Research On Collapse Behaviour Of Rc Beam-column Connections With Varying Reinforcement Detailing Subjected To Fires

As the most widely used structure,it is generally acknowledged that the reinforced concrete(RC)structures exhibit relatively good fire resistance.However,in recent years,there are large quantities of building fires,which had caused a considerable amount of fire damage on concrete structure or even structural collapse accidents,such as 11.3 Hengzhou building fire accident,Hengyang in 2013 and Harbin retail shop fire accident in 2015.The fire(high temperature)effect not only degrades the mechanical properties of steel rebars and concrete materials as well as the bond characteristics between them,but also has a significant influence on the internal force redistribution of the structure,the formation position and order of the plastic hinges as well as the rotational capacities.Presently,the research on the fire behaviour of reinforced concrete structures is mainly limited to the study of the fire resistance of individual members.It is still rare to consider the structurally restrained members and substructure fire resistance tests.On the other hand,under the coupling effect of fires and its secondary disasters,RC structures are more likely to suffer nonlinear large deformation.The accumulation of damage,(e.g.the failure of weak structural segments or key structural members),may arise severe damage to surrounding components,leading to the overall or partial collapse of the structure.The existing investigations on the structural collapse behaviour are mainly focused on steel structures,the investigations on the collapse behaviour of reinforced concrete structures are limited to normal temperature conditions.Therefore,it is necessary to study the force mechanism of reinforced concrete structure subjected to fire and collapse,as well as the rotational and tying capacities.The main research work in this dissertation includes the following aspects:(1)The push-down tests are carried out on the five pre-fire reinforced concrete beam-column connections with varying reinforcement detailing at beam ends.The development of cracks and the formation of plastic hinge are observed in detail throughout the whole process of test.The development of vertical load and horizontal reactions,as well as the variation in deflection,are also closely monitored.By comparing the axial force-moment curves(N-M curves)of critical sections,the influence of varying reinforcement detailing at beam ends on the rotational capacities of pre-fire reinforced concrete beam-column connections subjected to column removal is investigated.The push-down behaviour and underlying failure mechanism of reinforced concrete beam-column joints are revealed.(2)The push-down tests are carried out on the 4 in-fire and 4 post-fire reinforced concrete beam-column connections,respectively.The thermal/cooling responses of the RC beam-column connections in the whole process of fire tests are monitored,including the development of the temperature field and the response of the horizontal force and the deflection induced by heating or cooling process.The vertical reaction force,horizontal force development and the vertical deflection during the push-down tests are measured.The emphasis is attached to investigating the influence of the reinforcement detailing at beam ends on the rotation capacities of the RC beam-column connections under in-fire and post-fire conditions.Based on the experimental observations and results,the practical suggestions on the application of varying reinforcement detailing at beam ends in the fire collapse resistance design are proposed.(3)Base on the relevant experimental data,the influence of the different anchorage detailing of bottom reinforcement on the tying capacities of fire-exposed RC beam-columns is analyzed.The main variables to be investigated include: anchorage reinforcement details(different anchorage length,continuous longitudinal reinforcement and different anchorage forms),fire conditions(pre-fire,in-fire and post-fire),locations of column removal(interior or exterior column removal)and span-height ratio.In addition,the distribution of bond stress in the anch orage area of bottom rebar extended into column is analyzed,and the simplified calculation method is proposed.The influence factors of strain penetration effect on the strain distribution of the longitudinal reinforcement of the beam are discussed,and the importance of further study is emphasized.(4)As expected,large-scale structural tests are costly and time-consuming,and can only be used for validation and,similarly,detailed finite-element simulations require in-depth expertise and significant computing resources.Moreover,the previous theoretical analysis methods are rather complicated,but the calculation accuracy is still not high,and none of them accounted for the influence of fire conditions.Therefore,based on the sectional analysis of the combined loading states of axial force and bending moment at the critical section,and that considering the material damage caused by high temperatures and the internal force and deformation induced by fire,a practical calculation method is proposed.This calculation method selects the key transitions to simplify the bearing capacity-deflection curve into multi-line segments to characterize the responses of the frame beam-column connections subjected to column loss.The reliability of the simplified method is verified by comparing the calculation results with the experimental results.