| Reinforced concrete filled steel tube(R-CFST)is a composite member with small cross-sectional area,good ductility,high bearing capacity and excel ent seismic performance,which is formed by inserting steel cages on the basis of concrete filled steel tube structures(CFST).It is widely used in long-span bridges,earthquake-prone areas and large space structures at the bottom.The forms of such composite members are mostly round R-CFST,square R-CFST,rectangular R-CFST and opposite R-CFST.this paper takes square r-cfst as the research object.At present,the existing research results of square R-CFST all take the stirrup form of its members,the strength grade of concrete and the wall thickness of steel tubes as the change parameters,and study the mechanical properties of the members,hysteretic properties,earthquake resistance,fire resistance and the influence of internal components on the mechanical properties of square R-CFST,etc.,without in-depth study of longitudinal reinforcement and its reinforcement ratio.For this reason,taking the ratio of longitudinal reinforcement as the variable parameter,this paper has carried out experimental and numeric a l simulation research on R-CFST short columns with square section:(1)Experimental research: 10 square R-CFST specimens(5 kinds of reinforcement ratio),3 square CFST specimens and 3 C(plain concrete)specimens are made,and loading tests are carried out,and the measured data are comprehensive ly analyzed in terms of failure mode,bearing capacity,ductility,rigidity,combined effect and yield ratio.The results show that there is an optimal reinforcement ratio of longitudinal reinforcement,which makes the mechanical properties of square R-CFST specimens optimal.According to the experimental results,the lower limit of the optimal reinforcement ratio is 1%.(2)Numerical simulation research: The experimental results show that there is an upper limit of the optimal reinforcement ratio for square R-CFST specimens.In order to find out this upper limit,finite element simulation is established on the basis of experiments.Firstly,the reliability of the established structural model is verified by experiments,and then numerical simulation research on 8 different longitud ina l reinforcement ratios is carried out.The stress-strain curves of the longitud ina l reinforcement of the specimen are analyzed comprehensively by means of differe nce and fracture toughness,and the upper limit of the optimal reinforcement ratio of the longitudinal reinforcement is 3.2%.Combining the experiment with finite element method,the recommended range of the optimal reinforcement ratio of longitud ina l reinforcement is 1%～3.2%.At the same time,in order to explore the research of steel tube wall thickness on the mechanical performance of components,four kinds of steel tube wall thicknesses are simulated,and each kind of steel tube wall thickness is simulated with five kinds of different longitudinal reinforcement ratios.A total of 20 square R-CFST simulated specimens are simulated.Through quantitative analysis of the extracted load-displacement curves,it is concluded that the steel tube wall thickness should not be too large or too smal.When the width-to-thickness ratio is about 62.5,the mechanical performance of the square R-CFST specimens is optimal.(3)Research on calculation formula of bearing capacity: According to the results of previous experiments and numerical simulation,16 models are simulated with 16 different longitudinal reinforcement ratios as variation parameters based on the exis ting more accurate finite element square R-CFST model.Through simulation analysis,it is clear that the existing standard CFST bearing capacity calculation formula is no longer applicable to R-CFST bearing capacity calculation.In this paper,the formula for calculating the bearing capacity of R-CFST specimens is given based on the combined effect coefficient between the various parts of R-CFST.The formula is verified by combining the bearing capacity values of the tests.The results are conservative,simple in form and convenient to calculate.Through the above research,the following conclusions are finally summarized :(1)The failure mode and load-displacement curve of the established model are similar to those of the test and have higher reliability.(2)Through the combination of experiment and finite element method,it is found that when the longitud ina l reinforcement ratio is 1%～3.2% and the width-to-thickness ratio is about 62.5,the RCFST specimen has the best mechanical properties.(3)The bearing capacity calculation formula proposed in this paper is simple,easy to calculate,conservative and has a large safety factor. |
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