Ultimate Torsional Performance of Square CFDST Beams before and after Fire Exposure: Effect of Outer Tube Thickness

Before and after being exposed to fire, concrete-filled double-skin steel tubular (CFDST) beams were tested. Six square beams with outer tube thicknesses of 1.3, 1.7, and 2.6 mm were made and then put through pure torsion until they broke. Half of the samples were tested in normal conditions, and the other half were tested after being exposed to the ISO 834 standard fire for 90 minutes. The results showed that all the beams had stable and ductile torque–rotation behavior up to about 55° of twist, and there was no sign of brittle failure. After the heating and cooling cycles, the beams still had most of their torsional rigidity. The stiffness values were about two-thirds to nine-tenths of what they were before the fire. It was noted that making the outer steel wall thicker helped the beams keep more of their stiffness after being exposed to fire. The thicker walls clearly made the section less likely to lose torsional strength, which shows that wall thickness has a big effect on how things behave after a fire. The results showed that CFDST beams could still safely handle torsional loads after being in a fire. Because of this, they are good for use in bridge beams, transfer beams, and building cores, where both fire resistance and torsional strength are needed.