Research On The Continuous Collapse Process Of RC Frame Structure Under Extreme Fire

In recent years,the construction level of our country has been rapidly improved.Because the reinforced concrete frame structure has the characteristics of flexible space partition,light weight,excellent seismic performance and material saving,it is widely used in life.When a framed building is subjected to extreme fire,the strength of reinforced concrete decreases sharply with the increase of temperature.The whole structure is very easy to be destroyed because of a certain in extreme fire,which leads to the progressive collapse of the whole building.Therefore,it has important engineering significance and scientific significance to study the progressive collapse of reinforced concrete frame structure at high temperature.Based on the ABAQUS software,this paper has implemented the following work on the progressive collapse of the RC frame structure under extreme fire.(1)Based on the material properties and mechanical properties of steel bar and concrete under high temperature,the appropriate finite element unit is selected by using the ABAQUS software,and the experiment of fire-induced collapse of the reinforced concrete frame of Tsinghua University is simulated first.The experimental results and numerical simulation results are coMPared and analyzed,and the correctness of the material parameters is verified.(2)A two-story single-span frame model is established based on the verified material parameters.The effects of reinforcement ratio,fire position,column cross-section type,thickNess of protective layer and stiffness ratio of beam to column on the mechanical properties of the structure under fire are studied.Firstly,the frame structure with the thickNess of 20mm and 35mm is analyzed.The thicker the protective layer is,the smaller the deflection of beam is,and the difference of deflection is 3.58 mm.Meanwhile,the frame with 20mm thickNess of cover layer collapses in 152min,and the structure with a thickNess of 35mm collapses in 166min.For the influence factors of the location under fire,the P-? effect is more obvious when the lower part is under fire,the bending moment of the lower column is larger 49.18kN ·m than that of the upper fire,and the collapse time of the lower fire structure is 18 minutes earlier than that of the upper fire.For column cross section type,the circular column frame with the same volume under fire is more easily destroyed,and the bending moment difference between top and bottom reaches 134.7kN·m at 132 min.The maximum difference of bending moment difference of rectangular column is only 102.7kN·m,and circular frame is destroyed 19min earlier than rectangular frame.For the influence factors of linear stiffness ratio,the structure with small linear stiffness not only satisfies the weak beam with strong column,but also the structure with smaller linear stiffness ratio is smaller than the structure with larger stiffness,whether the extreme value of the bending moment extreme value or the deflection of beam.(3)Without considering the influence of floor slab,a typical three-story frame model is established.Through the method of whole process fire stress analysis,the fire in the corner room,the fire in the middle room in the long side and the fire in the interior room are simulated respectively.The collapse mode and mechanical response change of the structure under extreme fire are analyzed.The collapse time of the three conditions is 300 min,277min and 253min respectively.The results show that the deformation produces P-?effect,and the catenary mechanism is not easy to be formed because of the failure of steel bar at high temperature.Based on the results,the measures to improve the resistance of frame structure to progressive collapse under fire are put forward.