Fire Behavior of Composite Beam with Shear Connection
|關鍵字:||防火性能;合成梁;剪力接頭;擴孔設計;fire resistance;composite beam;shear connection;oversized bolt-hole|
The understanding of fire resistance and behavior of steel structural buildings in fire is extremely important because the material strength of the steel reduces greatly at elevated temperatures. Due to the restraining effects of the slab and girders, the steel beam in structural buildings will be subjected to axial compression caused by the thermal elongation in temperature rise and, however, the steel beams will be tensioned due to thermal contraction in the cooling phase. The thermal loading may result in failures on the shear tab and bolts of the shear connection. Corresponding to different types of shear connections, forces resulted from this thermal loading may differ and lead to different failure mode and fire resistance. In this study, two large-scale fire tests were carried out to investigate the deformation, temperature variation, failure mode, and fire resistance of the composite beam with shear connection under elevated temperatures. Two types of the shear connections, coped and un-coped beam ends, were used as the parameter of the specimens. In accordance with the specifications of the composite beam, the specimens were designed to have shear stud, reinforcing bar, slab, and oversized bolt-hole. Experiments were conducted following CNS 12514-1 and 12514-6 standards. Loading buckets were placed on the slab to simulate a load ratio of 0.6 in service condition of the steel beam. The furnace temperature followed standard heating curve. The steel beam and metal deck were exposed to fire while the girders were fire-protected. The tests terminated at 121 min considering the safety although the specimens didn’t reach the failure criteria. The test results showed that deflection of the steel beam increased with the rise of temperature. During the initial heating phase, the deflection varied quickly and increased linearly. In the middle heating phase, the change in deflection slowed down and stabilizing due to the loss of the steel beam strength and then the slab dominated the entire structural behavior. In the final phase of heating, the bending mechanism of the slab gradually shifted to membrane action mechanism, accompanied with more cracks on the top surface of the slab and large deflection of the steel beam. Because the membrane action of the slab provided the specimen load-carrying capacity, the whole structure remained stable. Meanwhile, axial force of the steel beam was speculated reducing greatly due to the oversized bolt-hole; thus, local buckling in the steel beam, rupturing of the shear plate, and shear failure of bolt were not observed. Overall, test results demonstrated that, exposed to standard fire, the specimens reached two hours fire rating. Different shear connection had insignificant effect on structural behavior, temperature distribution, failure mode, and fire resistance, attributed to oversized bolt-hole and membrane action developed on the slab.