| The concrete-filled steel tube(CFT)columns have been widely used in modern building structure due to the high capacity,favorable anti-seismic resistance and fire resistance,and high speed construction.The cross-sections are flexible and the connections are convenient for rectangular CFT columns;hence,the columns are suitable to be applied to residence.For high-rise and super high-rise buildings,the cross-section areas of CFT are too large for the living space,the welding is difficult for thick steel plates,and the structures are also over weighted.The optimization of cross sections and the use of high-strength materials are useful for design.High-strength steel(HSS)can be produced and applied at a reasonable cost and quality in China.The use of HSS in steel structures had been investigated by some researchers while the use of HSS in CFT structures has not been studied in China and the design of CFT columns using Q460 HSS is not included in the Chinese code.In this study,the experimental and theoretical investigations of rectangular CFT columns using HSS have been carried out:Firstly,the comprehensive test databases including the test of 455 rectangular CFT columns under axial load and 319 columns under eccentric load were compiled.The analysis was performed based on the databases.The experiments including 21 columns and finite element(FE)analysis of rectangular CFT columns using Q460 and Q420 steel under axial load had been conducted.The results indicated that the strength index(SI),the concrete contribution ratio(CCR)decrease with the increase of width-to-thickness ratio or aspect ratio,whereas the high-strength steel will not cost the ductility.The suitable FE model was obtained in which the elastic-perfectly plastic model of HSS was utilized.The uneven confining effect of steel to concrete was analyzed.The changing process of the gap and the contact pressure was obtained in the FE analysis.The confining effect exists only close to the corner when the ultimate load approaches.The tests including 35 specimens,FE simulations and the parametric studies of rectangular CFT columns using Q460 steel under eccentric load were performed.The results indicated that the capacity increases as the aspect ratio increases from 1.0 to 1.5while the aspect ratio has little influence on the capacity with the aspect ratio ranging from 1.5 to 2.0.The larger the width-to-thickness ratio is,the less conservative the design codes are.The appropriate FE model for accurate simulation was developed to study the behavior of the rectangular CFT columns under eccentric load.The results showed that the steel and concrete separate at most of the cross section when the ultimate load approaches,the contact pressure is relatively low and the confining effect could be ignored.The behaviors of rectangular CFT columns using compact and noncompact sections,with various aspect ratios and steel strength were investigated in the parametric study.The proposal on the limitations of width-to-thickness ratio and steel strength of different design codes was given.Based on the experimental database and the theoretical analysis,the limitations of width-to-thickness ratio incorporating the varying aspect ratios and the steel strength were proposed.The formulas for calculating the axial capacity were derived based on the superposition theory.The variations of the aspect ratios with the eccentricity ratios were derived theoretically,and the limitations of width-to-thickness ratio of the respective flange and the web were obtained.The proposed aspect ratio in practical design is below 1.5.Based on the superposition theory,the full N-M curve was developed to account for the plastic resistance.The curves were verified to be reasonably conservative.Finally,the artificial neural network models(ANN1 and ANN2)were developed and validated.The artificial neural network models can accurately predict the axial bearing capacity of rectangular CFT columns,and the generalization capacity of the models was favorable.The parametric study was implemented using the models.The variations of the capacity with the high-strength steel and concrete were predicted.The best composite effect can be achieved when the ratio of steel strength to the concrete strength is equal to 7.0,hence,the suitable match of the steel strength and the concrete was proposed.The proposed width-to-thickness ratio limitation is (?) which is similar to CECS159 and GB 50936. |
