Mass timber design for multifamily housing

San Francisco is currently experiencing a housing crisis that threatens its vitality. By 2030, the city’s population is estimated to increase by 150,000 people while adding only 16,000 new dwelling units. The gap between the amount of housing needed and housing provided for residents has widened for decades, due to both limited developable land and hesitance to densify the city. This has led directly to ballooning housing costs and has challenged two of the city’s most valuable resources: its community and culture.

Historic neighborhoods such as the Mission District have become battlegrounds of cultural identity and gentrification. Low-income residents are vulnerable targets for displacement, while consequential existing buildings are under constant threat of demolition due to both deterioration and new construction.

Our concept design for Mission: Housing showcases mass timber products as a tool for alternative development, providing additional housing in the heart of the city that does not require demolition of existing structures or displacement of existing residents. The El Capitan Theater & Hotel, built in the Mexican Baroque Churrigueresque style, is one of many exquisite yet neglected historic resources at risk. The original theater was demolished and used as a parking lot, while the hotel was converted into a single resident occupancy (SRO) dwelling. Building atop the original building with strategic intrusions to the existing structure embraces the affordability of its current use while reconnecting the site to the surrounding community.

The building’s form directly responds to its context through tapering and shearing; it is sensitive to the existing life of the street while increasing density. The skin system references Churrigueresque patterning on the existing base, yet expands and shifts across its height to reveal lush gardens and an expanding array of delicate canopies.

The building’s original light well is extruded through the full height of the building, providing passive ventilation and natural lighting throughout the core. Communal spaces are provided for urban agriculture, irrigated by an in-house greywater purification system. This embedded horticulture purifies the air inside the building, provides nourishment for inhabitants, and strengthens San Francisco’s tradition of civic parks.

Responding to San Francisco’s Population Needs

A phased construction plan would afford flexibility dependent on need. Up to five phases respond to structural and geological concerns, and can be executed over 15 years to meet the city’s changing population. The phasing also offers a unique opportunity for residents and visitors of San Francisco to observe and learn from a transparent construction process, providing greater opportunities for civic engagement and education.

Why Mass Timber?

The adaptability of urban development could be revolutionized through the inherent strength of mass timber construction. Conventional concrete and steel construction is often impractical and expensive for multi-story units, and high-rise construction with traditional wood balloon framing is not possible due to the risk of rapid spreading fire in lightweight lumber, which could quickly collapse. This issue is not a vulnerability in heavy timber construction, since fire creates a layer of charred wood on the exterior and extinguishes the fire before the structural integrity of the system is compromised.

Our design for Mission: Housing applies to an innovative new composite floor system from products which are already available: It uses a concrete topping with steel shear plates embedded into laminated veneer lumber floor panels, allowing for longer spans. Sandwiched joists in this floor design permit the bottom panel to be removed and replaced in the event of fire damage, enabling the building to be reoccupied quickly after a fire event. Additionally, heavy timber columns employ steel plate connections that minimize shrinkage between floors. Since the construction process uses a prefabricated panel kit-of-parts approach, this solution is highly efficient and flexible for growth.

Flexibility is enhanced by the lateral force resistance of post-tensioned laminated veneer lumber rocking shear walls, which can be recalibrated as necessary to maintain optimal strength or resolve shifts in structure after an earthquake. The high compressive capacity of these wall panels makes them incredibly resilient, handling large seismic forces while exhibiting minimal damage.

Flexible Programming

The unit’s modular design allows for dwellings of any size to be easily defined, allowing families and homes to grow in tandem. Creating narrower, multi-level units densifies the building and provides a variety of dwelling typologies. The modules can be changed over time depending on resident’s needs, adding stories as families grow. This aggregational method also works in conjunction with the building’s phasing.

Historically, urban growth has been obstructed by the false dichotomy between preservation and progress—but this no longer needs to be the case. The variety of mass timber technologies facilitates a strong and diversified strategy for sustainable evolution which can strengthen and mature the fabric of the city.