Comparative Assessment of Beam Configurations on the Embodied Carbon and Cost of Reinforced Concrete Two-Way Joist Slab Systems

The literature identifies concrete and steel as the primary contributors to embodied carbon in building structures and highlights a strong relationship between sustainability and structural system geometry. However, existing studies predominantly focus on one-way systems and flat slabs, while research on two-way joist slabs remains limited and often centred on strength optimisation. In particular, there is a lack of systematic life cycle comparisons of alternative beam configurations within this system. This gap is critical, as early-stage design decisions largely determine the environmental impact of structural systems. This study presents a comprehensive, span-dependent evaluation of four beam configurations, namely Without Beam, Internal Beam, Perimeter Beam, and Full Beam, for reinforced concrete two-way joist slabs used in office buildings. A parametric framework was developed using Eurocode-compliant structural design and nonlinear finite element modelling to assess 36 span combinations ranging from 4 × 4 m to 14 × 14 m. Material quantities were extracted from the final designs and converted into embodied carbon values using cradle-to-gate (A1–A3) emission factors derived from the ICE database. The results demonstrate that beam configuration has a significant influence on embodied carbon and construction cost. For spans below approximately 8 m, beamless systems provide the most material-efficient solution. For spans exceeding approximately 10 m, full-beam configurations offer improved structural efficiency and reduced embodied carbon due to enhanced stiffness and load distribution.