| Building fire is one of the most frequent disasters.The mechanical properties of concrete and steel will deteriorate to different degrees under fire exposure,thus weakening the bearing capacity of reinforced concrete(RC)members.Therefore,it is of great theoretical and practical significance to carry out the research on bearing capacity of RC members subjected to fire.Based on this hot research field,the bearing capacity at diagonal section of RC beams subjected to fire was selected as the research object in this thesis.The deterioration mechanism of the shear bearing capacity at diagonal section of RC beams under thermomechanical coupling and after fire exposure were revealed experimentally,numerically and theoretically.The shear capacity deterioration of RC beams subjected to fire was studied,and the shear capacity calculation method at diagonal section of RC beams under and after fire exposure was proposed.The main contents and results are as follows:(1)The whole process experimental study on shear capacity of RC beams with and without web reinforcement was carried out under the coupling of fire and constant loads.Thirty full-scale RC beams were tested to investigate the fire resistance of beams,including the seventeen RC beams for the fire resistance tests under thermo-mechanical coupling,and thirteen RC beams for the static load tests at room temperature.Additionally,the shear span ratio,load ratio,longitudinal reinforcement ratio and stirrup ratio were selected as the critical parameters to investigate the temperature field distribution,failure modes and fire resistance limit of RC beams.The experimental results show that: 1)The shear-bending failure and bending failure were observed in RC beams with web reinforcement under thermo-mechanical coupling.However,the failure of RC beams without web reinforcement was mainly shown as the shear failure.Regarding the comparison specimens at room temperature,the shear failures were found in RC beams.2)The measured fire resistance limit of specimens decreased with the increase of shear span ratio under thermo-mechanical coupling.The shear span ratio had significant influence on the fire resistance of specimens without web reinforcement when compared with the specimens with web reinforcement 3)The fire resistance of RC beams decreased with the increase of the load ratio,and its change was approximately linear with the load ratio.Compared with specimens with load ratio of 0.4,the fire resistance of specimens with load ratio of 0.5 and 0.6 decreased by about 5.0% and 12.7%,respectively.4)The fire resistance of specimens with or without web reinforcements increased with the increase of the longitudinal reinforcement ratio,indicating that the longitudinal reinforcement at the bottom can significantly improve the fire resistance of specimens under three-sides fire.5)When the load ratio of RC beams was constant,stirrup ratio had a great effect on the fire resistance of beams.The measured fire resistance of specimens increased with the increase of stirrup ratio,and the configuration of stirrup can reduce the sudden failure of beams when approaching the fire resistance.(2)The experimental study on shear capacity of post-fire RC beams with and without web reinforcement was carried out.The static load tests were carried out on 22 post-fire specimens to study the shear performance at diagonal section.The fire time,shear span ratio,load ratio and longitudinal reinforcement ratio were selected as the critical parameters to investigate the failure characteristics at diagonal section,crack development law and residual shear capacity of RC beams.The results show that: 1)The comparison beams at room temperature and post-fire beams both failed in shear failure,and the measured residual shear capacity of specimens decreased with the increase of fire time.2)The residual shear capacity and stiffness of post-fire specimens decreased with the increase of shear span ratio.When the shear span ratio and fire conditions were constant,the residual shear capacity reduction percentage of post-fire beams without web reinforcement was about twice than that of beams with web reinforcement.3)The residual shear capacity degradation of beams under the coupling of fire and constant load was higher than that of beams only under fire without constant load.That is,the shear capacity of beams undergoing thermo-mechanical coupling could be further degraded.4)When the stirrup ratio,fire time and shear span ratio were constant,the residual shear capacity of post-fire beams with and without web reinforcement increased with the increase of longitudinal reinforcement ratio,while the vertical deformation decreased in the static load test,which was the same as that of the reference specimen at room temperature.(3)The experimental study on shear capacity at diagonal section of indirectly loaded beams under and after fire exposure was carried out.Eight RC beams with the second beam overhanging section were designed and fabricated to simulate the indirect load with and without additional transverse reinforcement,including two beams for the diagonal section shear bearing capacity tested under thermomechanical coupling,four beams for the indirect loading testes with natural cooling after fire exposure,and the other two comparison beams for the tests at room temperature.The failure mechanism,failure modes and shear capacity degradation of indirectly loaded beams with and without additional reinforcement under thermo-mechanical coupling were analyzed.Besides,the mechanical characteristics of beams with and without additional reinforcement in the process of indirect loading test,such as the macro appearance,crack development law,failure modes and residual bearing capacity,were analyzed.The results show that:1)Under thermo-mechanical coupling,the beams without additional reinforcement were designed according to ‘strong bending and weak shear’,the diagonal crack development between the junction of primary and secondary beams and the main beam support made the main beam fracture along the diagonal crack.When reaching the fire resistance limit,the failure form was typical shear failure under thermo-mechanical coupling.Therefore,it is suggested that the additional reinforcement at the junction of the secondary and the main beam should be conservatively designed to avoid diagonal section failure or fracture brittle failure under elevated temperature.2)When reaching the fire resistance limit,the failure of beams with additional reinforcement designed according to ‘strong shear and weak bending’ was typical bending failure,and there was no obvious failure phenomenon at the junction of the secondary and the main beam,that is,the failure form of indirect loading beams meets the design expectation of ‘strong shear and weak bending’ under thermo-mechanical coupling.3)The static load test results after fire show that all beams without additional reinforcement occurred shear failure caused by the diagonal cracks at the junction of the secondary and main beam.However,the projection length of damaged diagonal crack of beam after fire was longer than that of comparison beam at room temperature,and the shear capacity of beam after thermo-mechanical coupling was lower than that of beam only subjected to fire.All beams with additional reinforcement exhibited bending failure,but the ultimate bearing capacity of post-fire beam was significantly lower than that of the comparison beam at room temperature.(4)The finite element simulation analysis(FEA)method for RC beams under and after fire was established,and the parameter study was conducted based on the proposed FEA method.Based on ABAQUS finite element analysis software,the numerical analysis method of RC beams under and after fire was established,and the accuracy of this method was verified according to the experimental results.Based on this FEA method,the shear span ratio,fire condition,fire time,temperature rise curve and stirrup ratio were selected as critical parameters to investigated the shear performance of forty-three directly loaded beams under fire,directly loaded and indirectly loaded beams after fire.The results show that:Under the same temperature curve,the fire resistance of RC beams under four-sides fire was lower than that of beams under three-sides fire.Under the same fire time,the residual shear capacity of RC beams after four-sides fire was lower than that of beams after three-sides fire.The relationship of residual shear capacity of postfire beams considering the shear span ratio,fire time and stirrup ratio was proposed by the data fitting analysis,which can provide a basis for establishing the shear capacity calculation method of RC beams under and after fire exposure.(5)The shear capacity calculation method of RC beams under fire was proposed.Based on the shear capacity calculation method of beams at room temperature,the shear capacity calculation method of directly loaded beams under elevated temperature,which considered strength deterioration of concrete and steel and the failure mechanism of beams under fire,was proposed using the composite strength criterion of concrete in shear compression zone.The feasibility of the calculation method was verified through the experimental data.The comparison results show that the average deviation between the theoretical calculation results and experimental results was 2.44%,which was in good agreement with the experimental results.Additionally,the effect law of normal section bending and diagonal section shear capacity on the overall bearing performance and failure modes of RC beams were analyzed under fire.(6)The residual shear capacity calculation methods of directly loaded and indirectly loaded beams after fire were proposed.Based on the diagonal section shear capacity calculation method of beams at room temperature,the strength reduction coefficient of material was introduced according to the test and numerical analysis results,and the two residual shear capacity calculation methods of directly loaded beams after fire exposure were proposed using the semi-empirical and semi-theoretical methods.The deviation between the theoretical calculation and experimental results of the above two methods were compared and analyzed,and the suggestion of the calculation method was put forward.In addition,the residual shear capacity calculation method of indirectly loaded RC beams after fire was proposed by introducing shear capacity reduction coefficient of indirectly loaded beams relative to directly loaded beams after fire.The theoretical calculation results were in good agreement with the experimental results.In summary,the shear failure mechanism,deterioration and shear bearing calculation method of RC beams under and after fire were studied systematically in this thesis.The calculation problem of the shear bearing capacity of RC beams under direct loading and indirect loading were solved,and the solution can provide a basis for the fire resistance design standards establishment of relevant structures and engineering practice. |
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