Limit of Width-to-Thickness Ratio of Rectangular Concrete-Filled Steel Tubular Column

Concrete-filled steel tubular(CFST) structures can give full play to the advantages of both concrete and steel materials, and have been widely used in commercial and public buildings. With the implementation of the prefabricated building policy in recent years, CFST columns with rectangular section are getting more and more attention in residential projects due to its advantages of excellent bearing capacity and simple joint structure. Compared with commercial and public buildings. residential steel structures have higher requirements for the effective use of building interior space (avoid " convex beams and columns" ) and economy (reduce the amount of steel used). In order to meet these requirements, the section of the rectangular concrete-filled steel tube column in the residential steel structure generally has a large aspect ratio, and the width to thickness ratio of the plate is as large as possible. Currently, the sectional aspect ratio of rectangular CFST column has reached about 3. 3 in the application of engineering, lager than the common range(no more than 2. 0). The local buckling of steel tubes is the one of the most important failure modes of CFST column, which will cause the significant decrease of bearing capacity of CFST column and affect the residual bearing capacity of the specimens. The limit of width-tothickness ratio is recommended in specifications for preventing the local buckling. Therefore, it is necessary to pay attention to the width-to-thickness ratio of CFST columns. However, there is difference of the limitation of width-to-thickness ratio between Chinese code and foreign codes. Meanwhile, the limit of width / thickness ratio of the existing code is mainly based on the study of the component whose section aspect ratio is not more than 2. 0, and for residential steel structure should be used in a wide range of aspect ratio greater than 2. 0 of the wide applicability of the concrete filled steel tube column need further research.
The limit of sectional width-to-thickness ratio of rectangular CFST column is studied by finite element(FE) analysis. Based on the confined concrete constitutive model and concrete damage plastic(CDP) model, considering the effects of section height to width ratio restraint cross section aspect ratio coefficient. FE model of rectangular CFST column is established for analyzing the members with large aspect ratio. The accuracy of the FE model is verified by comparing with the test. Through the parameter analysis, considering the influence of sectional aspect ratio, width-to-thickness ratio, steel strength and concrete strength on the ultimate bearing capacity of rectangular CFST columns, the limit of sectional width-to-thickness ratio of rectangular CFST columns is analyzed. The limits of sectional width-to-thickness ratio of rectangular CFST column in codes are introduced, and the analysis result is compared with the limit in codes. By defining the normalized ultimate bearing capacity coefficient and taking the turning point of the curve of the ultimate load as the basis to determine the limit of width-to-thickness ratio of the steel tube, it is found that the steel strength and concrete strength are negatively correlated and positively correlated with the ultimate bearing capacity of the specimens, respectively. But effect of section width-to-thickness ratio on the ultimate bearing capacity is more critical. The results show that the limit of width-to-thickness ratio of rectangular CFST columns is about 50√235/f_y, which is close to the recommendation values of AIJ, EC4 and BS5400, but obviously smaller than the limits of GB 50936, CECS 159, GJ / B 4142, DBJ / T 13-51 and AISC 360.