Influence of concrete strength and length/diameter on the axial capacity of CFT columns

This paper presents an experimental analysis of the confinement effects in steel–concrete composite columns regarding two parameters: concrete compressive strength and column slenderness. Sixteen concrete-filled steel tubular columns with circular cross section were tested under axial loading. The tested columns were filled by concrete with compressive strengths of 30, 60, 80, and 100 MPa, and had length/diameter ratios of 3, 5, 7, and 10. The experimental values of the columns’ ultimate load were compared to the predictions of 4 code provisions: the Brazilian Code NBR 8800:2008, Eurocode 4 (EN 1994-1-1:2004), AINSI/AISC 360:2005, and CAN/CSA S16-01:2001. According to the results, the load capacity of the composite columns increased with increasing concrete strength and decreased with increasing length/diameter ratio. In general, the code provisions were highly accurate in the prediction of column capacity. Among them, the Brazilian Code was the most conservative, while Eurocode 4 presented the values closest to the experimental results.     

Improving strength,stiffness and ductility of CFDST columns by external confinement

Concrete-filled double-skin tubular (CFDST) column is one of the most efficient forms of column construction in which the steel tubes provides both axial strength and confining pressure to enhance the strength and ductility of the in-filled concrete. Compared with confined reinforced concrete columns, CFDST columns had stronger and more uniform confining pressure provided to the in-filled concrete by the steel tubes, which reduces the steel congestion problem for better concrete placing quality. However, a major shortcoming of the CFDST columns is the imperfect interface bonding that occurred at the elastic stage that reduces the elastic strength and stiffness of columns. To improve the situation, it is proposed in this study to use external steel rings to restrict the dilation of outer steel tube of CFDST columns. For verification, a series of uni-axial compression test was performed on some CFDST columns with external steel rings. From the results, it was found that the elastic strength, elastic stiffness and ductility were enhanced by installing the steel rings as external confinement. Lastly, a theoretical model for predicting the axial strength of confined CFDST columns has been developed.     

Finite element analysis of CFT columns subjected to an axial compressive force and bending moment in combination

Proper material constitutive models for concrete-filled tube (CFT) columns subjected to an axial compressive force and bending moment in combination are proposed and verified in this paper by using the nonlinear finite element program ABAQUS compared against experimental data. In the numerical analysis, the cross sections of the CFT columns are categorized into three groups, i.e., ones with circular sections, ones with square sections, and ones with square sections stiffened with reinforcing ties.It is shown that the steel tubes can provide a good confining effect on the concrete core when the axial compressive force is large. The confining effect of a square CFT stiffened by reinforcing ties is stronger than that of the same square CFT without stiffening ties but weaker than that of a circular CFT. Nevertheless, when the spacing of reinforcing ties is small, a CFT with a square section might possibly achieve the same confining effect as one with a circular section.     

Elasto-plastic analysis of circular concrete-filled steel tube stub columns

This paper presents a full-range elasto-plastic analysis using continuum mechanics on circular concrete-filled steel tube (CFT) stub columns under concentric loading condition, covering concrete strengths from 30 to 120 MPa and diameter-to-wall thickness ratio (D/t) greater than 20. Firstly, a constitutive model was employed for laterally-confined concrete under axial compression. A continuum mechanics model was then established and the corresponding elasto-plastic analysis was performed through a FORTRAN program. This model is able to present full-range stress-strain developments in axial, radial and perimeter directions and further clarify the load sharing pattern between the steel tube and the concrete core. Based on the proposed model, parametric analysis was conducted to investigate the effects of material strengths and sectional steel ratio on the triaxial stress-strain developments and the load sharing pattern. In addition, the model was simplified to predict the ultimate capacity and the load-axial strain relationship of CFT composite sections and the results are in good agreement with experiments. Further comparisons were made of the approach developed by Han et al. 2004 and the existing international standards.     

Hysteretic behavior of multi-cell T- Shaped concrete-filled steel tubular columns

Several multi-cell improvement methods for solving existing problems of conventional T-shaped concrete-filled steel tubular (T-CFST) columns and for determining steel׳s optimal distributions for increasing the strength and ductility of the columns are presented. An experimental study with eight multi-cell T-shaped concrete-filled steel tubular (MT-CFST) columns and one conventional T-CFST column under low frequency cyclic loading was conducted. Effects of the multi-cell layout and the concrete strength on the hysteretic behavior of the specimens were investigated. Experimental results showed that the lateral load-displacement hysteretic curves of the columns were generally saturated with a slight pinching effect. Owing to the asymmetry of the T-shaped cross section, the hysteretic behavior of the composite columns is asymmetrical in different loading directions. The improved MT-CFST columns showed better seismic behavior due to high load bearing capacity, ductility and energy dissipation capacity. Furthermore, the non-linear finite element analysis was performed to simulate the hysteretic behavior of the specimens and the numerical results agreed well with the test results. In conclusion, with an increasing axial load ratio, the ultimate lateral load in the pushing direction gradually decreases and is reached earlier, whereas the ultimate lateral load in the pulling direction increases slightly under low axial ratio and decreases under high axial load ratio.     

Experimental investigation of thin-walled concrete-filled steel tube columns with reinforced lattice angle

Experimental investigation of thin-walled concrete-filled steel tube columns with reinforced lattice angle was conducted in this study. The lattice angle was designed to reinforce the concrete-filled steel tube columns by increasing the percentage of steel cross-sectional area. Column specimens having different lengths ranged from 500 mm to 3500 mm were tested. The behavior and strengths of concrete-filled steel tube columns with lattice angle were investigated. In addition, concrete-filled steel tube columns having the same size but without reinforced lattice angle were also tested for comparison. Material properties of the concrete and steel used in the test specimens were measured. The test strengths are compared with the design strengths calculated using the AISC Specification and Eurocode for the design of composite structural members. A new design method was also proposed for the concrete-filled steel tube columns with reinforced lattice angle. It is shown that the design predictions from the proposed method agree with test results well.     

Experimental behavior of circular concrete-filled steel tube stub columns

This paper presented an experimental study on the behavior of circular, concrete-filled, steel tube (CFT) stub columns with self-compacting concrete (SCC) and normal concrete (NC) concentrically loaded in compression to failure. Four measurement methods on the axial deformation of specimens were compared. Seventeen specimens were tested to investigate the effects of concrete strength, notched holes or slots, and different loading conditions on the ultimate capacity and the load-deformation behavior of the columns. The behavior of these stub columns in confinement was discussed.It is concluded that for the specimens with the entire section loaded, strain gauges with different dimensions could record the strains of the steel tubes, and electronic displacement transducers with certain gauge lengths could record the axial displacement. By using higher strength concrete, the specimens with the entire section loaded experienced a significant increase in the ultimate capacity, but their residual capacity after failure is almost constant. However, once the steel tube was notched, the axial compressive stiffness of specimens was reduced; in some cases the ultimate capacity was also reduced, and the steel tube acted more as a transverse confinement than an axial compression component. Eurocode 4 predicted a reasonable capacity for the unnotched CFT stub columns with both SCC and NC if the entire section of the specimen is loaded.     

Mechanical performances of concrete-filled steel tubular stub columns with round ends under axial loading

An experimental study of 22 concrete-filled round-ended steel tubular (CFRT) stub columns under axial compression is conducted compared with 4 circular concrete-filled steel tubular (CFT) stub columns. The influences of width–thickness ratio, concrete strength, steel yield strength and wall-thickness of steel tube on the ultimate bearing capacity of the CFRT columns are discussed. The 3D finite element (FE) model is also developed to analyze the behavior of the CFRT columns under axial compression. From the results, local buckling of the round-ended steel tube associated with shear failure of in-filled concrete could be observed. With the increasing width–thickness ratio, the corresponding load–strain curves have a shorter elastic–plastic stage. The parametric studies indicate that the concrete strength, tube thickness and width–thickness ratio of the steel tube also have a great effect on the ultimate bearing capacity. The numerical results also show that the confinement effect of the stub columns decreases with the increasing width–thickness ratio. A practical calculation formula for the bearing capacity of the CFRT stub columns is proposed, which is well in agreement with the experimental results.     

设纵向加劲肋后矩形钢管混凝土柱屈曲机制分析及构造建议

针对搜集的38根设肋试件的轴压试验结果,采用ABAQUS进行了有限元计算。计算结果表明,轴压承载力与试验轴压承载力误差在7%以内,破坏模式与试验破坏模式吻合较好。根据相关试验数据,对加劲肋的工作机理及受力状态进行了分析。分析结果表明,平板加劲肋截面抗弯刚度对试件承载力有明显的影响,应选取合适加劲肋抗弯刚度及截面尺寸,才能既使组合构件的承载性能得到有效发挥,同时用钢量得到合理优化;钢管屈曲模式受加劲肋刚度的影响较大,随着加劲肋刚度的增大,钢管板件逐渐由在板件横向的一个半波转变为两个半波;当加劲肋刚度达到临界刚度后,加劲肋截面面积不再影响钢管屈曲模式,但试件的轴压承载力随加劲肋面积的增大而增大。     

Numerical simulation of mega steel reinforced concrete columns with different steel sections

In recent years, the mega steel reinforced concrete (SRC) columns have been applied in super‐tall buildings. The previous research on SRC columns mainly focuses on the components with simple arrangement of encased steel, such as H‐shaped steel and cross‐shaped steel, with little attention paid to mega SRC columns that always have complicated encased steel and large steel ratio. The cyclic loading tests were carried out on scaled mega SRC column specimens with different cross‐section type of encased steel and steel ratio. The nonlinear three‐dimensional finite element and fiber element models were established respectively to simulate the inelastic behavior of mega SRC columns. The equivalent plastic strain of concrete, steel and rebar as well as the confinement effect on the concrete caused by the rebar and the steel plate were analyzed subsequently. By using the verified numerical model, the seismic behavior of specimens with different axial compression ratio was also studied. The test and analysis results indicate that the steel ratio and axial compression ratio have significant effects on the seismic performance of mega SRC columns, while the effect of cross‐section type of encased steel is not significant. Copyright © 2016 John Wiley & Sons, Ltd.     

Ultimate capacity of rectangular concrete-filled steel tubular columns under unequal load eccentricities

The paper describes 36 experimental tests conducted on rectangular and square tubular columns filled with normal and high strength concrete and subjected to a non-constant bending moment distribution with respect to the weak axis. The test parameters were the nominal strength of concrete (30 and 90 MPa), the cross-section aspect ratio (square or rectangular), the thickness (4 or 5 mm) and the ratio of the top and bottom first order eccentricities e_top/e_bottom (1, 0.5, 0 and − 0.5). The ultimate load of each test was compared with the design loads from Eurocode 4, presenting unsafe results inside a 10% safety margin. The tests show that the use of high strength concrete is more useful for the cases of non-constant bending moment, whereas if the aim is to obtain a more ductile behavior the use of concrete-filled columns is more appealing in the cases of normal strength concrete with non-constant bending moments because, although they resist less axial force than the members with HSC, they obtain a softened post-peak behavior.     

Experimental study of high strength concrete-filled circular tubular columns under eccentric loading

The paper describes 37 tests conducted on slender circular tubular columns filled with normal and high strength concrete subjected to eccentric axial load. The test parameters were the nominal strength of concrete (30, 70 and 90 MPa), the diameter to thickness ratio D/t, the eccentricity ratio e/D and the column slenderness (L/D). The experimental ultimate load of each test was compared with the design loads from Eurocode 4, which limits the strength of concrete up to 50 MPa. The aim of the paper is to establish the advisability of the use of high strength concretes as opposed to that of normal strength concretes by comparing three performance indices: concrete contribution ratio, strength index and ductility index. The results show for the limited cases analyzed that the use of high strength concrete for slender composite columns is interesting since this achieves ductile behavior despite the increase in load-carrying capacity is not greatly enhanced.     

Suggested empirical models for the axial capacity of circular CFT stub columns

In this paper, a total of 250 experimental tests of axially loaded circular concrete-filled steel tube (CFT) stub columns, published in the literature was summarized. The applicability of the current design codes such as ACI, Australian Standards, AISC, AIJ, Eurocode 4, DL/T and some available empirical models proposed by various researchers for calculating the axial capacity of circular CFT stub columns was examined using these experimental data. Based on the investigations, four new empirical models for predicting the axial capacity of circular CFT stub columns are proposed. The comparisons between the experimental results and the predictions of these models show that the proposed empirical models provide a direct, compact, and efficient representation of the ultimate strength of circular CFT stub columns made with not only normal strength but also high strength steel tubes and concrete. Finally, the limiting values of the maximum effective length, the compressive strength of concrete, the yield strength of steel tubes and the diameter-to-thickness for circular CFT stub columns with respect to the present empirical models are suggested. It is expected that engineers can easily use the present empirical models to estimate the axial capacities of circular CFT stub columns for engineering designs.     

Experimental study on seismic performance of mega steel‐reinforced concrete columns subjected to cyclic loads

In recent years, mega steel‐reinforced concrete (SRC) columns have been applied in super tall buildings. The previous research on SRC columns mainly focuses on the components with simple arrangement of shaped steel, such as H‐shaped steel and cross‐shaped steel, with little attention paid to mega SRC columns, which always have complicated encased steel and large steel ratio. The cyclic loading tests were carried out on scaled mega SRC column models with different cross‐section type of encased steel and steel ratio. The principal damage states were investigated throughout the entire testing process. Based on the test results, the effects of steel ratio and the cross‐section type of encased steel on the damage characteristics, hysteretic behavior, ductility, secant stiffness degradation, energy dissipation capacity and maximum crack width were analyzed. The test results indicate that the steel ratio has significant effect on the seismic performance of mega SRC columns while the effect of cross‐section type of encased steel is not significant. Copyright © 2015 John Wiley & Sons, Ltd.     

A novel steel section for concrete-filled tubular columns

In modern structural constructions, concrete-filled steel tubular (CFT) columns have gradually become a central element in structural systems like tall buildings, bridges and so forth. The effective parameters on load carrying capacity of CFT columns are the bond between the steel and internal concrete, local buckling strength of steel tube, creep of concrete and loading conditions of column at connections. Considering these effective parameters, a novel section is suggested which can be used for columns of tall buildings and bridges with large spans. The main characteristic of the suggested steel section is internal longitudinal symmetric stiffeners. In the present study, a comparative investigation into the behavior of this novel section (with circular and octagonal shapes) and the most common used sections of CFT columns has been carried out under axial and cyclic loading. Having verified the finite element modeling, several different analyses have been undertaken. The results of the analyses clearly exhibit the increase in strength and ductility of the suggested novel section under axial and cyclic loading and therefore, its application is recommended in construction practice.     

Behaviour of multi‐cell composite T-shaped concrete-filled steel tubular columns under axial compression

To improve the behaviour of conventional T-shaped concrete-filled steel tubular (CFST) columns, multi-cell composite T-shaped concrete-filled steel tubular (MT-CFST) columns are proposed in this paper. Experimental study of 25MT-CFST columns, including 13 short specimens with various cross sections and material properties and 12 slender specimens with different slenderness ratios, subjected to axial loads was conducted. The failure modes, axial load–strain curves for short specimens and axial load–lateral deflection curves for slender specimens were investigated. The test results were compared with design approaches for conventional CFST columns presented in Eurocode 4, AISC specification, Australian standard AS51006, Chinese code CECS159, and Hong Kong steel code, and it was found that all the design codes underestimate the bearing capacity of both short and slender MT-CFST columns to some extent.     

不等截面三肢格构式钢管混凝土柱的换算长细比计算

根据弹性稳定理论,同时考虑弯矩和剪力对结构临界荷载的影响,推导了不等截面三肢格构式钢管混凝土柱的换算长细比,并进行了适当的简化,可供工程设计人员参考。     

Nonlinear analysis of circular concrete-filled steel tubular short columns under axial loading

The confinement effect provided by the steel tube in a circular concrete-filled steel tubular (CFST) short column remarkably increases the strength and ductility of the concrete core. The reliable prediction using nonlinear analysis methods for circular CFST columns relies on the use of accurate models for confined concrete. In this paper, accurate constitutive models for normal and high strength concrete confined by either normal or high strength circular steel tubes are proposed. A generic fiber element model that incorporates the proposed constitutive models of confined concrete is created for simulating the nonlinear inelastic behavior of circular CFST short columns under axial loading. The generic fiber element model developed is verified by comparisons of computational results with existing experimental data. Extensive parametric studies are conducted to examine the accuracy of various confining pressure models and the effects of the tube diameter-to-thickness ratio, concrete compressive strengths and steel yield strengths on the fundamental behavior of circular CFST columns. A new design formula accounting for concrete confinement effects is also proposed for circular CFST columns. It is demonstrated that the generic fiber element model and design formula adequately predict the ultimate strength and behavior of axially loaded circular CFST columns and can be used in the design of normal and high strength circular CFST columns.     

Simulation and design recommendations of eccentrically loaded slender concrete-filled tubular columns

This paper proposes an efficient numerical model for the simulation of the behavior of slender circular concrete-filled tubular columns subjected to eccentric axial load with single curvature, for the cases of both normal and high strength concrete. The paper focuses on the study of the influence that the variables affecting beam-column behavior (length and relative slenderness) and the variables affecting section behavior (diameter/thickness ratio, mechanical capacity of steel) have on the overall buckling of this type of column. An extended parametric study is carried out to propose design recommendations, primarily to establish the importance of the use of high strength concrete compared with that of normal strength concrete. The results show that for slender elements the optimum design is reached when the mechanical capacity of the steel is slightly lower than that of the concrete contribution.     

多边形空心钢管混凝土轴压短柱非线性分析

在轴压试验结果的基础上,选择合理的材料本构关系模型,应用大型通用ANSYS有限元软件对四边形和八边形空心钢管混凝土短柱进行了有限元模拟,着重讨论了有限元模型的建立方法,并通过模拟结果与试验结果的对比,验证了有限元模型的合理性。