In the Central District of Seattle, USA, a 67,000-square-feet mass timber middle income housing project represents a significant development in tall timber construction. The eight-storey Heartwood building was designed by Seattle-based architectural studio atelierjones for Community Roots Housing, a non-profit organisation that builds, preserves, and manages affordable homes for over 2,000 residents across the Seattle area.
Changing the building code in favour of tall timber buildings
Heartwood is the first Type IV-C International Building Code workforce housing building to be constructed in the USA. The anticipated demand for taller mass timber buildings led to the addition of three new construction types to the 2021 International Building Code: IV-A (18 storeys), IV-B (12 storeys), and IV-C (9 storeys). Type IV construction relates to buildings constructed using mass timber. Susan Jones, founder of atelierjones, represented the American Institute of Architects and its 90,000 members on the International Code Council’s Tall Wood Building Committee. The committee’s efforts led to changes in the building code, enabling the construction of mass timber buildings up to 18 storeys. This work paved the way for Heartwood.
The first to test the water
Work on Heartwood commenced in 2019, with a two-year design phase followed by a two-year construction phase. Support from a $250,000 U.S. Forest Service Wood Innovations Grant funded research into the development of a mass timber structure. Despite the grant, potential client uptake was low due to the perceived risk of constructing in mass timber. As an entrepreneurial and affordable housing provider, Community Roots Housing was drawn to the prospect of mass timber development. Working with Skipstone Development, the organisation recognised the value proposition, the lower carbon footprint, and the merit of being the first to test the water.
Heartwood was constructed on a parking lot, adjacent to an existing three-storey affordable housing project owned by Community Roots Housing. The two buildings share a courtyard, designed in the style of a pedestrian street.
Heartwood’s eight storeys provide a total of 126 workforce housing apartments, including 113 studio units and 13 one-bedroom units. The building provides bicycle parking and a lobby lounge; the shared courtyard has a community fire pit.
Innovative mass timber construction and sustainable design
Heartwood’s hybrid mass timber post-and-beam superstructure features glulam beams and columns in the apartments, hallways, and lobby lounge, with exposed cross-laminated timber (CLT) ceilings. The building has a concrete foundation with steel rebar and a steel Buckling-Restrained Braced Frame (BRBF) core, partly for seismic reasons. A unique set of Mass Plywood Panel exit stairs adds to the building’s timber essence.
CLT floor and ceiling panels are perpendicular to the glulam post and beam structure. Services, including 17 vertical plumbing shafts per floor, are distributed horizontally and vertically throughout the building.
The soundproofing between the units uses a combination of wood and lightweight gypsum concrete. All windows are triple-glazed.
Timberlab, an Oregon-based company pioneering innovation in the mass timber sector, collaborated with Portland-based DCI Engineers to create a beam-to-column connection. This innovative design uses the inherent bearing strength of wood, eliminating the need for steel connectors. The all-timber assembly meets a 2-hour fire-resistance rating and helps reduce embodied carbon. Additionally, the timber connection can be exposed within the structure.
The site’s narrow urban plot provided little room for unloading as well as a time-restricted unloading lane. The use of prefabricated mass timber components helped to minimise any disturbance through just-in-time delivery.
Heartwood’s components were sourced within a 400-mile radius of the site, from southern British Columbia to southern Oregon. The exposed wood columns, beams, and ceilings of the building extend the biophilic nature of the project to all units and shared spaces, improving the quality of life and well-being of Heartwood’s residents.
