Reuse beneath Infrastructure: balancing structural fire risk and the upcycling of overlooked urban space

• Urban infrastructure residual spaces (B-ROADs) hold untapped potential for reuse. • B-ROADs are classified into use categories to assess fire risk via HRR curves. • Unattended spaces pose highest fire hazard due to unpredictable ignitable material. • Vehicle fires set benchmark; dry vegetation fires threaten steel structures. • Proactive strategies can mitigate fire risks in under-bridge urban spaces. This interdisciplinary study explores the untapped potential of residual urban spaces beneath elevated infrastructure, termed “B-ROAD spaces,” and their associated fire hazards. As urban expansion faces land-use constraints, transforming these spaces into habitable areas becomes crucial. To transform them into true urban spaces, it is essential to adopt an approach that combines the recognition of their architectural formal and functional value with recovery strategies capable of addressing technical, regulatory, and safety challenges. The research integrates findings from B-ROAD1, focusing on sustainable urban development, and SaFeBIMAs2, which assesses seismic and fire risks for infrastructure. Through analysis of Naples’ East End Industrial District, B-ROAD spaces are classified into four categories – public, commercial, residential, and informal/illicit activities – and their fire risks are evaluated using heat release rate (HRR) curves derived from Eurocode standards and experimental data. Results show that unattended spaces, such as illegal dumps and makeshift homes, pose the highest fire risks due to unpredictable combustible materials and lack of fire prevention measures. While vehicle fires remain the most severe benchmark, uncontrolled vegetation fires also present significant hazards, particularly for steel structures. The study emphasizes proactive management of under-bridge spaces, integrating fire safety into urban design to mitigate risks and enhance resilience. By combining urban morphology with structural fire analysis, this research provides a framework for transforming neglected urban voids into safe, sustainable public spaces, aligning with principles of circular economy and urban mining.