Design Feasibility: More living units achieved with mass timber framing system

As the first Type IV-C building in North America, Heartwood utilizes mass timber to provide a lightweight construction solution for the workforce housing development. Comprised of glulam beams and posts supporting cross-laminated floor panels through eight floors, Heartwood is able to provide more units compared to traditional light-frame construction, which is limited to five stories. The project is a direct product of the 2021 IBC prescriptive code.

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Seattle, Washington
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Square Feet:
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Structural, Civil
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Mass Timber, Multi-Family Residential, Sustainable Design
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Primary Material:
Steel, Wood

Heartwood's 114 units average about 400-sf with the majority taking advantage of the exposed mass timber. In addition to structural services, DCI’s civil team prepared demolition, temporary erosion & sedimentation control, horizontal control, drainage and wastewater control, storm drain pipe schedule, bioretention details, grading, utility, and street improvement plans for the project.

The building has a couple of underground utilities which are coordinated with the traditional concrete spread footings for the foundation system.

Heartwood originated as a study that was part of a US Department of Agriculture Wood Innovation grant. DCI was part of that integrated project team to determine the feasibility of a 114-unit mass timber workforce housing solution.

Project Highlights

  • The initial feasibility study required DCI to discuss different unit layouts and the implications of unit dimensions with the architect and contractor team. The team had to coordinate the acoustical considerations of the glulam post and beam frame between the units, as well as how to facilitate a visual timber aesthetic while still hitting the client’s acoustical goal.
  • DCI worked with atelierjones, Timberlab and Hilti to develop a new, all-wood mortise tenon beam connection that meets the modern two-hour fire rating. It is now being used on other tall mass timber projects.
  • Heartwood's lateral system includes a steel buckling restrained brace frame.
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