Fire resistance of steel fiber reinforced concrete beams after low-velocity impact loading

Engineering structures in the city environment surrounded with combustible materials are prone to be in the danger of combined effects of blast-induced impact loading and fire. Four steel-fiber reinforced concrete (SFRC) beams were tested after pre-impact loading (impact velocity = 5.4 m/s) to explore their fire-resistance. The beams were first subjected to impact loadings and then exposed to fire with a constant load. The failure patterns of beams were observed, and the time histories of mid-span deflections, temperature field as well as rebar strains were recorded. Moreover, the fire resistance of these beams were discussed. A three-dimensional finite element numerical model considering the effects of strain rate and high temperature were established. In the simulations, a two-step analysis method was utilized. In the first-step, the impact loading process was simulated. While, in the second-step, and the failure behavior of the SFRC beams subjected to both fire and constant mechanical loading was modeled with a sequentially coupled thermal-stress analysis method, in which the simulation results obtained in the first-step were taken as the initial state. Good agreement between the simulation results and the test results illustrates the validation of the simulation method. It is found that the failure patterns of SFRC under the low-velocity impact load is altered from shear type to bending type with the increase of steel fiber dosage. When the impact energy is relative lower, though weakened by the pre-impact, the beam still work in an elastic stage and shows good fire resistance. Moreover, limited to the low-energy impact conditions, the influence of steel fiber content on the thermal and mechanical behaviors of damaged beam can be ignored.