Experimental study on fire resistance of a full-scale composite floor assembly in a two-story steel framed building

Abstract

This paper presents the first of four planned fire experiments of a full-scale two-story steel framed building constructed at the National Fire Research Laboratory. This first experiment was aimed to quantify the fire resistance and behaviour of the steel composite floor system commonly built in the United States, incorporating prescriptive approaches for a 2-hour fire resistance rating. The 9.1 m × 6.1 m composite floor assembly, situated in the edge bay on the first floor of the prototype building, was tested to failure under a natural gas fuelled compartment fire and simultaneously applied mechanical loads. The test showed that the protected floor beams and girders of the test assembly achieved matching or superior fire resistance based on the acceptance criteria of standard furnace testing. However, the test floor slab exhibited a potential fire hazard within a specified fire rating period because of the use of a minimum code-compliant shrinkage reinforcement (59 mm2 /m). The heated slab cracked around the interior edges of the test bay less than 30 min into heating, followed by centre cracks along the secondary beam at 70 min. This centre breach, accompanied by ruptures of wire reinforcement, was caused by tension (due to catenary action) developed along the shorter span of the test floor assembly. This result suggests that the minimum slab reinforcement prescribed for normal conditions may not be sufficient to activate tensile membrane action of a composite floor system under the 2-hour standard fire exposure.