Integrated optimization framework for fire-resilient and sustainable hybrid steel-concrete composite footbridges

While structural optimization is essential for sustainable design, its potential for complex structures to enhance resilience to extreme events remains underexplored. Hybrid steel-concrete systems offer significant performance benefits, yet their efficiency under these critical scenarios requires further investigation. This study addresses these gaps by proposing a straightforward and integrated multi-objective optimization framework that treats fire performance as a primary design driver alongside cost, environmental impact, and pedestrian comfort. The framework is demonstrated on a 20 m simply supported steel-concrete composite footbridge composed of two hybrid welded I-girders and a 3 m-wide concrete deck, with design variables defined by geometry, material strengths, and the degree of composite interaction. Results demonstrate that hybrid girders are highly effective across all scenarios, reducing total costs relative to conventional girders while maintaining comparable environmental performance. These benefits are particularly significant for designs that prioritize dynamic efficiency, where cost reductions reach up to 21% even under high comfort requirements. Optimal configurations typically employ lower-strength steel for the web and higher-strength steel for the flanges, with a yield strength ratio of around 1.6 as a practical design recommendation. Fire safety is improved with more compact girders, whereas slender geometries enhance dynamic performance, highlighting the distinct nature of these objectives. Incorporating fire performance considerations at the conceptual design stage leads to safer, more resilient, and cost-effective footbridges with minimal environmental impact. Overall, the proposed framework promotes performance-based structural design by supporting informed decision-making across multiple competing criteria and extreme conditions. • Novel framework enables multiobjective optimization including fire. • Fire optimal sections are compact, dynamic optimal sections are slender. • Hybrid girders reduce cost by 21% with equivalent environmental impact. • Yield strength ratio of 1.6 identified as a practical hybrid sizing guideline.