The purpose of this building study is to use whole building life cycle assessment (WBLCA) and cost estimates to understand the embodied carbon and dollar cost differences between functionally equivalent mass timber/light-frame wood hybrid (MT/LF hybrid) and steel structural systems. it compares a MT/LF hybrid building— the Nez Perce-Clearwater National Forests Supervisor’s Office, located in Kamiah, Idaho to an alternate designed in steel. Comparisons between the two structural 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 Nez Perce-Clearwater 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, LCA, and dollar cost analyses, and the importance of embodied and biogenic carbon as part of the building industry’s strategy to address climate change and environmental degradation. This study details information specific to the Nez Perce-Clearwater office and variations from the series introduction.

Project Background and Alternative Designs

The Nez Perce-Clearwater office is a Type V-B, two-story building owned by the U.S. Forest Service (USFS). The Architect of Record is Mosaic Architecture, the Structural Engineer of Record is Morrison-Maierle, and the general contractor is Quality Contractors, LLC. The USFS chose an MT/LF hybrid system because of its alignment with their values and sustainability goals, as described in the WoodWorks case study of this project (WoodWorks, 2022). The CLT was designed and supplied by SmartLam North America from their Columbia Falls, Montana plant, which also supplied the glulam. The wood trusses were designed by The Truss Company and supplied by Early Bird Supply. The authors of this study utilized information from completed construction documents and shop drawings from the built project. 

A steel system was chosen for the alternate design based on local construction practices. It is commonly used for buildings of this scale in this part of the country.

Comparative Building Systems 

The two structural systems compared in this study are illustrated in Figure 3. The MT/LF hybrid reference building was designed by the project’s original design team. KL&A designed the alternative steel building in collaboration with Mosaic Architecture.

Reference Building – Mass Timber/Light-Frame Hybrid  

The MT/LF hybrid building has a total gross floor area of 18,540 ft2 (1,722 m2) and includes a lower level partially below grade and one partial story above grade at the main level. The lower level is mixed-use, with a café, storage, and office space, while the main level includes office space, conference rooms, open work areas, and mechanical rooms. The MT/LF structural system can generally be described as a post-and-beam framing system with locations of load bearing walls. A schematic section drawing of the reference building is shown in Figure 4.

The east portion of the building consists of a two-story open space extending from the lower level to the roof. The floor assembly for the partial main level at the west half of the structure consists of floor finishes on 3/4-in. plywood sheathing over prefabricated wood trusses, dimensional lumber joists, and wood I-joists (Figure 5). Typical floor truss depths are 18 in. and 24 in. and span to exterior and interior wood bearing walls or glulam beams, which range in size from 6-3/4 in. x 12 in. to 6-3/4 in. x 18 in. The glulam beams are supported by glulam columns, which range in size from 6-3/4 in. x 7-1/2 in. to 8-3/4 in. x 9 in. In the cavities between the floor framing, there is 6 in. of sound batt insulation. Kinetic wave hangers suspend two layers of gypsum board below the insulation with acoustic ceiling tiles suspended below the gypsum board.

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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.