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 and steel structural systems. The study compares a reference mass timber building—Denver Office in Denver, Colorado—with an alternate steel design. Comparisons between the two systems are made in terms of embodied carbon, construction dollar cost, and construction speed. The architectural variations (construction type, enclosures, fire protection, acoustic performance, and ceiling finishes) between these designs are included in the analyses. 

The Denver Office 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 carbon and biogenic carbon as part of the building industry’s strategy to address climate change and environmental degradation. This study details the information specific to Denver Office and variations from the series introduction. 

Project Background and Alternative Designs 

Denver Office is a Type III-A office building with four levels above grade and no below-grade construction. Level 1 is retail, restaurant, and back-of-house storage space. Levels 2 through 4 are office space. The design team includes Shears Adkins Rockmore (SAR+) as Architect of Record, KL&A Engineers and Builders (KL&A) as Structural Engineer of Record, and construction partner PCL Construction. The design of Denver Office was completed through the Design Development (DD) phase, and the authors of this study made use of the DD documents. 

A steel system was chosen for comparison because it is the most viable alternative based on common construction practices for this type of project in the Denver market.  

Comparative Building Systems

Figure 3: Schematic 3D images of the two alternate buildings – mass timber (left) and steel (right)

Reference Building – Denver Office 

The mass timber reference building has a total gross floor area of 98,280 ft² (9,130 m²) and includes four levels above grade and no below-grade construction. Level 1 is retail, restaurant, and back-of-house storage space. Levels 2 through 4 are office space. See Figure 4 for a schematic building section. 

Figure 4: Schematic building section of Denver office

The foundations consist of 8-in. cast-in-place concrete stem walls on spread footings and 30-in.- thick mat slab foundations with 10 psf reinforcement under the core walls and braced frame. The mat slab foundations for the cores are isolated from one another. A rammed aggregate pier system is used for ground improvement due to soft soils and undocumented fill at the site, making it unsuitable to support foundations and floor systems in its current condition. Continuous spread footings are 1 ft thick and either 3 ft or 4 ft wide. Isolated footings vary in size from 4 ft x 4 ft x 1 ft thick to 8-ft-2-in. x 5 ft x 1-ft-6-in. thick. Level 1 is a 5-in.-thick concrete slab-on-grade.  

Level 2 through Level 4 are mass timber, which can generally be described as a post, beam, and plate framing system with beams framing in one direction. The floor assembly consists of floor finishes on a 3-in. cast-in-place concrete topping slab over a 3/4-in. acoustic underlayment mat over 5-ply (6-7/8-in.- thick) CLT floor panels supported by glulam beams and columns (Figure 5). The concrete topping slab and acoustic underlayment mat are used to meet the sound transmission class (STC) and impact insulation class (IIC) specifications of 56 and 54 respectively, allowing the CLT panels to remain exposed at the underside. Glulam beams are between 6-3/4 in. and 14-1/4 in. wide and 12 in. to 27 in. deep. Glulam columns are 16 in. wide x 24 in. deep. At the restaurant/retail space on the south end of the building, there are exterior exposed glulam columns at the first level; these will be manufactured with Alaskan cedar. The grid system is typically 20×34 ft (6.1×10.4 m) at exterior bays and 20×22 ft (6.1×6.7 m) at the interior bay. The glulam beams span 34 ft and cantilever from both exterior bays of the building into the short 22-ft bay, creating a 5-ft-wide “mechanical highway” with no interruption by glulam framing (Figure 6). This is possible because of the two-way spanning capability of CLT panels. 

Figure 5: Mass timber floor assembly

For more information, see the Mass Timber Comparative Life Cycle Assessment Series Introduction and comparative LCA building studies on the following projects:

• Return to Form
• Nez Perce-Clearwater National Forest Supervisor’s Office
• Burwell Center for Career Achievement

Return to the top of the page to download the full study.

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.