Fire assessment of steel beam members with partial passive fire protection coverage

In offshore structures there are instances where the application of passive fire protection (PFP) is not possible or desired on certain portions of a structural member's surface area. The most common cases are those where the top surface is left unprotected due to the presence of deck grating or plating. Current code and standard provisions on heat transfer and strength assessment of restrained flexural members are not directly applicable to these cases. Thus, a case study is presented for performing a fire assessment of a restrained plate girder subjected to jet fire impingement with the top flange surface left unprotected. To assess residual strength and perform non-linear analyses under combined thermal and static loading, a heat transfer analysis was first performed to obtain the time histories of the two dimensional heat distributions throughout the studied cross-section. The results showed that the top flange heats up rapidly and the heat conducts very slowly down the web to the rest of the cross-section, with a very large thermal gradient occurring over the height of the section. Approximate screening calculations for the cross-section, based on AISC capacity equations, indicated that the member will quickly exceed its elastic capacity and that local buckling may occur prior to yielding. Advanced non-linear finite element analysis of the mechanical response confirmed large amounts of plasticity and local buckling occur, but showed that global integrity of the member is maintained for the duration of the fire due to redundancy and catenary action. Recommendations and conclusions on analysis methods for partially protected deck members are made based on the results of this study.