Comparative life cycle assessment for two structural framing plan alternatives in a composite hybrid steel—cross laminated timber (CLT) building

The construction industry is increasingly focused on reducing embodied carbon emissions to address climate change. Steel—cross laminated timber (CLT) composite hybrid structures, where CLT floors and steel framings work in composite action to resist gravity forces, offers benefits such as carbon storage, recyclability, reduced use of carbon-intensive materials, and improved project schedule and quality control. This composite hybrid system can accelerate progress toward net-zero embodied carbon by integrating carbon-storing materials within the existing AEC (Architecture, Engineering, and Construction) ecosystem for commercial and high-rise buildings, where timber use is limited. This study analyzes two structural patterns within the composite hybrid system and uses life cycle assessment (LCA) to identify trade-offs in embodied carbon. A 12-story office prototype is designed using two framing spans of 12.5 feet (3.8 m, Basic Hybrid ) and 25 feet (7.6 m, Stretch ), resulting in a change in the wood-to-steel ratio. In the Stretch design, the structure’s mass increases by 20% due to thicker CLT panels for the longer span, despite reduced steel framing, resulting in a 5% heavier foundation. The LCA considers upfront emissions from the product and transportation stages (A1–A4). Excluding biogenic carbon, the Stretch design has 3% higher embodied carbon than Basic Hybrid ; however, including biogenic carbon storage shows an 83% greater carbon benefit for Stretch . A dynamic assessment of biogenic carbon storage reveals that the building must be in service for 23 years for forest regrowth to offset initial forestry emissions, while the 7-ply system achieves net-zero carbon for the whole building in 67 years, compared to 80 years for the 5-ply system.