The purpose of this building study is to use whole building life cycle assessment (WBLCA) and cost estimating to understand the embodied carbon and dollar cost implications of choosing between functionally equivalent mass timber, steel, and concrete structural systems. It compares a mass timber reference building—Return to Form, located in Denver, Colorado—to alternatives designed in cold-formed steel (CFS) and concrete. Comparisons between the three systems are made in terms of embodied carbon, construction dollar cost, and speed of construction. The variations between architectural designs (construction type, enclosures, fire protection, acoustic performance, and ceiling finishes) are included in the analyses.

The Return to Form building study is intended to be read together with the Mass Timber Comparative Life Cycle Assessment Series Introduction, which details aspects common to all studies in the series, including methodology, approach, scope, and code compliance of the comparative designs, life cycle assessment (LCA) and dollar cost analyses, and the importance of embodied carbon and biogenic carbon as part of the building industry’s strategy to address climate change and environmental degradation. This study details information specific to Return to Form and any variations from the series introduction.

Project Background and Alternative Designs

The Return to Form study compares three structural systems for a multi-unit residential building: mass timber, CFS, and concrete. The reference mass timber building is a Type IV-B, 12-story, 84-unit residential building, five of which are affordable 2-bedroom units. The building is owned by Katz Development in partnership with Wynkoop Investors LLC and is located in the River North Art District of Denver, Colorado. The Architect of Record is tres birds, the Structural Engineer of Record is KL&A Engineers & Builders, and the general contractor is Swinerton with support from Timberlab. The development is anticipated to be the tallest mass timber building in Colorado, at around 148 ft (45 m), utilizing Denver’s early adoption of the 2024 International Building Code (IBC) Type IV-B construction type and exposure allowances for tall wood.

Return to Form had been submitted for permit application at the time of this analysis, and the authors made use of the completed Construction Documents. Katz Development’s preference for mass timber is due to its market differentiation, low embodied carbon, and biophilic attributes. Mass timber was identified as the project’s structural system early in the design process. The project was awarded a grant through the Softwood Lumber Board’s 2022 Mass Timber Competition: Building to Net Zero Carbon and a USDA Forest Service Wood Innovation Grant.

Comparative Building Systems

The three building systems compared in this study were designed by KL&A in collaboration with the Return to Form project architect, tres birds.

The reference building, Return to Form, totals approximately 139,000 ft² (12,900 m²) gross floor area, with 12 stories above grade and no below-grade construction. The foundation system consists of grade beams spanning between concrete drilled piers ranging in diameter from 30 to 48 in. Level 1 is a concrete slab-on-grade for mixed use that includes a loading dock, gym, entryway, lobby, and future tenant space. Levels 2 and 3 consist of 8-in.- thick post-tensioned cast-in-place concrete slabs for vehicle parking. Level 4 is the first residential floor, consisting of a post-tensioned cast-in-place concrete podium transfer slab, the thickness of which varies for each alternative building design. These elevated concrete slabs are supported by concrete columns ranging from 18 to 20 in. wide and 30 to 34 in. deep.

Levels 5 to the roof total about 90,000 ft² (8,350 m²) gross floor area and utilize a mass timber framing system (cross-laminated timber and glulam), which is described in the Mass Timber Building System section. These levels are relatively repetitive residential floors with penthouse units and an exterior amenity space on Level 12. The alternative structural systems are employed at the residential levels—i.e., Level 5 to the roof. A schematic building section is shown in Figure 4.

Denver is a region of low seismicity and moderate winds with a design ultimate wind speed of 115 mph. Return to Form’s lateral design is governed by wind loads. The lateral system at all levels is comprised of ordinary reinforced concrete shear walls supported by cast-in-place concrete mat slabs and drilled piers at the foundation. The shear walls are located at the two stair/elevator cores; see Figure 5 for the plan layout.

The vertical enclosure finish is a combination of glass, metal panel, and stucco. The finishes are typically backed with polyisocyanurate insulation, sheet vapor barrier, fluid-applied water barrier, and gypsum sheathing board. The exterior wall is framed using platform-framed non-load-bearing CFS studs with glass fiber batt insulation in the stud cavities and gypsum board at the interior. The levels of vehicle parking utilize a partial height, fully grouted, exterior masonry crash wall, instead of a fully enclosed vertical wall.

The roof enclosure consists of an ethylene propylene diene monomer (EPDM) finish over cover board, polyisocyanurate insulation, and a sheet vapor barrier atop the mass timber roof panel. At the Level 12 exterior amenity space, the finish is concrete pavers on pedestals. The pavers are backed by the typical EPDM roofing membrane over gypsum board, polyisocyanurate insulation, and a sheet vapor barrier.

The alternative buildings are functionally equivalent and meet the same design criteria as the reference mass timber building, meaning equivalent floor area, site orientation, occupancy, programmatic layout, geographic location, load criteria, and performance requirements, in accordance with ISO 14044 4.2.3.7 (ISO, 2006) and ASTM E2921 (ASTM, 2022).

All three buildings utilize the same structural system from the Level 4 podium and below, including the foundation, with design modifications for the different loads imposed from above. The residential floors begin at Level 4 and employ alternative structural systems for Levels 5 through 12 and the roof (mass timber, CFS, and concrete). Structural designs were optimized for each material system to show each alternative realistically and fairly. The alternative designs consider the effects of the building’s weight on the foundation, as well as gravity and lateral systems.

All three buildings use the same vertical enclosure system, interior wall locations, and residential unit layouts. In general, assumptions regarding aesthetic preferences were avoided; the same exterior finishes, exposure structure, and dropped ceiling locations were maintained unless noted otherwise. The comparative designs consider the building’s construction type and its effect on the fire rating and fire protection assemblies, exposure, and material requirements. All three buildings have the same occupancy, which in some cases has more stringent fire ratings than required by the construction type; this was considered in the designs.

The alternative structural systems and their effects on the fire rating, fire protection systems, foundation systems, gravity systems, lateral systems, and floor-to-floor heights are described in the following sections.

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Authors:

This series is being developed collaboratively by KL&A Engineers & Builders / KL&A Team Carbon and WoodWorks with funding from the USDA U.S. Forest Service and the Softwood Lumber Board.