Fire behavior of axially loaded slender high strength concrete-filled tubular columns

This paper describes sixteen fire tests conducted on slender circular hollow section columns filled with normal and high strength concrete, subjected to concentric axial loads. The test parameters were the nominal strength of concrete (30 and 80 MPa), the infilling type (plain concrete, reinforced concrete and steel fiber reinforced concrete) and the axial load level (20% and 40%). The columns were tested under fixed-pinned boundary conditions and the relative slenderness at room temperature was higher than 0.5 in all of the cases. A numerical model was validated against the tests, in order to extend the results and understand the failure mode of such columns. It is the aim of this paper to study the influence in a fire situation of the use of high strength concrete, as opposed to normal strength concrete. The results have shown that for slender columns subjected to high temperatures, the behavior of high strength concrete was different than for stub columns, spalling not being observed in the experiments. Furthermore, the addition of steel fibers was not found very advantageous in slender columns, since no increment in terms of fire resistance was obtained for the columns which used this type of reinforcement. However, the addition of reinforcing bars seems to be the solution in some cases, where the use of external fire protection wants to be avoided in the design of HSS structures, since the reinforcing bars allow the tube to resist a higher axial load.     

Fire behavior of eccentrically loaded slender high strength concrete-filled tubular columns

This paper describes a series of 24 fire tests conducted on slender circular hollow section columns filled with normal and high strength concrete, subjected to eccentric axial load. It is a continuation of a previous research paper (Romero et al., 2011 [1]), where test results on centrally loaded columns were presented. The test parameters covered in this fire testing program were the nominal strength of concrete (30 and 90 MPa), the infilling type (plain, bar-reinforced and steel fiber reinforced concrete), the axial load level (20% and 40%) and the load eccentricity (20 and 50 mm). The columns were tested under fixed-pinned boundary conditions, with a relative slenderness at room temperature higher than 0.5 for all the specimens. The aim of this paper is to study the influence of eccentricity in combination with the type of concrete infill. The results show that the addition of steel fibers does not improve the fire resistance of slender columns under eccentric loads, as compared to columns filled with plain concrete. However, the addition of reinforcing bars increases the fire resistance of the columns in this situation. Filling the steel hollow section columns with concrete increases their fire resistance, the increase in load bearing capacity being more noticeable for columns filled with high strength concrete. A comparison with the current simple calculation model in Eurocode 4 Part 1.2 shows that, although the method is safe for eccentrically loaded columns, it produces a high error in the predictions for columns filled with plain or steel fiber reinforced concrete.     

圆钢管高强混凝土轴压短柱受剪承载力分析

为了深入研究钢管高强混凝土轴压短柱破坏模式与破坏机理,提出适合钢管高强混凝土轴压短柱极限承载力计算方法,针对圆钢管高强混凝土轴压短柱大都发生剪切破坏这一典型现象,引入莫尔-库仑强度理论,从理论上分析其发生剪切破坏的原因和受力机理,并从剪切破坏的角度提出了钢管高强混凝土轴压短柱承载力计算方法。利用基于圆钢管高强混凝土轴压短柱试验研究和有限元分析回归得到的处于复杂应力场中的钢管纵向应力σv-纵向应变ε关系曲线和钢管横向应力σh-纵向应变ε关系曲线的数学表达式,得到了钢管高强混凝土轴压短柱承载力包络线的简化计算方法,简化计算曲线与试验曲线吻合良好,可用于分析钢管高强混凝土轴压短柱的受剪承载力。     

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.     

Influence of ultra-high strength infill in slender concrete-filled steel tubular columns

This paper describes 24 tests conducted on slender circular tubular columns filled with normal, high, and ultra-high strength concrete for plain, bar reinforced and steel fiber reinforced columns. These were reinforced and subjected to both concentric and eccentric axial load. It is a continuation of a previous research paper (Portoles et al., 2011 [1]), which presented test results on eccentrically loaded plain concrete columns. The test parameters are nominal strength of concrete (30, 90 and 130 MPa), eccentricity e (0, 20 and 50 mm) and type of reinforcement. A comparison with the corresponding empty tubular columns is performed, as the aim of the paper is to analyze the influence of each type of infill and establish the best option for practical application. For the limited cases analyzed the results show that the addition of high or ultra-high strength infill is more useful for concentric loaded cases than for eccentric loaded ones, where it seems that the best design option is the utilization of bar reinforced concrete filling rather than steel fiber to reinforce CFST columns. The experimental ultimate load of each test was compared with the design loads from Eurocode 4, accurate for the eccentrically loaded tests.     

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。     

多室式钢管混凝土T形短柱轴压性能试验研究

对钢管混凝土T形柱进行合理的改进,提出多室式钢管混凝土T形柱(MT-CFST柱)。进行12个短柱试件的轴压试验并对其破坏过程作了详细描述,给出试件的轴压应力-纵向平均应变曲线,同时也考察试件的破坏形态及截面各尺寸、板件布置方式、钢板厚度、混凝土强度等因素对试件力学性能的影响。试验结果表明:多室式钢管混凝土T形柱能较好地增强T形截面内钢材对混凝土的约束作用,发挥两种材料的组合性能;试件的破坏以局部鼓曲和整体剪切两种形式为主,轴压性能受腹板高度、钢板厚度及混凝土强度的影响较大。同时,参考国内外4种钢管混凝土规范中的计算方法对试件的承载力进行计算,经过对比后发现,按EC4规范计算的结果与试验结果符合最好。     

Behaviour of high strength axially loaded concrete columns confined with helices

This paper presents an experimental study of the behaviour of four HSC columns confined by helical reinforcement of 500 MPa and with 500 MPa longitudinal reinforcement. Two NSC columns confined with a 500 MPa-grade helix and two confined with a 250-grade helix, all with 400-grade longitudinal bars were also tested. The helices were made of 10 mm diameter bars and had a constant diameter of 160 mm. A comparison on the effect of the pitch sizes for the helical reinforcement is made with the use of 50 and 75 mm pitches as well as the effect of the distribution of the longitudinal bars. With this variation in design, a higher strength, yet more ductile column can be achieved.     

Behaviour of short and slender concrete-filled stainless steel tubular columns

In this paper, a series of tests were carried out on short and slender concrete-filled stainless steel tubular columns to explore their performance under axial compression or combined actions of axial force and bending moment. Empty short steel hollow sections were also tested for comparison. The test results showed that the performance of the composite columns was quite good and have the potential to be used extensively as structural members. Comparisons of the test results were also made with several existing design methods for conventional concrete-filled carbon steel tubular columns as presented in Australian standard AS 5100 (2004), American code AISC (2005), Chinese code DBJ/T 13-51-2010 (2010), and Eurocode 4 (2004), which indicates that all the codes are somewhat conservative in predicting the load-carrying capacities of both short and slender columns.     

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.     

High strength circular concrete-filled steel tubular slender beam-columns, Part II: Fundamental behavior

There is relatively little experimental and numerical research on the fundamental behavior of high strength circular concrete-filled steel tubular (CFST) slender beam-columns. In a companion paper, a new numerical model for predicting the nonlinear inelastic behavior of high strength circular CFST slender beam-columns under axial load and bending was presented. The numerical model developed accounts for confinement effects on the strength and ductility of the concrete core and on circular steel tubes as well as initial geometric imperfections of beam-columns. This paper presents the verification of the numerical model and extensive parametric studies on the fundamental behavior of high strength circular CFST slender beam-columns. The ultimate strengths and axial load-deflection responses of circular CFST slender beam-columns under eccentric loading predicted by the numerical model are verified by corresponding experimental results. The computer program implementing the numerical model is used to investigate the fundamental behavior of high strength circular CFST slender beam-columns in terms of load-deflection responses, ultimate strengths, axial load-moment interaction diagrams, and strength increase due to concrete confinement. Parameters examined include column slenderness ratio, eccentricity ratio, concrete compressive strengths, steel yield strengths, steel ratio and concrete confinement. It is demonstrated that the numerical model developed is an efficient computer simulation and design tool for high strength circular CFST slender beam-columns. Benchmark numerical results presented in this paper are valuable in the development of composite design codes for high strength circular CFST slender beam-columns.     

Behaviour of eccentrically loaded high-strength rectangular concrete-filled steel tubular columns

This paper presents an experimental and analytical study of the behaviour of high-strength rectangular concrete-filled steel tubular (CFT) columns subjected to eccentric loading. Four slender and 16 stub CFT columns were tested to investigate their structural behaviour. The test parameters were material strengths (), cross-sectional aspect ratio (1.0-2.0), slenderness ratio (10 and 60) and load eccentricity ratio (e/H=0.10-0.42). Favourable ductility performance was observed for all specimens during the tests. Experimental failure loads are employed to calibrate the specifications in the design codes EC4, ACI and AISC. Results show that EC4 overestimates the failure loads of the specimens by 4%. ACI and AISC conservatively predict the failure loads by 14% and 24%, respectively. An analytical model is developed to predict the behaviour of high-strength rectangular CFT columns subjected to eccentric loading. Calibration of the model against the test results shows that it closely estimates the ultimate capacities of the columns by 3%.     

Nonlinear behavior of concrete-filled stainless steel stiffened slender tube columns

This paper investigates the nonlinear behavior of concrete-filled high strength stainless steel stiffened slender square and rectangular hollow section columns. The stiffened slender tubes had overall depth-to-plate thickness (D/t) ratios ranging 60–160. The concrete strengths covered normal and high-strength concrete. The investigation focused on short axially loaded columns. A nonlinear finite element (FE) model has been developed to study the behavior of the concrete-filled stiffened tube columns. A parametric study was conducted to investigate the effects of cross-section geometry and concrete strength on the behavior and strength of the columns. The results of the concrete-filled stiffened tube columns were compared with the results of the companion concrete-filled unstiffened tube columns. It is shown that the concrete-filled stiffened slender tube columns offer a considerable increase in the column strength and ductility than the concrete-filled unstiffened slender tube columns. The column strengths obtained from the FE analysis were compared with the design strengths calculated using the American specifications and Australian/New Zealand standards. A design equation was proposed for concrete-filled stainless steel stiffened slender tube columns. It is shown that the proposed modified equation provides more accurate design strengths compared to the American and Australian/New Zealand predictions.     

Numerical modelling of concrete-filled lean duplex slender stainless steel tubular stub columns

Major technological advances in materials processing have led to the development of duplex stainless steels with exceptional mechanical properties. Duplexes have great potential for expanding future structural design possibilities, enabling a reduction in section sizes leading to lighter structures. The duplex grades offer combination of higher strength than austenitics as well as a great majority of carbon steels with similar or superior corrosion resistance. However, high nickel prices have more recently led to a demand for lean duplexes with low nickel content, such as grade EN 1.4162. Extensive work is needed to include the lean duplex grade EN 1.4162, into design standards such as EN 1993-1-4 and ENV 1994-1-1. Accordingly, finite element modelling for concrete-filled lean duplex slender stainless steel tubular stub columns of Grade EN 1.4162 is presented in this paper. The paper is predominantly concerned with two parameters: cross-section shape and concrete compressive strength, which have not yet been investigated. The non-linear displacement analysis of the columns was constructed herein based on the confined concrete model provided by Hu et al. (2003) [15]. The behaviour of the columns was investigated using a range of concrete cylinder strengths (25–100 MPa). The overall depth-to-width ratios (aspect ratio) varied from 1.0 to 1.8. The depth-to-plate thickness ratio of the tube sections varied from 60 to 90. The concrete-filled lean duplex slender stainless steel tubular columns were subjected to uniform axial compression over the concrete and stainless steel tube to force the entire section to undergo the same deformations by blocking action. The ABAQUS 6.6 program, as a finite element package, is used in the current work. The results showed that the design rules specified in the ASCE are highly conservative for square and rectangular concrete-filled lean duplex slender stainless steel stub columns while they are conservative in the case of European specifications. A new design strength is, therefore, proposed that is accurately found to represent the behaviour of concrete-filled lean duplex stainless steel tubular stub columns.     

Behaviour of concrete-filled steel tubular stub columns subjected to axially local compression

The behaviour of concrete-filled steel tubular (CFST) stub columns subjected to axially local compression was experimentally investigated in this paper. A total of thirty-two specimens were tested. The main parameters varied in the tests are: (1) sectional types: circular and square; (2) local compression area ratio (concrete cross-sectional area to local compression area): 1.44 and 16; and (3) thickness of the endplate: from 2 to 12 mm. A finite element analysis modelling was used for the analysis of CFST stub columns subjected to axially local compression, and a comparison of results calculated using this modelling shows generally good agreement with the test results. The theoretical modelling was then used to investigate the mechanism of the composite columns subjected to axially local compression.     

Fire resistance of axially loaded concrete filled steel tube columns

The behaviour of axially loaded square and circular concrete-filled steel tube (CFST) columns when exposed to elevated temperatures is investigated in this paper. The fire resistance of this kind of composite tubes is calculated. Comparison of the square and circular columns in the fire resistance shows that, for columns with the same steel and concrete cross-section areas, the circular column has slightly better fire resistance than the square column.     

Nonlinear behaviour of eccentrically loaded FR concrete-filled stainless steel tubular columns

This paper investigates the nonlinear behaviour of eccentrically loaded fibre reinforced (FR) concrete-filled stainless steel tubular composite columns. A nonlinear 3-D finite element model for the axially loaded composite columns, recently reported by the author, was extended to study the structural performance of the eccentrically loaded composite columns. The columns were pin-ended subjected to an eccentric load acting along one axis. The model accounted for the inelastic behaviour of the composite column components, effect of FR concrete confinement and interface between the stainless steel section and concrete. The measured initial local and overall geometric imperfections were carefully incorporated in the model. The finite element model has been validated against tests previously reported by the author. Furthermore, the variables that influence the eccentrically loaded composite column behaviour and strength comprising different eccentricities, different column slenderness and different concrete strengths were investigated in an extensive parametric study comprising 72 columns. The composite column strengths and moment resistances predicted from the finite element analysis were compared with the design composite column strengths and moment resistances calculated using the Eurocode 4. The study has shown that finite element modelling could effectively assess the accuracy of the design rules in current codes of practice.     

双向压弯方钢管高强混凝土构件滞回性能试验与分析

进行了4个双向压弯方钢管高强混凝土构件在低周反复荷载作用下的试验研究。试验中的主要参数为轴压比(0.433～0.624)、宽厚比(30和50)和长细比(18.5和25.4)。试验结果表明,随轴压比的提高,构件的承载力和延性降低,随钢管宽厚比的增加,构件的承载力提高;试件的位移延性系数μ=3.63～5.18,抗震性能良好。利用纤维模型法计算了构件的荷载位移滞回曲线,计算结果与试验结果吻合较好。进行了压弯构件的截面参数分析,研究了轴压比、套箍系数、加载角度和荷载类型对构件滞回性能的影响。建立了方钢管混凝土压弯构件的弯矩曲率和荷载位移恢复力模型,基于恢复力模型的计算结果与试验结果吻合较好,可为结构的弹塑性动力分析提供参考。     

多腔钢管混凝土异形柱不同方向抗震性能试验研究

为研究多腔钢管混凝土异形柱在不同加载方向的性能差异,设计并制作了5个1/30缩尺多腔钢管混凝土异形柱模型试件,截面构造包括13腔体基本型截面和5腔体简化型截面,基本型试件包括沿截面长轴加载、短轴加载和与长轴成45°方向加载的试件,简化型试件包括沿截面长轴加载和短轴加载试件。采用拟静力试验方法,对各试件的破坏特征、滞回性能、承载力、刚度退化、延性以及耗能能力进行研究。结果表明：各试件屈服位移角均值为1/92,破坏位移角均值为1/30,具有良好的延性;对于基本型试件,随着加载方向由长轴转向短轴,承载力及耗能能力逐渐降低,不同加载方向的性能差异显著;简化型试件的承载力较基本型试件的小,但具有更好的延性,耗能能力相当,且不同加载方向性能差异较小。使用ABAQUS有限元软件进行数值模拟,所得模拟结果与试验结果吻合良好,由此进一步分析了钢筋笼、轴压比以及混凝土强度对异形钢管混凝土柱受力性能的影响,结果显示：钢筋笼能够提高试件承载力;随着轴压比增大,试件延性和承载力逐渐降低;试件承载力随混凝土强度增大而增大。