) gap was left between panels to conceal sprinkler lines. These gaps are bridged by plywood on top and have corrugated cover panels beneath made from vertically staggered 2×3 members to match the main panel soffits. The panels use a total of 580 m³ (20,483 cf) of locally harvested dimension lumber and 1320 sheets of plywood.

The lumber was sourced from five regional mills to meet the demands of the schedule and to allow for competitive pricing. Shop prefabrication of the wood components also enabled the work to be carried out over the winter, providing off-season employment for carpentry crews. The simplicity of the design meant special tools or skills were not required.

The finished panels were hoisted onto the steel beams in the summer; there was no need for painting or electrical work. Sprinkler installers were able to kneel on the roof to drop the pipes in the provided slots, rather than working above their heads from scaffolding in the building. A second layer of 13-mm plywood was installed in the field to create a roof diaphragm.

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The panels did not contain insulation. Instead, continuous rigid insulation was laid on top of the field-installed plywood sheathing before the roofing membrane was applied. The vapour retarder is also located on top of the plywood sheathing. The maximum volume of the voids within the panels is less than the threshold for sprinklering of concealed spaces.

Since the lumber used has simply been cut from raw logs and not undergone any further processing, it has very low embodied energy and the benefits of natural carbon sequestration and storage within the wood fibre are maximized. It is estimated the roof structure contains 546 metric tonnes of embodied carbon. In addition to the tangible benefits of low-carbon construction and local employment, the form, materials, and organization of the building express the unique qualities of the site, and enhance the role the building plays as a social hub for the community of Salmon Arm.

Doubling as a retaining structure, the south wall of the building reconciles the change in level between the street and the future urban square below. The sweeping roof with its exposed Douglas fir soffit creates a warm ambience while the expansive windows admit abundant daylight and offer views of the surrounding mountains. The projecting eave soffit is protected by a flashing on its leading edge.

Perhaps appropriately for a grocery store, the design of Askew’s, in some respects, parallels the ambitions of the slow food movement. Just as slow food has promoted the virtues of quality and taste over the agri-business model of uniformity and convenience, the design of Askew’s puts the ecological, economic, and cultural sustainability of the community first.

Whistler Community Services Society building

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The Whistler Community Services Society (WCSS) is a nonprofit providing a variety of social services to the residents of the Resort Municipality of Whistler, B.C. WCSS operates a food bank and a range of social and community programs, partially funded by a ‘Re-use It Centre’ that sells second-hand sports equipment and other donated items.

Rapid expansion of its retail operations and community services resulted in WCSS programs being spread across multiple locations, thus increasing operational inefficiencies and overheads.

With the complexities of a mixed-use program, including high-clearance storage areas and retail and office spaces, together with a desire to be close to the town centre and accessible by transit as well as private vehicles, WCSS concluded the best option for consolidating its operations was to construct a new building on a vacant site in the municipal works yard.

Due to the heavily trafficked, industrial nature of the site, the WCSS board decided the building should be constructed by using the traditional combination of a steel frame structure, with tilt-up concrete (structural insulated) panels for the exterior walls. AKA architecture + design headed the project’s design team and Fromme Engineering was the structural engineers.

The project went through the schematic design and design development phases, received both development and building permits, and was put out to tender as a construction management contract based on this steel and concrete structural system. However, it soon became apparent the concrete structural insulated panels (SIP) supplier was unable to meet the required construction schedule.

Whistler has long, cold winters with significant snowfall, restricting the construction season for concrete work in particular to the six months from May to October. Faced with this untenable situation, the WCSS board instructed the design team to investigate the feasibility of a mass wood alternative.

AKA architecture + design brought in a cross-laminated timber (CLT) fabricator (Penticton, B.C.) who recommended Fast + Epp design the wood structure. The first priority was to determine whether a wood alternative could be designed and delivered within the client’s tight time and cost constraints. From a cost perspective, the design team had a known tender price for the steel and concrete building as a benchmark.

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In the end, the two solutions were almost identical in cost. Poor soil conditions meant large foundations were required for the concrete structure, and these could be substantially reduced for the lighter wood option. Since the CLT exterior wall panels could be loadbearing, it was possible to eliminate the perimeter columns required in the steel and concrete option. Additionally, interior finishes could also be removed in some places, as the CLT panels could be left exposed.

The mass wood structure is as simple as possible, with vertical CLT panels, a glulam post-and-beam interior frame, CLT ground and upper floors, and a glulam and CLT roof. The floor-to-floor heights had to be increased slightly, as services designed to run through open web steel joists now had to be suspended below the glulam beams.

The CLT panels were exposed in the counselling rooms and in the storage areas, where racks could be secured directly to the walls. For this project, the client was happy to specify a less expensive industrial grade of CLT, rather than an architectural ‘appearance’ one, and to leave building services exposed.

With only three months between the award of the contract to the required start date onsite, speed was of the essence. According to Carla Dickof, senior technical specialist at Fast + Epp, “The dimensions of the building as originally designed made it perfect for CLT.”

Both the height and width were just short of 12 m (39 ft)—the maximum length of a CLT panel that is currently available. It also happened to be the maximum permitted height of the building given the pre-existing development permit. The five-ply CLT wall panels were aligned vertically, running the full height of the building from grade beam to eaves, as in balloon frame construction. The elevator and stair shafts were constructed in the same way. This meant the panels could be lifted off the delivery truck and dropped directly into place by Seagate, the mass wood subcontractor. The wood at the elevator and stair shafts were also fireproofed with fire-resistant drywall.

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The exterior wall panels had dimension lumber ledgers pre-installed to provide bearing for the five-ply CLT upper floor panels. At mid-span, the floor panels bear on a pair of glulam beams running either side of a central line of glulam columns.

The lateral system uses the exterior walls along the 34-m (112-ft) length of the building, with the panels connected by plywood strips nailed across the joints. For these tall panels to contribute to the lateral system, a creative interpretation of the Canadian Standards Association (CSA) 086-14, Engineering design in wood, was required (CSA 086-14 requires the aspect ratio and platform-framed approach when using the noted ductility values. However, it does provide an alternative approach with lower ductility if the criteria [such as aspect ratios, sliding of panels governing, etc.] are not met. This is a very conservative approach but does offer a path forward for alternate approaches to CLT structures [such as ‘balloon’ style construction] within the current code limitations.). Across the width of the building, elevator, and stair shafts contribute to lateral stability.

For lateral load distribution, a simple envelope approach was used, considering both rigid and flexible diaphragms. Diaphragm action was achieved by ‘stitching’ the floor and roof panels together using strips of plywood to bridge the seams—the same detail as used for the exterior walls. The revised foundation design is a raft slab, with short perimeter walls to create a crawl space. These concrete walls support both the CLT floors and exterior walls.

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The building program is arranged on three levels. The first (ground) level is largely devoted to the sale of second-hand items, with receiving, storage, and sales areas, and seasonal storage is located on the second level (mezzanine) above. The food bank, counselling, and other community support programs operate out of offices and meeting rooms on the third level.

Since the mezzanine occupies more than 40 per cent of the floor area in which it is located, it is not considered a mezzanine, as defined in BCBC. Therefore, the structure is deemed to be three storeys in height. In the specific case of WCSS, the higher hazard retail-use had to be located only 1.5 m (5 ft) from the north property line. Vancouver-based engineering firm GHL Consulting devised an alternative solution that permitted the use of CLT within the exterior wall assembly in the locations where a two-hour noncombustible wall assembly was prescribed by the building code (see “Firewall Alternative Solution,” page 12). Had the mezzanine conformed to the BCBC definition, the building would been subject to less stringent fire performance requirements.

This is the first retail building of its type in Canada. It demonstrates the potential of mass wood construction to compete in this market sector, with comparable costs and enhanced environmental performance relative to ‘traditional’ steel and concrete construction. The wood solution is, in the author’s opinion, cost competitive and highly repeatable.

FIREWALL ALTERNATIVE SOLUTION

For the three-storey structure of the Whistler Community Services Society (WCSS) in Whistler, B.C., Vancouver-based engineering firm GHL Consultants provided an alternative solution to address the exterior wall construction due to the proximity of the building to the property line, which is as little as 1.5 m (5 ft) in some places. Given the higher fire load for the retail occupancy and the size of the fire compartment in this location, the British Columbia Building Code (BCBC) 2012 prescribes no more than 10 per cent of the wall adjacent to the property line be unrated window openings and/or wall area. The remaining (minimum 90 per cent) of the wall assembly must have a two-hour fire-resistance rating and be of noncombustible construction. The functional objective of this requirement is to limit the risk of a fire in the WCSS building spreading to the neighbouring property before emergency responders could perform their duties (e.g. fire suppression and wetting of nearby structures). This could be due to the fire spreading to the adjacent property through a window opening or unrated wall assembly or through ignition of the wall itself. The alternative solution permitted the use of cross-laminated timber (CLT) within the exterior wall assembly in the locations where a two-hour noncombustible wall assembly was prescribed by the building code. The solution demonstrated a non-loadbearing CLT wall could achieve the required level of performance. This was achieved using a char analysis and specifying mineral fibre insulation and a noncombustible cladding on the exterior side of the CLT. Similar measures could be used on future projects to address the same situation, if reorganizing the program or increasing the distance from the property line were not viable options.

 

[11]Jim Taggart is a Vancouver-based journalist who has written on the subject of contemporary architecture in wood for over 20 years. His credits include more than 100 articles for national and international magazines, numerous technical case studies for wood industry organizations, and the books Toward a Culture of Wood Architecture (2011) and Tall Wood Buildings: Design, Construction and Performance (with Michael Green, 2017). He can be reached via e-mail
at architext@telus.net[12].

Source URL: https://www.constructioncanada.net/employing-wood-in-commercial-buildings/

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