This course is part of the Mass Timber Academy

Photo courtesy of Lawrence Anderson Photography

Demand for multifamily housing continues to play an important role in the overall U.S. construction market. The National Multifamily Housing Council estimates that more than 325,000 new apartment homes are needed each year to meet demand.¹ Multifamily projects include apartments and condominiums as well as other residential uses like affordable housing, student housing, senior living, hotels and motels, and vacation timeshare properties.

One of the most fundamental decisions facing a multifamily design team is choosing the building’s structural material. While dominant in single-family residential construction, the cost-effective, code-compliant, and sustainable attributes of wood construction apply to mid-rise, multifamily projects, too. This CEU explores the reasons for the increasing popularity of wood in multifamily buildings, reviews code compliance and fire safety technical considerations, and discusses techniques for successful wood building designs. In addition, it addresses trends expanding the opportunities for wood use in multistory design.

Why Wood?

Developers and design professionals are increasingly recognizing the benefits of wood construction as an effective way to build high quality multifamily housing at a lower cost while reducing the carbon footprint of their projects.

Stella

Photo courtesy of Lawrence Anderson

Location: Marina del Rey, California
Architect: DesignARC

This six-story, mixed-use project demonstrated the versatility and practicality of using wood in mid-rise, multifamily construction. Stella’s unique construction approach used both Type III-A and V-A construction on one podium to build the 244-unit complex. The original building design included two four-story buildings, but plans were modified to accommodate two nested, wood-framed L-shaped structures–one with five stories of wood/Type III-A construction and the other with four stories of wood/Type V-A. Building designers used the area allowed by the extra story to create more open space and add resort-style amenities more typically found in luxury housing projects. Stella was built with Douglas fir dimensional lumber, parallel strand lumber (PSL), laminated veneer lumber (LVL), glue-laminated timber (glulam) beams, and engineered wood I-joists for the floor and roof structures.

Cost Savings, Increased ROI

Building professionals often find wood-frame structures to be more economical, for reasons including speed of construction and lower material costs. Since wood is lighter weight than other structural materials like concrete or steel, a wood structure may have lower foundation costs.² Cost of wood construction can also be lowered by using prefabricated components and assemblies, which speed installation.

After completing the first phase of a five-story student housing project at Illinois State University using steel construction, OKW Architects in Chicago switched to wood for the remaining phases. “Because the structure had so many openings, the metal studs for the first building, Flats on Main, needed to be 12-gauge, but they were heavy and it was difficult for the crews to screw and attach our finishes. So, the contractor encouraged us to switch to wood bearing walls for Flats on Osage, the second building. As a result, it ended up being a more economical project,” said OKW’s Eileen Schoeb.³

1430 Q, a multifamily, mixed-use development in Sacramento, California, is another example of how wood was used to improve the return on investment for a mid-rise project. The developer collaborated with WoodWorks and a code consultant to file an Alternate Means and Methods Request (AMMR) with the City of Sacramento, which gave its approval for six and a half stories of wood, including the top-level mezzanine, over a two-story podium. This allowed more tenants and more lease income from the one- and two-bedroom apartment complex. It also allows the developer to realize premium rents for the top floor view units. When it was completed, 1430 Q was the first residential building in the U.S. to be constructed using a 6.5-over-2 building configuration and the country’s tallest light-framed wood building.

Code-Compliant, Marketable Structures

Design professionals familiar with wood construction for two-to-four-story residential structures may not be aware that the Table 504.4 of the 2021 International Building Code (IBC), Allowable Number of Stories Above Grade Plane, allows up to five stories of Type III wood-frame construction for residential occupancies and six stories for business occupancies. Type V buildings have a maximum of four stories.

Designers can also use concrete podiums under wood-frame construction to add additional stories. Table 510.2 limits the height above grade to the most restrictive building’s normal height, though additional stories can be fit within that height-above-grade restriction. For instance, a Type VA of a 1-A podium can have two stories of 1-A plus four stories of VA as long as the VA building does not exceed 70 feet above grade (sprinklered), its normal height-in-feet limit.

Since the 2009 edition, the one-story limit for the 1-A lower building was removed but the upper building still cannot exceed height above grade for its construction type, as noted above. This means developers can build four or five stories of wood over a single- or double-level podium (which would be referred to as a 4-over-1 or a 5-over-2).

Mahlum Architects worked with engineers at Coughlin Porter Lundeen to design five buildings of student housing for the University of Washington. They made the most of the urban campus location in Seattle, designing each of the buildings with five stories of light-frame Type V-A wood construction over a two-story Type I-A concrete podium. This approach helped them meet code requirements, ambitious design goals and the university’s tight budget. The 668,800-square-foot project, completed in 2011 and 2012, was constructed for $177 per square foot.⁴

Building codes are meant to be material neutral, which means that mid-rise wood-frame buildings are required to meet all the same safety and performance requirements as a similar building made from any other material. With the assurance that wood meets code, building owners and designers often use wood because of its cost benefits. Wood buildings can usually be constructed more quickly, which helps developers increase the return on their investment through earlier completion. Wood’s versatility, sustainability, and aesthetic attributes help make it a desirable choice for many design teams.

Innovative Multifamily Wood Construction Case Study

Photo courtesy of Kevin Korczyk

Project: WREN
Location: Los Angeles, California
Architect: Togawa Smith Martin

The $144 million WREN multifamily project in the fast-rising South Park district of Los Angeles helped to rewrite a neighborhood's narrative through thoughtful innovation, commercial discipline, and the use of a renewable, sustainable building material: wood. This pair of seven-story buildings transforms the skyline along Pico Boulevard through a series of structural/building innovations, including use of the city’s first Type III wood construction with a double-podium design. The challenge for wood-framed buildings in high seismic zones like southern California is being able to provide large glass areas while still providing walls with sufficient shear. To achieve this, the WREN team worked with the structural engineer to determine the minimum length of shear wall required at each floor. Any area not required for shear wall was used for windows. While the large windows added structural complexity, the design versatility of wood allowed them to vary lengths and locations of shear panels easily.