Designing with engineered wood products

by Katie Daniel | October 16, 2017 3:26 pm

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By Cory McCambridge and Marilyn Thompson
Multifamily construction represents nearly half of Canada’s housing starts, and the trend toward more urban housing is pushing developers and designers to adopt design methods that meet the burgeoning demand for innovative apartment and condominium dwellings. This article examines three ways engineered wood products and wood systems are being used to bring efficiency, flexibility, and striking designs to multifamily construction:

• panelization with both prefabricated wall and
  floor assemblies;
• a mass timber hybrid system using cross-laminated timber (CLT) and other structural wood building materials in building assemblies; and
• podium construction.

According to Canada Mortgage Housing Corporation (CMHC), multifamily construction accounted for 46 per cent of all housing starts in Canada last year. (For more, visit www03.cmhc-schl.gc.ca/hmiportal/en/#TableMapChart/1/1/Canada[2], and then click on the down arrow next to 2017 Q2.) With increasing affordability challenges due to high prices and tighter lending standards, this strong performance in apartment and condominium construction is expected to continue for the next five years.

In fact, if the first half of 2017 is an indication, the share for multifamily may grow slightly. “The first half of the year, starts came in at an annual rate of 216,000 units, with multifamily starts accounting for almost 50 percent of the starts,” says Joe Elling, market research director for APA−The Engineered Wood Association. “We saw especially strong activity in the Toronto market.”

Engineered wood for engineered solutions in panelized walls and floors
Prefabricated construction addresses one of the key constraints impacting multifamily construction—the increased pressure a shortage of skilled labour places on project schedules. Prefabricated subcomponents can be engineered and precisely manufactured in a controlled environment and assembled at the building site in a process known as ‘panelization.’ This trend is being observed all across Canada in both the single- and multifamily markets, with large builders committing to major investments in equipment and manufacturing facilities.

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Engineered wood products are well-suited for use in prefabricated components because they are consistent in both properties and performance characteristics. These building materials are designed and manufactured to maximize the natural strength and stiffness characteristics of wood by optimally orienting the wood strands, veneers, or laminations and by combining it with moisture-resistant adhesives. Additionally, engineered wood products—including I-joists, glued-laminated timber (glulam), and structural composite lumber (SCL)—have consistent dimensions and are much less susceptible to warping, cupping, shrinking, twisting, or bowing.

A crucial aspect of the panelization process is the opportunity for builders to fully optimize the structural framing elements in their buildings early in the design stages. With an experienced team, this process can significantly reduce the overall cost of materials and labour, as well as allow for seamless installation at the jobsite. In this ‘optimization’ process, structural designers work with project architects to reconfigure the load path based on the structural properties of engineered wood products. Floor and roof panels are fabricated using oriented strand board (OSB) or plywood sheathing attached to engineered wood I-joists. Finished floor and roof panels can be up to 3.7 m (12 ft) in width and up to 15.2 m (50 ft) in length, offering major advantages over conventional framing methods.

With careful attention to detail in the design and fabrication stages, builders can take advantage of continuous spans with a single floor panel, incorporating openings to eliminate labour-intensive connection details. Fabrication with a thicker subfloor deck allows the builder to increase the on-centre (o.c.) spacing of the I-joists, thereby realizing savings in materials and reducing obstructions for the mechanical, electrical, and plumbing (MEP) trades. Some similar efficiencies can be achieved with panelized roof assemblies.

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The improper placement of holes in I-joist webs can potentially reduce the floor assembly’s performance, leading to future customer complaints. By co-ordinating HVAC plans in the design stages of the panelization process, designers can ensure the proper placement of holes in I-joist webs to accommodate electrical wiring, plumbing lines, and other mechanical systems. This not only reduces the possibility of floor issues, but also allows for a seamless flow of trades on the jobsite.

In many four- to six-storey projects, flat roof systems are included in the architectural design to create recreation spaces for residents.

Designers sometimes choose to take this a step further, with vegetated roof designs that incorporate landscaping and grass along with walkways and community barbecue areas on portions of these flat roofs. Here, benefits of engineered wood products are realized once again. Available in depths from 240 mm (9 ½ in.) to greater than 405 mm (16 in.), engineered joists, glulam, and structural composite lumber such as laminated veneer lumber (LVL) and laminated strand lumber (LSL) provide the strength and rigidity required to satisfy the needs of designers trying to incorporate creative ideas into their projects.

Wall panels are carefully fabricated and assembled, incorporating openings for windows and doors, cut-outs for plumbing and electrical, and housewrap. Efficient, well-planned fabrication ensures accuracy and speed of assembly at the construction site. Wall panels are typically 4.3 to 4.9 m (14 to 16 ft) in length, which is usually determined by the available length of plate material.

In order for prefabricated wall panels to remain rigid throughout the manufacturing, shipping, and site erection process, OSB or plywood wall sheathing material is typically employed. This creates a major benefit for prefabricated wall panels—not only do they offer superior racking strength, but they are also square and straight throughout the structure, allowing for faster installation and a better fit of trim, moulding, and cabinets.

Due to the accuracy of the panelization process, building envelope leakage is significantly reduced. This makes it easier for architects and designers to meet any airtightness requirements and take advantage of available tradeoffs for energy performance. The performance of shear walls and diaphragms is much more dependable as each fastener is driven in the correct location and with the correct amount of force.

Advancements in engineered wood products and the assemblies in which they are used makes the prefabrication of building subcomponents at an offsite factory setting a natural evolution in the multifamily and midrise construction process.

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Incorporating mass timber in B.C. projects
Vancouver-based real estate developer Adera is using cross-laminated timber (CLT) to speed construction times and take advantage of the strength and performance of mass timber. On the University of British Columbia (UBC) campus in Vancouver, Adera’s 106-unit Virtuoso project is on track for completion later this year. The project is designed by Rositch Hemphill Architects.

The six-storey condominium structure differs from conventional wood-frame construction in that it features CLT floor panels that are lifted into place and connected to steel columns, making for quicker and simpler assembly. Three-layer CLT (104 mm [4 in.] thick) was used in the floor sections.

“One of CLT’s primary benefits is the way it influences design and scheduling,” explains Ron McDougall, a mass timber specialist with the project’s CLT manufacturer.

McDougall noted at the Virtuoso project site, a 400-m² (4300-sf) CLT floor panel could be installed every 10 minutes. At times, the construction crew was able to install more than 836 m² (9000 sf) of CLT floors in a day.

In addition to speeding construction time, the CLT floor assemblies also lend acoustic benefits.

“If you compare the noise level on the Virtuoso site to the site right next door, there’s a significant difference,” McDougall says.

In other structural elements, the Virtuoso design uses OSB sheathing in the exterior walls and some loadbearing interior walls. This combination of mass timber and wood structural panel sheathing is an example of hybrid construction, where multiple building materials are used together in a single project design. This allows developers and builders to capitalize on the benefits of mass timber while also employing traditional construction techniques and materials in the wall assemblies.

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The OSB wall assemblies enable the building designers to take advantage of the lateral and shear capacities of wood structural panels. Continuous wood structural panel sheathing contributes to a structure’s ability to handle uplift loads, lateral loads, and wind pressures. It also provides flexibility when bracing wall segments that have window and door openings. The engineered shear walls are also designed to resist seismic and wind forces—important considerations on the West Coast.

In all, 2400 sheets of OSB were used for the exterior sheathing, and another 3000 panels were employed for shear walls throughout the project.

“We used OSB in almost every party wall and every corridor wall,” explains Adam Weir, GSC, senior project manager with Adera. “On the lower floors, we used OSB in double-sided shear walls.”

All the OSB sheathing was 15⁄32 Performance Category thickness. (The Performance Category designates the panel thickness range that is linked to the nominal panel thickness used in the building codes. In this case, it is 12 mm [15⁄32 in.].) Weir noted OSB was the most cost-competitive material to meet the structural requirements of the exterior walls and shear walls.

Adera’s design included unique connections detailing where the shear walls were tied to the CLT floors.

“We use only one top plate and one bottom plate in the wall design,” says Weir. “This allowed for the plates to be nailed from the underside of the plate up to the CLT and from the bottom plate down to the CLT.”

While meeting these important structural requirements, the engineered wood and mass timber products also provide enhanced design flexibility and the esthetic advantages of a wood structure.

“The Virtuoso project is an excellent example of how we are fundamentally changing how residential and multifamily housing is being built,” said Stephen Tolnai, a director with the manufacturer of the CLT used on the project. “We’re using new wood building materials and systems in efficient, cost-effective ways, in developments that meet the criteria of discerning homebuyers. They are getting that feeling of the strength and mass of having all this timber around them. They have a healthy home. They have a quiet home.”

The mass of the CLT panels give the floor has a solid feel, similar to what one would expect with concrete. The acoustical benefits of sound-absorbing timber can be recognized even when walking on the CLT immediately after installation, and before the ceiling assembly and acoustical matting is installed.

A Canadian study that evaluated the biophilia effect in building materials showed wood in interiors was perceived by a majority of subjects as more “warm,” “inviting,” “homey,” and “relaxing” than all other tested materials. (The study was summarized in a 2006 issue of Wood and Fiber Science [38(4)], published by the Society of Wood Science and Technology. The article, “Appearance Wood Products and Psychological Well-being,” by Jennifer Rice, Robert Kozak, Michael Meitner, and David Cohen, can be read at www.woodworks.org/wp-content/uploads/Appearance-Wood-Products-and-Psychological-Well-Being.pdf[7]. For more information on biophilia, visit www.apawood.org/designerscircle-nature-in-design-the-biophilia-effect[8].)

Podium construction
Mid-rise podium construction, consisting of two to five residential storeys of wood framing above a concrete nonresidential first storey (i.e. the podium) is common throughout North America, and current trends for urban living have led to a greater demand for podium structures. Designers can take the concept a step further by using wood instead of concrete for the podium. Engineered wood podium design can reduce overall construction cost and construction time, while creating a more sustainable and less massive building.

Examples of two U.S. wood podium projects are the Oceano Luxury Apartments and Galt Place buildings in California. In both cases, the podiums are composed of gypsum concrete topping over wood structural panels supported by I-joists and glulam beams; the former vary in size from 130 to 311 mm (5 1⁄8 to 12 1⁄4 in.) wide and 273 to 1143 mm (10 3⁄4 to 45 in.) deep.

Galt Place specified one stress class: 24F-1.8E (2400 psi bending stress, 1.8×106 psi modulus of elasticity), while Oceano specified two: 24F-1.8E and 30F-2.1E glulam. The 30F-2.1E stress class is considered a high-strength composite (HSC) glulam, in which the beam’s structural properties are increased by replacing the sawn lumber tension lamination with high-strength LVL. Steel beams were an early consideration for both design teams, but were found to be more expensive. Steel columns were used to support the glulam girders with custom steel saddles designed for both projects. As the glulam beams are deeper than typical stocked sizes, it is essential to consult a manufacturer in advance to ensure availability of the materials when needed.

Both project teams were comfortable with the overall durability of wood podiums, as they provide a building envelope that does not expose the wood members to the environment. Oceano further protected the wood elements with drywall, fibreglass mat gypsum sheathing, and stucco on all subterranean wood surfaces, including the shear walls. The Galt Place designers took a different approach, leaving the glulam exposed for the esthetic appeal.

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A key to solving design challenges was heightened communication. Both design teams noted that they worked closely with the entire architecture, engineering, construction team from the beginning.

“We’ve been building this way a long time, but we just haven’t called it a wood podium,” explained Tom VanDorpe, Oceano structural engineer.

In the Galt Place project, the structural engineer consulted with the framer in the field to direct the cutting of allowable hole sizes in the glulam beams for utility runs.

The design teams agreed benefits of all-wood podiums outweigh the challenges. In both projects, they reported enhanced constructability, decreased mass for lateral design, improved sustainability, and more economical building.

Several enhancements to construction were also noted. Field modifications of a wood deck away from the beam line are easier to accommodate because it is not necessary to X-ray the slab for rebar and/or post-tensioned strand placement. Additionally, having fewer building materials decreases the number of trades on the job and, as a result, mobilization time and construction delays. Further, the redundancy of building each floor with the same trade and materials improves framing efficiency and decreases the amount of detailing required by the designers. At Galt Place, the design team noted access to a large pool of experienced and competitive labour for wood framing.

A wood podium also offers many structural benefits. It is less massive than a concrete podium, which is important in high seismic zones where building mass impacts lateral design loads. Additionally, the wood podium design allowed both projects to use light-framed shear walls on the first level, as well as smaller foundations than would be sized for a concrete podium. Instead of different structural engineering firms handling the wood and concrete, a single entity was able to design each building, simplifying the process. Finally, due to the emphasized stacking of the shear walls of the superstructure, through the podium to the foundation, these projects benefitted from fewer discontinuities in the lateral system.

In both the Oceano and Galt projects, cost savings were realized in the building materials and construction time. The Oceano team estimated the wood podium was approximately two-thirds the cost of a concrete podium. At the Galt site, fewer trades and faster construction reduced changes and delays. Only one change order was documented. The project received a $2.5-million credit and the architect believes most of this was due to the use of wood.

Sustainable design solutions
Used in all three design approaches, wood is a structural building material that grows naturally and is renewable. It is also biodegradable, nontoxic, energy-efficient to manufacture, recyclable, and reusable. Wood is unique in that more carbon is removed from the atmosphere by the growing tree than is emitted during its manufacture into products and transportation to its point of use.

Cory McCambridge is an engineered wood specialist who represents APA in Canada. Based in Mississauga, Ont., he is an architectural technology graduate of Algonquin College. McCambridge has more than a decade of architectural and structural design and analysis experience, and has spent most of his career working in the prefabricated panel industry as a designer or design manager for various facilities located across Canada. He can be reached at cory.mccambridge@apawood.org[10].

Marilyn Thompson is the market communications director and corporate secretary for APA. Based in Tacoma, Wash., she is a graduate of Oregon State University, where she studied technical journalism and forestry. Thompson can be reached via e-mail at marilyn.thompson@apawood.org[11].

Source URL: https://www.constructioncanada.net/designing-engineered-wood-products/

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