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.     

An experimental behaviour of concrete-filled steel tubular columns

In this paper results of tests conducted on 27 concrete-filled steel tubular columns are reported. The test parameters were the column slenderness, the load eccentricity covering axially and eccentrically loaded columns with single or double curvature bending and the compressive strength of the concrete core. The test results demonstrate the influence of these parameters on the strength and behaviour of concrete-filled steel tubular columns. A comparison of experimental failure loads with the predicted failure loads in accordance with the method described in Eurocode 4 Part 1.1 showed good agreement for axially and eccentrically loaded columns with single curvature bending whereas for columns with double curvature bending the Eurocode loads were higher and on the unsafe side. More tests are needed for the case of double curvature bending.     

Experiments on rectangular concrete-filled steel tubes loaded axially on a partially stressed cross-sectional area

The present paper aims to experimentally investigate the behaviour of rectangular concrete-filled steel tubular (RCFST) stub columns loaded axially on a partially stressed cross-sectional area. A total of twenty-two specimens, including thirteen partially loaded composite specimens, six partially loaded plain concrete reference specimens and three corresponding fully loaded composite specimens, were tested under concentric loads. The main parameters varied in the tests are: (1) depth-to-width ratio of the rectangular sections: from 1.2 to 1.8; (2) partial compression area ratio (concrete cross-sectional area to partial compression area): from 1.44 to 17.42; (3) steel wall thickness of top endplate: from 0 mm to 12 mm; and (4) shape of the bearing plate for partial compression: rectangular and strip. The test results showed that, while being loaded on partial cross-sectional area, RCFST stub column had a favourable bearing capacity and ductility compared with the corresponding plain concrete specimens. Simplified formulae were used for the prediction of the ultimate strength of CFST stub columns loaded on partial cross-sectional area.     

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.     

Eccentrically loaded concrete encased steel composite columns

This paper presents a nonlinear 3-D finite element model for eccentrically loaded concrete encased steel composite columns. The columns were pin-ended subjected to an eccentric load acting along the major axis, with eccentricity varied from 0.125 to 0.375 of the overall depth (D) of the column sections. The model accounted for the inelastic behaviour of steel, concrete, longitudinal and transverse reinforcement bars as well as the effect of concrete confinement of the concrete encased steel composite columns. The interface between the steel section and concrete, the longitudinal and transverse reinforcement bars, and the reinforcement bars and concrete were also considered allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the column. The initial overall geometric imperfection was carefully incorporated in the model. The finite element model has been validated against existing test results. The concrete strengths varied from normal to high strength (30–110 MPa). The steel section yield stresses also varied from normal to high strength (275–690 MPa). Furthermore, the variables that influence the eccentrically loaded composite column behaviour and strength comprising different eccentricities, different column dimensions, different structural steel sizes, different concrete strengths, and different structural steel yield stresses were investigated in a parametric study. Generally, it is shown that the effect on the composite column strength owing to the increase in structural steel yield stress is significant for eccentrically loaded columns with small eccentricity of 0.125D. On the other hand, for columns with higher eccentricity 0.375D, the effect on the composite column strength due to the increase in structural steel yield stress is significant for columns with concrete strengths lower than 70 MPa. The strength of composite columns obtained from the finite element analysis were compared with the design strengths calculated using the Eurocode 4 for composite columns. Generally, it is shown that the EC4 accurately predicted the eccentrically loaded composite columns, while overestimated the moment.     

钢管初应力对内配型钢的圆钢管混凝土柱受压性能影响

为研究钢管初应力对内配型钢钢管混凝土构件后期受力性能的影响,共设计了12根内配型钢圆钢管混凝土柱进行受压性能试验研究,试验主要参数为初应力系数和荷载偏心率。通过试验研究了内配型钢圆钢管混凝土柱的变形过程、破坏形态及不同参数对构件荷载-变形曲线和受力性能的影响。试验结果表明：初应力系数及荷载偏心率对内配型钢圆钢管混凝土柱最终的破坏形态影响较小,但不同长细比的构件破坏过程有所差异;初应力系数的变化对轴压短柱承载力影响较小,但对偏压柱受力性能有一定影响,随着初应力系数的增大其承载力下降;试验参数范围内,偏心率对柱承载力的影响明显大于初应力系数的影响。基于试验结果对有限元模型的可靠性进行验证,在模型验证基础上,分析钢管初应力对钢管与混凝土之间的相互作用和各部件承载力分配的影响,并进一步分析不同参数（如长细比、偏心率、型钢截面尺寸等）对柱承载力影响系数的影响。结果表明,钢管初应力系数、偏心率和长细比对柱承载力影响系数影响显著,基于此提出考虑钢管初应力时内配型钢钢管混凝土柱受压承载力计算方法,其计算结果与试验和数值结果吻合较好。     

薄壁圆钢管再生混合中长柱的轴压与偏压试验研究

通过36根薄壁圆钢管再生混合中长柱的轴压与偏压试验,考察废弃混凝土混合比、钢管壁厚、荷载偏心距等因素对试件受压性能的影响。基于新、旧混凝土的组合强度,根据国内外钢管混凝土结构设计标准,对试件的受压承载力进行计算对比。试验和分析结果发现:①采用25%或40%的废弃混凝土替换新混凝土后,钢管再生混合中长柱的受压承载力分别比全现浇钢管混凝土中长柱降低5.77%～9.71%和10.65%～15.63%,但前者的初始刚度、屈服性能和延性特征与后者基本相当;②为使钢管再生混合中长柱的计算受压承载力具有与全现浇钢管混凝土中长柱相近的安全性,可对前者计算结果乘以调整系数0.95;③总体来看,设计标准DBJ 13-51-2003和ANSI/AISC 360-05对轴压试件受压承载力的预测结果相对较好,JCJ 01-89对偏压试件受压承载力的预测结果相对较好。研究表明,薄壁圆钢管柱内放置废弃混凝土块体是废弃混凝土循环利用的一条有效途径。     

圆CFRP-钢管混凝土压弯构件静力性能试验研究

进行了32个圆CFRP-钢管混凝土压弯构件的试验研究。结果显示,构件的载荷-中截面挠度曲线可以分为弹性阶段、弹塑性阶段和下降阶段,构件的破坏属于延性破坏。分析表明,从加载之初直到极限荷载,钢管和CFRP在纵向和环向都可以协同工作,环向应变沿截面周边分布不均匀;钢管纵向应变沿截面高度的分布基本符合平截面假定;纵向受压的钢管存在内力重分布过程,纵向受拉区钢管对核心混凝土的环向约束效果不显著;构件的挠曲线大致为正弦半波曲线。采用纤维模型法分析圆CFRP-钢管混凝土压弯构件的荷载-中截面挠度曲线,分析结果与试验结果吻合较好且偏于安全。     

圆中空夹层钢管混凝土柱力学性能研究

分别进行了十四个轴心受压构件和十二个偏心受压构件的试验研究 ,二种构件的试验参数分别为钢管径厚比和空心率 ,构件长细比和偏心率。在确定组成圆中空夹层钢管混凝土的钢材和核心混凝土应力 -应变关系模型的基础上 ,利用数值解法对圆中空夹层钢管混凝土轴压和压弯构件的荷载 -变形关系进行了全过程分析 ,分析结果与试验结果吻合良好。最后在参数分析的基础上 ,提供了圆中空夹层钢管混凝土轴压和压弯构件承载力实用验算方法。     

Tests and calculations for hollow structural steel (HSS) stub columns filled with self-consolidating concrete (SCC)

The behaviour of self-consolidating concrete (SCC) filled hollow structural steel (HSS) stub columns subjected to an axial load was investigated experimentally. A total of 50 specimens were tested. The main parameters varied in the tests are: (1) sectional types: circular and square; (2) steel yielding strength: from 282 to 404 MPa; and (3) tube diameter or width to wall thickness ratio (D/t or B/t): from 30 to 134.A mechanics model is developed in this paper for concrete-filled HSS stub columns. A unified theory is described whereby a confinement factor (ξ) is introduced to describe the composite action of the steel tube and the filled concrete. The predicted load versus deformation relationship was in good agreement with test results. The theoretical model was used to investigate the influence of important parameters that determine the ultimate strength of the composite columns. The parametric and experimental studies provide information for the development of formulae for the calculation of the ultimate strength and the axial load versus axial strain curves of the composite columns. Comparisons are made with predicted stub column strengths using the existing codes, such as ACI-1999, AISC-LRFD-1999, AIJ-1997, BS5400-1979 and EC4-1994.     

钢管超高强混凝土长柱及偏压柱的性能与极限承载能力的研究

本文报导了钢管超高强混凝土长柱和偏压柱的试验研究工作。长柱试验研究结果表明,钢管超高强混凝土长柱的承载能力和极限纵向变形率随长细比Ｌｅ／ Ｄ的增大而下降,在所研究的 Ｌｅ／ Ｄ范围内,所有的钢管超高强混凝土长柱都有一定的延性,但延性随Ｌｅ／Ｄ的增大而降低。普通钢管混凝土长柱的承载能力考虑长细比影响的折减系数计算公式也适用于钢管超高强混凝土长柱。偏压柱试验研究结果表明,在偏心率为０．２２～０．６５范围内,加载后所有偏压柱试件横向无明显的外形变化。在相同的长细比下,随着偏心率的增加,试件的承载能力降低,极限纵向变形率降低,但总体来说,偏压短柱的纵向变形率比轴压短柱的极限应变要大一些。在相同的偏心率下,长细比越大,试件的承载能力和纵向变形率也越低。钢管超高强混凝土耐偏压能力等于或优于普通钢管混凝土偏压柱。经过适当修正的普通钢管混凝土偏心率折减系数可以用于钢管超高强混凝土偏压柱承载能力计算。     

Experimental study on steel tubular columns in-filled with plain and steel fiber reinforced concrete

An experimental investigation on the structural behaviour of steel tubular columns in-filled with plain and steel fiber reinforced concrete is presented in this study. A total of 16 concrete-filled steel tubular columns were constructed and tested subjected to biaxial bending and short-term axial load. The main variables considered in the test study were the cross section, slenderness, concrete compressive strength and the load eccentricity. In the presented study, a theoretical method for the prediction of ultimate strength capacity and load-deflection curves of concrete filled steel tube columns is proposed. In the analysis procedure, the nonlinear behaviour of the materials is considered and the slenderness effect has been taken into account. The experimental ultimate strength capacities and load-deflection curves of both plain and steel fiber concrete-filled tube columns have been compared with the analysis results and discussed in the paper. The results indicate that the addition of steel fibers in core concrete has considerable effect on the behaviour of concrete-filled steel tube columns.     

带约束拉杆方钢管混凝土短柱轴压性能试验研究

进行了10个带约束拉杆和5个不设约束拉杆方形钢管混凝土短柱轴心受压构件的试验研究。主要研究参数为约束拉杆直径和间距、钢管厚度、钢材强度等。试验研究表明,在轴心压力作用下,方形钢管混凝土短柱在设置了约束拉杆后,能有效改善横截面周边中部钢管对核心混凝土的约束作用,延迟或避免钢管在应力达到屈服强度前发生失稳性的局部屈曲而导致构件的过早破坏,从而使方形钢管混凝土轴压短柱的承载力和延性均有较大幅度的提高。同时,应用现有规范及规程有关方形钢管混凝土承载力计算公式,对带约束拉杆方形钢管混凝土轴压试件承载力进行计算,计算结果与试验结果相比较为保守。     

大长细比钢管混凝土轴心受压柱承载力的试验研究

进行了12个钢管混凝土和4个空铜管轴心受压构件的试验研究．研究结果表明,由于填充了混凝土,可以有效地延缓钢管的局部屈曲,从而使钢管混凝土长柱具有较高的承载力．通过试验结果表明,对于长细比为130～155的钢管混凝土柱,《钢-混凝土组合结构设计规程》（DL5058／T）中有关承载力计算公式的计算结果与实测结果相比偏于保守,浇灌混凝土以后,钢管混凝土构件的承载力较空钢管可提高30％左右．     

Experimental investigation on concrete-filled stainless steel stiffened tubular stub columns

This paper presents an experimental investigation on concrete-filled normal-strength stainless steel stiffened tubular stub columns using the austenitic stainless steel grade EN 1.4301 (304). The stiffened stainless steel tubes were fabricated by welding four lipped angles or two lipped channels at the lips. Therefore, the stiffeners were formed at the mid-depth of the sections. In total, five hollow columns and ten concrete-filled columns were tested. The longitudinal stiffener of the column plate was formed to avoid shrinkage of the concrete and to behave as a continuous connector between the concrete core and the stainless steel tube. The behavior of the columns was investigated using two different nominal concrete cubic strengths of 30 and 60 MPa. A series of tests was performed to investigate the effects of cross-section shape and concrete strength on the behavior and strength of concrete-filled stainless steel stiffened tubular stub columns. The measured average 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. Different lengths of columns were selected to fix the length-to-depth ratio to a constant value of 3. The concrete-filled stiffened stainless steel tubular columns were subjected to uniform axial compression over the concrete core and the stainless steel tube to force the entire section to undergo the same deformations by blocking action. The column strengths, load–axial strain relationships and failure modes of the columns are presented. Several comparisons were made to evaluate the test results. The results of the experimental study showed that the design rules, as specified in the European specifications and the ASCE, are highly conservative for square and rectangular cold-formed concrete-filled normal-strength stainless steel stiffened stub columns.     

Evaluation of passive confinement in CFT columns

This paper presents the experimental results of 32 axially loaded concrete-filled steel tubular columns (CFT). The load was introduced only on the concrete core by means of two high strength steel cylinders placed at the column ends to evaluate the passive confinement provided by the steel tube. The columns were filled with structural concretes with compressive strengths of 30, 60, 80 and 100 MPa. The outer diameter (D) of the column was 114.3 mm, and the length/diameter (L/D) ratios considered were 3, 5, 7 and 10. The wall thicknesses of the tubes (t) were 3.35 mm and 6.0 mm, resulting in diameter/thickness (D/t) ratios of 34 and 19, respectively. The force vs. axial strain curves obtained from the tests showed, in general, a good post-peak behavior of the CFT columns, even for those columns filled with high strength concrete. Three analytical models of confinement for short concrete-filled columns found in the literature were used to predict the axial capacity of the columns tested. To apply these models to slender columns, a correction factor was introduced to penalize the calculated results, giving good agreement with the experimental values. Additional results of 63 CFT columns tested by other researchers were also compared to the predictions of the modified analytical models and presented satisfactory results.     

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.     

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.     

Reliability-based evaluation of design code provisions for circular concrete-filled steel columns

This paper presents an investigation of design code provisions for steel-concrete composite columns. The study covers the national building codes of United States, Canada and Brazil, and the transnational EUROCODE. The study is based on experimental results of 93 axially loaded concrete-filled tubular steel columns. This includes 36 unpublished, full scale experimental results by the authors and 57 results from the literature. The error of resistance models is determined by comparing experimental results for ultimate loads with code-predicted column resistances. Regression analysis is used to describe the variation of model error with column slenderness and to describe model uncertainty. The paper shows that Canadian and European codes are able to predict mean column resistance, since resistance models of these codes present detailed formulations for concrete confinement by a steel tube. ANSI/AISC and Brazilian codes have limited allowance for concrete confinement, and become very conservative for short columns. Reliability analysis is used to evaluate the safety level of code provisions. Reliability analysis includes model error and other random problem parameters like steel and concrete strengths, and dead and live loads. Design code provisions are evaluated in terms of sufficient and uniform reliability criteria. Results show that the four design codes studied provide uniform reliability, with the Canadian code being best in achieving this goal. This is a result of a well balanced code, both in terms of load combinations and resistance model. The European code is less successful in providing uniform reliability, a consequence of the partial factors used in load combinations. The paper also shows that reliability indexes of columns designed according to European code can be as low as 2.2, which is quite below target reliability levels of EUROCODE.     

圆锥形钢管混凝土长柱力学性能研究

利用有限元分析软件ABAQUS建立圆锥形钢管混凝土长柱力学性能分析的三维实体有限元计算模型,并与相应的试验结果进行对比。结果表明:有限元计算与试验所得的荷载-3H/4挠度曲线、承载力及破坏形态吻合较好。在此基础上,对圆锥形钢管混凝土长柱受力全过程中钢管与混凝土应力、应变分布情况进行分析,同时给出钢管与混凝土之间的相互作用规律,并对长细比、锥度、偏心距、材料强度及钢管壁厚等参数对圆锥形钢管混凝土长柱初始刚度、承载力及荷载-3H/4挠度曲线的影响进行比较。结果表明:圆锥形钢管混凝土长柱在3H/4处发生侧向挠曲破坏,长细比、锥度、偏心距、混凝土抗压强度及钢管壁厚等参数对其力学性能影响显著,钢管屈服强度对其力学性能影响不明显。