Investigation of the structural performance of fire-protected castellated beams under cyclic loads and fire effects

In this study, the post-fire structural performance of IPE 140 and IPE 200 steel beams under cyclic loads was experimentally investigated. Within the scope of the study, solid web and castellated section types, fire-protected and unprotected states, and stiffened and unstiffened production configurations were comparatively evaluated. The specimens were subjected to high temperatures using a fire furnace, followed by the determination of moment–rotation relationships, energy dissipation behaviors, and failure modes using a FEMA-based increasing amplitude loading protocol. The findings indicated that while increasing the web depth in castellated sections enhanced the pre-fire capacity, significant strength losses occurred post-fire due to rapid heat conduction around the web openings. It was determined that fire-protective paint delayed the time to reach critical temperatures in the specimens, retarded stiffness degradation, and increased energy dissipation capacity. The application of stiffeners expanded the stress distribution in the opening regions, causing the failure mechanism to evolve from sudden local instability to global plastic deformation behavior. Consequently, it was demonstrated that section geometry, fire protection, and stiffener conditions are decisive factors for ductility, load-bearing capacity, and failure safety under cyclic loading. This study contributes to the engineering understanding of the combined fire–cyclic loading effects, addressing a gap in the existing literature.