Interpretable Optimization Design of Polygonal Pretensioned Concrete Bulb-T Girders Considering Tendon Section Coordination and Performance Cost Efficiency

The design of polygonal pretensioned concrete girders involves a practical conflict among load-carrying capacity, stiffness, ductility, damage control, and material cost. In conventional design, these characteristics are strongly coupled: increasing web thickness may improve stiffness but reduce ductility, while modifying tendon inclination or inflection-point position may improve prestress efficiency but may also induce local stress concentration near tendon-deviation regions. This coupling makes it difficult to identify rational design solutions through trial-and-error procedures alone. To address this problem, this study proposes a mechanism-informed and interpretable design framework for 30 m polygonal pretensioned concrete Bulb-T girders by integrating nonlinear finite-element analysis, surrogate-assisted modeling, multi-objective trade-off evaluation, and SHAP-based feature-attribution analysis. The scientific problem addressed in this study is the insufficient understanding of how tendon geometry and sectional parameters interact to govern structural response, while the applied problem is the lack of transparent design guidance for balancing performance and cost in polygonal pretensioned girders. The results show that girder behavior is controlled by coordinated parameter interactions rather than isolated parameter changes. Tendon inclination, inflection-point location, and web thickness are identified as the dominant variables affecting load-carrying capacity, damage evolution, stiffness–ductility balance, and cost-effectiveness. Compared with the conventional design, the representative optimized design increased the ultimate load-carrying capacity by approximately 26%, reduced the peak concrete damage index by approximately 24%, and increased the structural performance index by approximately 8.5%, lowered the normalized material–cost indicator by approximately 5%, and improved the performance–cost index by approximately 14–15%. These findings indicate that the proposed framework is not a fundamentally new girder form, but an improved interpretable design methodology that converts numerical optimization results into transferable engineering design principles for polygonal pretensioned concrete girders.