LEED-Aligned Embodied Carbon Decision Support for Hospital Expansion: An LLM-Assisted Comparison of Mass Timber, Structural Steel, and Reinforced Concrete Structural Strategies

Abstract

Hospital expansion projects require early carbon decisions that do not confuse material embodied carbon with hospital operational energy. This paper presents a LEED-aligned embodied-carbon decision-support workflow for comparing mass timber (MT), structural steel (SS), and reinforced concrete (RC) structural strategies during early expansion planning. The WBLCA experiment uses the public 2024 bill-of-materials and whole-building life-cycle-assessment dataset for nine functionally matched U.S. building alternatives: MT, SS, and RC at 8, 12, and 18 stories. Because the source WBLCA dataset is a structure-and-enclosure benchmark rather than a hospital energy model, a separate hospital operational-energy boundary check is added using the 2018 CBECS public-use microdata and DOE/PNNL ASHRAE 90.1-2022 hospital prototype output files. The embodied-carbon analysis evaluates material GWP for modules A-C and A-D; B6 operational energy and B7 water use are not inferred from structural material choice. MT reduced A-C embodied GWP by 39.15-50.87% relative to RC and by 28.08-34.48% relative to SS. Seven of nine pairwise comparisons exceeded a 20% GWP-only LEED screen, and the remaining two SS-versus-RC comparisons exceeded the 10% screen. The hospital-energy check found a CBECS inpatient-health-care floor-area-weighted site EUI of 190.30 kBtu/ft2-yr and DOE Appendix G proposed-hospital site EUIs of 81.29-120.00 kBtu/ft2-yr across 19 climate locations. These operational-energy results are used only to define the separate B6 workflow. The results support a table-grounded LLM-assisted explanation layer for hospital owners and architects while preserving a strict boundary between embodied-carbon screening and operational-energy simulation.