Restrained steel beam behaviour in case of fire, depending on load ratio, performed in case study of a bus station

A new bus station infrastructure, in San Sebastian city, in Spain, was seriously compromised by the strict requirements of fire safety regulations, due to the necessity of maintaining its aesthetic appearance, not jeopardizing a beautiful steel structure. The structural system is compounded by several twenty meters long radial beams and an iconic central multi-column. The infrastructure is a 7,500 m² bus station hall located underground with a 7.2 m maximum height. The steel structure is made of Corten steel and it is compounded by restrained beams connected to columns with a bracing system based on concrete rings. A comprehensive fire safety engineering study was required, specially focusing on structural analysis, defining and validating an integrated solution based on fire suppression systems. During this study, restrained beams happened to fail before what it was expected. So, a more detailed analysis was developed, because it is normally supposed that restrained steel beams have better fire-resistant capability than isolated steel beams, due to "catenary action". When the steel beams reached temperatures below 350°C, a lateral buckling failure happened, before the tension force initiates and catenaria action does not occur. A real application is presented, in which the length (20m) and properties (hollow section) of steel elements make that the real critical temperature was load ratio dependent, and the results could be considered unsafe if the analysis was developed according to the sound principle appearing in the Eurocodes regarding the verification of the fire resistant of an element as isolate, submitted to an ISO fire curve without considering indirect effects caused by elongations.