| SRC structure has the advantages of good ductility,high bearing capacity,good seismic performance,convenient construction and so on.It is widely used in high-rise buildings,and the research on its fire resistance is particularly important.Based on finite element method and analytical method,this paper studies the calculation method of bearing capacity of SRC column under fire.The specific contents and results are as follows:1.Summarize the research status of thermal properties of concrete and steel and mechanical properties under fire and after fire at home and abroad.According to the existing test results,the value of mechanical properties of the material is analyzed,and the relevant parameters suitable for this paper are selected.2.On the basis of relevant tests,the finite element model of SRC column was established by using ANSYS analysis platform,and the temperature field in the whole process was analyzed when it was subjected to fire on all sides.The temperature time-history curves of the whole process and the distribution law of the maximum temperature in the section of the three measuring points inside the longitudinal reinforcement,inside the flange plate and the web center were obtained.3.Based on the distribution law of cross-section temperature field and concrete step conversion model,combined with the calculation method of SRC column bearing capacity under normal temperature,the equivalent section method and finite element method are used to calculate SRC column bearing capacity under high temperature for specific examples.The results show that the three-step model of RC structure is not suitable for SRC structure,and relevant parameters need to be adjusted.It is suggested that the concrete strength reduction coefficient should be 0.9 and 0.35 when the temperature is 300℃～500℃ and 500℃～800℃ respectively,and the three-step calculation model is improved.Due to the rough calculation of equivalent section method,in order to meet the requirements of higher accuracy,a simple calculation method based on Matlab software platform was established:"Firstly,the temperature field of section was fitted with Matlab surface,and then the fitted temperature function was substituted into the corresponding constitutive relationship,and finally the ultimate bearing capacity under fire was determined".4.In view of the shortcomings of using "slightly convex straight truncated pyramid"to describe the characteristics of concrete strength distribution in section,this paper proposes two simplified calculation methods based on temperature field:(1)Two-step conversion method.The basic idea of the method is as follows:the temperature field of the section is divided into temperature zones.Zone 1 is the zone from the inside of the stirrup to the section steel and its surrounding concrete,and its temperature is the converted temperature.Zone 2 is the section steel and its surrounding concrete area,and its temperature is the highest temperature in zone 2.(2)Surface fitting method.The maximum temperature of a certain number of nodes was obtained through the finite element post-processing interface(the temperature of the extracted nodes should reflect the temperature distribution of the section to a certain extent),and then the fitting function of the maximum temperature field of the section was obtained by surface fitting of the scattered points.Based on the two simplified calculation methods of temperature field of SRC column after fire,the simplified calculation formulas of axial and bias ultimate bearing capacity of SRC column after fire were further deduced,and a concrete example was given to verify the applicability of the two simplified calculation methods. |
