Fire resilience of a steel-concrete composite floor system :

Abstract

The National Institute of Standards and Technology is currently conducting a series of large compartment fire tests to investigate the behavior and fire-induced failure mechanisms of the full-scale composite floor systems situated in the two-story and two-bays by three-bays structural steel frame. This report presents the research background, details of experimental design involved with the two-story test building, and the results from the first fire experiment (Test #1) conducted at the National Fire Research Laboratory. The Test #1 was aimed to quantify the fire resistance and behavior of a 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 test floor assembly built in the middle edge bay in the floor plan of the test building was tested to failure under a natural gas fueled compartment fire simulating the standard furnace temperature-time relationship while sustaining hydraulically applied mechanical loads. This study showed that the protected floor beams and girders of the test floor 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 prior to reaching 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 slab breach in the middle of the test bay at 70 min. This center breach, accompanied by ruptures of embedded wire reinforcement, was caused by 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.