Introduction to Light-Frame Structural Systems

Whether designing for light-frame or mass-timber structural systems, project teams that opt for wood can benefit from the material’s versatility, sustainable supply chain, and benefits to occupants, as well as thermal, acoustic, seismic, and fire performance. Wood can also help to maximize value through gains made in square footage and building height.

Photo courtesy of CBPhotography & Designs

Orchards at Orenco | Ankrom Moisan Architects

Light-frame wood construction has long been the go-to framing choice for low-rise and, increasingly, midrise residential and commercial buildings. Cost-effectiveness, material-use efficiency, ease of assembly, minimal environmental impact, and the ready availability of labor and materials make light-frame construction the most common type of wood construction in North America. Typically, nail-assembled light-frame construction uses a formulaic combination of dimensional lumber, I-joists, trusses, structural composite lumber, and plywood and oriented strand board (OSB) decking and sheathing for floors, walls, and roof decks.

While mass-timber structures are often built as components off-site and assembled at the project site, light-frame construction typically occurs entirely on-site. Increasingly, however, elements of light-frame buildings are fabricated off-site and assembled on the job. Off-site construction offers greater control over construction conditions and improved safety oversight for all material types, while requiring less skilled labor on-site and contributing to faster construction timelines.

Light-Frame Roofs and Floors

Typical light-frame roof and floor systems consist of repetitive framing members, such as rafters or trusses with wood structural panel decking. Framing components include solid sawn dimension lumber, I-joists, structural composite lumber, and parallel chord and pitched trusses. OSB and plywood are used interchangeably as decking material.

As part of the lateral resisting system, roofs and floors are designed as horizontal diaphragms and may require special considerations for high loads or irregularly shaped structures. Diaphragm design can also affect the structure’s load distribution, so in making an assumption as to whether a diaphragm is flexible or rigid, it is important to analyze the lateral deflections.

Roofs shed water and act as part of the thermal building envelope. In many nonresidential applications, low-slope roofs are common and may need special attention to ensure building longevity. How the roof is insulated can often affect the building’s thermal performance. Prescriptive paths within the energy codes prefer continuous insulation, but performance paths offer opportunities for other insulation techniques.

Photo courtesy of Think Wood

Typical light-frame roof and floor systems consist of repetitive framing members such as rafters or trusses with wood structural panel decking.

Light-Frame Walls

Typical light-frame walls consist of studs spaced at 12 to 24 inches on center (o.c.) with a double top plate and single bottom plate, and they are often sheathed with wood structural panels. Construction depends on the amount of load bearing on the wall, its height, and whether it is part of the lateral force-resisting system.

Walls higher than 12 feet are often designed as tall walls and may require additional engineering.

Shear walls refer to wall components that participate in the lateral resistance of the structure and can be designed a number of ways; methods include segmented, perforated, and force transfer around openings.

Approaches to Light-Frame Construction

There are several approaches to light-frame wood construction, and each is suited for a specific application, most often in the Type III (four to five stories) and Type V (two to three stories) categories. These approaches are distinguished by the wall-to-floor connection and include platform, balloon, semi-balloon, plank and beam, and truss framed.

Platform

In platform construction, the floors are framed separately, and the walls of the story beneath each new level bear the load. The rim board, which is typically made of structural composite lumber, transfers lateral and vertical loads. Framing members are typically spaced between 12 and 24 inches o.c. Due to its relative ease of building, platform framing is the most common form of light-frame construction for Type VA residential buildings.

Balloon

In balloon framing, vertical structural members extend from the foundation to the rafters, and framing members are spaced between 12 and 24 inches o.c. The wall extends two or more stories, and the floor is hung off a ledger connected to the wall. Balloon framing is often used in industrial and retail applications where a parapet is needed.

Semi-Balloon

Often used for multistory projects due to its fire-management abilities, semi-balloon framing sees floors suspended from the double top plates of the walls below them, with the walls stacked directly on the wall below. This approach limits shrinkage throughout the building, although it requires more hardware than other framing types. Framing members are typically spaced between 12 and 24 inches o.c. Semi-balloon framing is an alternative to platform framing for Type VA and Type IIIA buildings.

Plank and Beam

Historically used in heavy-timber buildings, plank-and-beam construction has since been adapted for light-frame projects. It uses larger wood members and spaces them more widely than does platform or balloon construction. This type of framing sees 2-inch subfloors or roofs on post-or pier-supported beams spaced up to 8 feet. Additional framing allows for interior and exterior cladding and finishes.

Truss Framed

Truss-framed light-frame construction employs engineered wood systems made up of roof and floor trusses as well as wall studs for additional strength. These assemblies are fabricated off-site, offering the additional advantages previously discussed. Truss-framed systems also boast greater resilience, making them particularly well-suited for areas that experience high winds. Spacing of 24 inches between frames is possible. A lack of headers, floor beams, and interior columns contributes to material and cost savings, maximizing value with gains in square footage and height.

LIGHT-FRAME CASE STUDY

Photos courtesy of William P. Wright

Yobi Microhousing | Neiman Taber Architects

The Yobi represents a very important housing type, providing private-market affordable housing in a desirable location, designed to promote community and featuring a carbon footprint per occupant that is leaps and bounds ahead of conventional housing. The commons provides a public counterpart to the private rooms—a place that brings people together for movie nights, weekend dinners, and group meetings—to build the social glue that transforms living together into a positive, desirable experience.

Yobi Apartments: Reinventing Microhousing

The Yobi begins with a simple assertion that a large number of people living together in a communal setting deserve architecture that is designed from the ground up to build community among residents. Inspired by community space, the Yobi is a different kind of microhousing project than what has come before. Whereas earlier microprojects were designed as a cluster of townhouses with multiple entries and stairwells, the Yobi is designed as a single building with one entry at the street. The ground floor features a generous commons with a lounge, fireplace, kitchen, study room, laundry, and media area. The route to the private rooms goes through the commons, activating the space and creating opportunity for chance encounters.

The commons provides a public counterpart to the private rooms—a place that brings people together for movie nights, weekend dinners, and group meetings—to build the social glue that transforms living together into a positive, desirable experience. While the commons on the first floor is a living room at the building scale, the kitchen provided on all other levels acts as a “pajama commons,” where immediate neighbors can casually interact. This layering of public and private space fulfills the need for a sense of community as well as solitude, when desired.

Congregate microhousing development is widely recognized as the least-expensive form of housing that the market can produce, with typical rents ranging from 45–65 percent of area median income (about $700 to $1,000 per month). Seattle’s Housing Authority and Livability Agenda (HALA) committee recognized congregate microhousing as an important tool that can help cities with housing supply, affordability, and access to desirable neighborhoods for low-income renters. The Yobi is a new prototype for how to build this housing in a way that promotes livability as well.

The Yobi is certified Built Green 4 Star, which means that it has met performance criteria over a broad range of categories, including stormwater management, water use, energy use, indoor air quality, and waste reduction. Conventional wood-frame construction with dimension lumber shear walls combined with high insulation levels and sealed openings helped create a high-performance exterior envelope. The Yobi has a number of additional energy saving features, including spray foam insulation, passive solar design, high-efficiency lighting, and high-efficiency gas boilers. Altogether, the project has an energy-use budget that is 60 percent of what is required under the current energy code. When you take space efficiency into account, you start to see just how energy efficient small apartments can be. A person living in the Yobi uses roughly one-third of the energy of someone living in a conventionally designed apartment building.