Plastic-limit topology optimization of perforated steel beam webs: Structural and environmental perspectives

Topology optimization of perforated steel beam webs is commonly performed within a linear elastic framework, while ultimate limit states and environmental impacts are rarely addressed simultaneously. In addition, limited research has compared conventional fabrication methods with additive manufacturing solutions for perforated steel beams in terms of environmental cost. This paper presents a multi-objective topology optimization framework in which plastic collapse behaviour, constructability and Wire Arc Additive Manufacturing (WAAM) constraints are explicitly incorporated, while the material embodied carbon is evaluated as an additional design objective. The methodology combines a Bidirectional Evolutionary Structural Optimization (BESO) algorithm with nonlinear finite element analyses. Plastic failure is controlled through an ultimate load multiplier evaluated at each optimization step, whereas structural stability is assessed using buckling-based indicators. To improve manufacturability and practical applicability, constraints on member inclination and design standardization are introduced, promoting rationalization and constructability from the early design stage. Beyond the embodied carbon associated with material consumption, a cradle-to-gate Life Cycle Assessment (LCA) was conducted to compare conventionally manufactured circular-opening steel I-beams with topology-optimized minimum-weight perforated beams featuring irregular openings enabled by WAAM technology under different deposition rates. The results demonstrate that the proposed TO-WAAM solution constitutes a viable and more sustainable alternative to conventional circular-opening steel beams, combining enhanced structural efficiency, lower stress concentration, and enhancing environmental performance.