The Punching Shear Mechanism in Reinforced-Concrete Slabs under Fire Conditions

Abstract

Punching shear transfer at ambient temperature is a complex phenomenon and continues to be the subject of research. The addition of elevated temperature makes the problem of punching shear even more challenging. The shear behavior of reinforced-concrete in fire is dependent upon the degradation of the individual material properties with temperature, their interaction, and the effects of restrained thermal expansion. This paper reports the experimental findings of fifteen 1400×1400mm slabcolumn specimens, tested in punching shear at elevated temperatures. A purpose built reaction frame allowed the support conditions to be either restrained or unrestrained. Load was applied to the column stub and the slabs were heated from above using a 960×990mm array of propane gas radiant panels. Instrumentation included strain gauges, thermocouples, displacement transducers and digital cameras for displacement measurement using digital image correlation (DIC). Clear differences between the behavior of slabs with different support conditions were observed. Unrestrained slabs failed soon after the heating started, whereas the equivalent restrained slabs endured up to two hours of heating. One of the restrained slabs (the most heavily reinforced) went on to fail during cooling. The tests indicated that the diameter of the shear cone does not depend upon the restraint condition, and DIC allowed the crack locations and slab rotation angles to be visualized throughout testing