Mid-rise wood-frame construction gets innovative

by Katie Daniel | January 15, 2018 3:45 pm

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By Jim Taggart, FRAIC
In the nine years since the British Columbia Building Code (BCBC) was revised to permit six-storey residential wood construction, architects, engineers, municipal authorities, and local fire departments have become familiar with the basic parameters of this new building type. Over the same period, market conditions and technological advances in wood products and building systems have continued to evolve, creating new challenges and opportunities.

There is an increasing interest in ultra-low energy buildings that comply with the Passive House standard, along with a growing need to explore new approaches to project delivery, particularly when building on constricted infill lots. There is also a desire to explore hybrid wood construction in anticipation of impending code changes that will permit six-storey wood construction for some Group D occupancies.

This article looks at three six-storey wood-frame projects in the Vancouver area that are addressing these emerging changes on the country’s urban development landscape. (The full case study titled Mid-rise 2.0: Innovative Approaches to Mid-rise Wood Frame Construction is now available online. Visit wood-works.ca/bc/case-studies-videos[2].)

The Heights
Scheduled for completion in early 2018, The Heights (shown above), owned by 8^th Avenue Properties, is a mixed-use residential building, with a basement parking garage and ground-level commercial space constructed in concrete, and five storeys of light wood-frame apartments above.

The project was rezoned from its existing C-2 commercial zoning under the City of Vancouver ‘Rental 100’ program, which offers incentives to new projects undertaking to provide rental accommodation for a minimum of 60 years. Since this arrangement means the developer is responsible for long-term operating costs, it makes low-energy solutions particularly attractive.

With this in mind, Scott Kennedy of Cornerstone Architecture proposed that the project pursue Passive House certification. Among some of the most important strategies for Passive House buildings are:

• the correct orientation, sizing, and shading of windows to optimize passive solar heating;
• super-insulated building envelope with minimal thermal bridging;
• airtightness to reduce heat loss through air leakage;
• reduction of ventilation rates to maintain healthy indoor air quality (IAQ); and
• recapture of heat from exhaust air using heat recovery ventilation.

The cumulative effect of these strategies is to reduce the building’s energy demand to such an extent internal sources provide much of the energy required for heating, and conventional mechanical systems can be smaller than in traditionally designed buildings.

Kennedy wanted to prove a Passive House building could be constructed using traditional methods, and worked with Doug Wilson of Peak Construction to develop details that would minimize uncertainty, risk, and additional cost.

First, he had to convince his client the Passive House approach made sense from a business perspective. Kennedy argued a conventional hydronic heating system would have a capital cost of approximately $450,000 and maintenance and repairs over the 60-year life of the building had a net present value of $150,000. If this money could instead be spent on a superior building envelope, hydronic heat could be replaced with cheaper electric baseboards, and the net cost would be the same. The advantage would accrue to the developer in the form of greatly reduced operating costs.

Cornerstone designed the building with a conventional nominal 50 x 152-mm (2 x 6-in.) wood stud exterior wall, sheathed in plywood and supporting a rainscreen system of brick veneer. These walls contain no services—such components are contained within a secondary service wall—facilitating the installation of insulation.

Inboard of the 50 x 152-mm exterior wall, and separated from it by a 50-mm (2-in.) gap, is a secondary nominal
50 x 101-mm (2 x 4-in.) wood-stud service wall. These were built after installation of a 50-mm expanded polystyrene rigid insulation on the interior face of the exterior wall. The rigid insulation was factory finished with a polymer coating and acts as both a vapour and air barrier when appropriately sealed. Sealing takes the form of proprietary tapes at joints, and caulking where necessary.

The specified slider does not exist.

The expanded polystyrene air barrier is sealed to theplywood floor, which extends out to the exterior face of the 20 x 152-mm wall, then wrapped around the floor joists and back to the top of the inner wall on the next level down (Figures 1 and 2). To ensure this detail and others relating to the airtightness of the building were executed correctly, two members of the contractor’s crew were trained in Passive House construction at the nearby British Columbia Institute of Technology (BCIT).

To eliminate thermal bridging, sunshades and balconies are hung off the 20 x 152-mm exterior wall—the fasteners neither penetrate the insulation nor puncture the air barrier (Figure 3). The interior service wall is also insulated (i.e. the vapour barrier is in the centre of the wall), and the drying out of any moisture that might become trapped within the wall happens to either the interior or exterior, depending on where it occurs. The wall assembly’s performance was verified using simulation software.

The careful attention to detailing and sealing for airtightness has resulted in a building envelope in which thermal bridging has been eliminated (Figure 4). In this way, The Heights has confirmed traditional wood-frame construction techniques can successfully be applied to the new generation of high-performance buildings.

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King Edward Villa
Completed in the fall of 2017, King Edward Villa is a mixed-use residential building, with a basement parking garage, ground-floor commercial space constructed in concrete, and 77 apartments occupying five floors of wood-frame construction above.

As with The Heights, GBL Architects applied to rezone this former C-2 zoned site under the City of Vancouver’s Rental 100 program. Permission was granted for a six-storey building with an overall floor area determined by the application of the pre-existing setback requirements to the new structure.

With a 1-m (4-ft) setback from the sidewalk, the plan of the building follows the property line, making an oblique angle where Kingsway and King Edward Avenues meet. The ends of the building abut the property line at either side of the site, giving the long front and rear elevations a north-northeast and south-southwest orientation, respectively. The only vehicular access to the site is through the rear yard, where a large tree reduces the width to 12 m (40 ft).

Once building permit drawings were in process, the project was let as a construction management contract. Up to this point, it was assumed the building would be constructed using conventional nominal 20 x 152-mm wood-framed exterior walls to meet the energy requirements of American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, and achieve a Leadership in Energy and Environmental Design (LEED) Gold certification as mandated by the City of Vancouver.

The contract was awarded to Performance Construction, which persuaded Richard Wong, the owner, to consider a low-energy option based on passive design principles, using a similar business case to that described previously for The Heights.

At King Edward Villa, the super-insulated envelope was achieved by using two nominal 20 x 101-mm wood-stud walls with a 25-mm (1-in.) space between them. The exterior sheathing is plywood with a vapour-permeable (breathable) peel and stick membrane that also acts as an air barrier. The entire wall depth is filled with two layers of spray-applied cellulose insulation achieving an R-value of 28 (Figure 5). The cellulose insulation eliminates heat transfer by convection; because it is hygroscopic (i.e. able to absorb and release moisture), it also provides added insurance against interstitial condensation.

The wall’s inside is lined with airtight drywall along with vapour barrier paint on the interior face. Gaskets were installed around power receptacles and other openings, and the edges were sealed with fire-resistant caulking.

Since the floor structure runs parallel to the long front and rear exterior walls, the only component penetrating the insulation layer is the plywood flooring. Windows and doors are steel-reinforced vinyl ‘tilt-and-turn’ units, with low-emissivity (low-e) double glazing. The average window-to-wall ratio is approximately 32 per cent, but feels higher because the two end walls of the building are fire-resistance-rated walls with no window openings.

The drywall was installed before the gypsum concrete floor screed was poured. This increases the airtightness between inside and outside as well as between suites. Baseboards were glued in place. The air change rate is controlled by a pair of heat-recovery ventilators (HRVs) installed in each suite.

Above the concrete basement and ground floors, the structure consists entirely of light wood-frame components factory-prefabricated by Mitsui Homes. These include 20 x 152-mm nail-laminated timber (NLT) panels used for the elevator shaft, floor, and roof trusses as well as wall panels.

Of these components, the 304-mm (12-in.) deep parallel chord floor trusses are the most ingenious. Running parallel to the exterior walls, their open webs permit all the main mechanical and electrical services to be run horizontally throughout each suite (Figure 6), with vertical drops only where needed for HRV grilles, light switches, and plugs. This proved quicker and less expensive than the traditional roughing-in process.

Another advantage of the floor trusses is they could be hung from their top chords only, enabling the drywall finish on party walls to be notched around them, but otherwise taken up to the underside of the floor above. This simplified the fire separation between suites, with intumescent caulking used to seal the joints, and mineral wool insulation employed in the ceiling cavities.

Demising walls between suites were also constructed using a double-wall system to improve sound isolation. On the southwest side of the building, these walls are topped with a 304-mm-deep parallel strand lumber (PSL) beam that cantilevers approximately 1.5 m (5 ft) beyond the exterior wall to support the balconies, which also function as sun shades. This detail means that, rather than a series of header joists penetrating the building envelope to support the balconies, only the PSL beams project, significantly reducing thermal bridging (Figure 7).

From the architectural and structural drawings, Mitsui Homes created a 3-D model that identified each individual wall panel and truss, giving it specific attributes and a unique position within the model. Co-ordinated with mechanical and electrical drawings, wall panels that were to be superimposed one above the other were framed with identical placement and spacing of studs. This enabled vertical drops for plumbing pipes, electrical conduits, and tie-down anchors to be continuous, requiring only minimal drilling of plates and headers on site (Figure 8). The net result was to reduce uncertainty, mistakes, and mess while speeding up installation. It also enabled drywall to be installed from bottom to top—a safety precaution that can limit the extent of a construction fire.

Framing lumber with a moisture content of 15 per cent or less has become the norm for five- and six-storey construction. The quality and consistency of the material used by Mitsui lends itself to precise fabrication, with frames being factory-produced to tolerances of 3 mm (0.12 in.) or less.

Speed and accuracy are the most notable advantages of prefabrication. The unique code given in the factory to each completed panel identified where it fit in the construction sequence. In turn, this enabled deliveries to be made on a ‘just-in-time’ basis and improved the flow of work on a tight and congested site. This was particularly important on the King Edward Villa project, where the site had only one narrow point of access, and lane closures on the arterial road were particularly disruptive.

The specified slider does not exist.

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Virtuoso
Located on the University of British Columbia (UBC) campus, and scheduled for completion this year, Virtuoso—designed by Rositch Hemphill Architects with Wicke Herfst Maver as structural engineers—contains 106 condominium apartments in two buildings arranged around a central garden. The six storeys of wood-frame construction sit on top of a two-storey basement parking garage.

As both owner and contractor for the project, Adera Development Corporation chose to explore the possibilities of hybrid light wood-frame/mass-timber construction, recognizing that with further code changes imminent, this technology would soon find a broader application in buildings with a major Group D commercial occupancy.

The project combines light wood-frame walls, with cross-laminated timber (CLT) floors, roof, and elevator shafts. Additionally, glued-laminated timber (glulam) beams were used to support cantilevered balconies, also made from CLT. Exterior walls throughout the building, together with the interior loadbearing walls on the first four floors are of nominal 50 x 152-mm (2 x 6-in.) wood-stud construction. On the ground and second floors, the framing used is Douglas fir, whereas spruce/pine/fir is employed on the upper floors. All the lumber is dried to a moisture content of 15 per cent or less.

Interior demising walls, which perform as both fire and acoustic separations between suites, consist of two separate 50 x 101-mm (2 x 4-in.) wood-stud walls with a 25-mm (1-in.) gap between them. They are lined on both sides with two layers of 15-mm (⁵/8-in.) fire-rated drywall.

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Rather than the familiar I-joist and plywood floors that are found in most mid-rise wood-frame buildings, the floors in the Virtuoso project consist of three-ply CLT panels spanning either 2 or 4 m (9 or 14 ft) between the interior walls. The panels are 101 mm (4 in.) thick and 1 m (4 ft) wide. Their adjacent long edges are stitched together with closely spaced stainless steel screws set at opposing 45-degree angles in what is known as a ‘dragon’s claw’ pattern
(Figure 9). The floor panels are concealed by a suspended ceiling below, and a hardwood or ceramic tile floor finish above.

The ends of the floor panels are supported either on the 50 x 152-mm loadbearing interior walls, or on the double-stud demising walls. In the latter case, a gap is left between the panel ends to maintain the discontinuity required for acoustic separation. (Figure 10).

Where required, interior loadbearing walls were topped with a glulam beam projecting approximately 2 m (6 ft) beyond the building to support exterior balconies. This detail is similar to the one described previously for King Edward Villa, but here the balcony floors are made from CLT panels.

Three-ply CLT was also used for the walls of the elevator shafts. These panels are set vertically and, like the floor panels, connected with a ‘dragon’s claw’ arrangement of screws.

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Considerable time was expended at the design development stage to optimize panel layout and dimensions and details, as well as to minimize waste and maximize economy and efficiency. A 3-D model was produced to identify each individual panel, its place within the building, and the location of any holes needed to be pre-drilled in the factory.

The Virtuoso project has demonstrated the use of CLT floor and roof panels in a hybrid mass timber/light frame application can offer shorter construction times. The just-in-time delivery and easy installation of the prefabricated panels can be quicker and more efficient than traditional site construction.

Conclusion
Since its introduction in the late 19^th century, light wood-frame construction has continued to evolve—whether in response to new code requirements, technological advances, or market expectations. As a renewable resource that sequesters carbon and has low embodied energy, wood has an increasingly important role to play in the creation of high-performance buildings and more sustainable built environments.

Now, with wood construction recognized as a key component in the long-term mitigation of climate change, the technology is being asked to deliver larger buildings that are more durable and use less energy over their service life. (This is based on the 2007 fourth assessment report by Intergovernmental Panel on Climate change. Visit www.ipcc.ch/report/ar4[7].) As The Heights, King Edward Villa, and Virtuoso have demonstrated, light wood-frame construction, in pure or hybrid form, can meet or exceed these expectations.

Jim Taggart, FRAIC, teaches wood design  at the British Columbia Institute of Technology (BCIT) in Vancouver. He is also the editor of Sustainable Architecture and Building Magazine (SABMag) and the author or editor of more than a dozen books, including the award-winning Toward a Culture of Wood Architecture (2011). Taggart has also lectured extensively on the role of wood in contemporary architecture throughout North America, Scandinavia, and Australasia. He is a Fellow of the Royal Architectural Institute of Canada (RAIC) and the recipient of the 2012 Premier of British Columbia’s Wood Champion Award. He can be reached at architext@telus.net[8].

Source URL: https://www.constructioncanada.net/mid-rise-wood-frame-construction-gets-innovative/

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