Steel framed structures often utilize concrete due to several advantages composite construction offers over other types of construction. The composite action that develops between steel and concrete significantly enhances structural performance under ambient and fire conditions. However, the beneficial effects of composite action are not often taken into consideration in evaluating the fire response of structures due to poor understanding on the behavior of composite structural systems, and lack of design methodologies for evaluating fire resistance. With the aim of developing an understanding on the behavior of composite structural systems under fire exposure, both experimental and numerical studies were carried out as part of this study. The experimental studies consisted of analyzing the response of four composite beam slab assemblies under fire exposure. The assemblies consisted of a network of five steel beams, atop which was cast various types of concrete slabs. The assemblies were tested under design fire exposure and realistic load levels. In the fire tests, special attention was given to monitor the development of composite action and tensile membrane behavior under realistic loading and fire conditions.Data from the fire resistance tests and the literature were utilized to validate the response of composite column, beam slab assembly, and full-scale steel framed structural models created in SAFIR finite element based computer program. The validity of the program is established by comparing measured temperatures, deflections, and failure modes observed in testing with those predicted by SAFIR. These validated models were then applied to study the influence that critical factors have on the fire response of composite structural systems at the element, assembly, and system levels. In total, more than 2000 numerical simulations were conducted to quantify the effect of critical parameters on the fire response of steel framed structures. In each of the simulations, the failure times were evaluated based on strength limit states.Data generated from the parametric studies was applied to develop design methodologies for evaluating the fire resistance of concrete filled HSS columns, and composite beam slab assemblies. The design methodology for concrete filled HSS columns is based on equivalent fire severity principals, and utilizes the equal area concept to establish equivalency between severity of a design fire, and that of ASTM E-119 fire exposure for predicting failure of concrete filled HSS columns under design fire exposure. For beam-slab assemblies, a relationship between maximum design fire temperature and fire response is utilized to establish a correlation between fire resistance under standard ASTM E-119 fire exposure and under design fire exposure. The validity of the proposed methods is established by comparing the predictions from these methods with results from SAFIR analysis. To further demonstrate the validity of the proposed methodologies, fire resistance calculations have been carried out for a typical eight story steel framed office building, and compared with SAFIR predictions. The building was analyzed under various fire scenarios and structural configurations to illustrate the improvements in fire resistance achieved through composite construction. Initially, with unprotected steel member, failure occurred in less than 20 minutes in the structure, after incorporating concrete filled HSS columns and SFRC beam-slab assemblies, fire resistance was enhanced to such an extent that fire protection can be eliminated from columns and secondary beams while still providing the required level of fire resistance.
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