Concrete-filled steel circular tubes subjected to constant amplitude cyclic pure bending

This paper describes an experimental investigation of the cyclic inelastic flexural behaviour of concrete-filled tubular (CFT) beams made of cold-formed circular hollow sections and filled with normal concrete. Cyclic bending tests were performed using a constant amplitude loading history on different CFT specimens with diameter-to-thickness ratios (D/t) ranging from 20 to 162. The CFT beams exhibited stable hysteresis behaviour up to the formation of plastic ripples and then showed considerable degradation in stiffness, strength, and ductility depending on the D/t ratio. Seismic capacity parameters are presented including strength, stiffness, rotation capacity, hysteresis loops and modes of failure of the specimens. Peak moments obtained in the cyclic tests are compared with those obtained previously in monotonic tests and also with design moments predicted using the available design rules for composite beams. The deformation ductility demand was determined and used to derive new fully ductile section slenderness limits suitable for seismic design.     

圆钢管轻集料混凝土构件抗弯性能的试验研究

进行了21根钢管轻集料混凝土以及8根空钢管的纯弯试验。通过对试件在加载过程中的挠度及表面应变的测试分析,研究钢管轻集料混凝土构件在弯矩作用下的宏观变形特征、弯矩-曲率关系和破坏模式,分析了轻集料混凝土强度、含钢率、剪跨比等主要参数对构件抗弯性能的影响。试验研究结果表明,钢管轻集料混凝土构件在纯弯矩的作用下,其宏观变形特征和破坏形态与钢管普通混凝土构件相似;内填的轻集料混凝土可以大大延缓或避免钢管局部屈曲的发生,钢管轻集料混凝土的极限抗弯承载力较空钢管提高了20%～30%,抗弯刚度也有所提高;在钢管和轻集料混凝土相互作用下,钢管轻集料混凝土受弯试件具有较高的抗弯承载力和良好的延性性能;含钢率是影响钢管轻集料混凝土抗弯承载力和初始抗弯刚度的主要因素,含钢率越大,初始刚度越大和构件极限抗弯承载力也越大;而内填混凝土强度和剪跨比对构件抗弯承载力的影响较小;与钢管普通混凝土研究结果的对比分析表明,在相同条件下,钢管轻集料混凝土与钢管普通混凝土的极限抗弯承载力大致相当。     

Performance of concrete filled steel tube reinforced concrete columns subjected to cyclic bending

This paper reports nine test results of concrete filled steel tube reinforced concrete (CFSTRC) columns, which were tested under constant axial load and cyclically increasing flexural loading. The main parameters varied in the experiments were axial load level and cross-sectional type. The influence of these parameters on strength, ductility, stiffness and energy dissipation was investigated. It was found that, in general, CFSTRC columns exhibit favourable energy dissipation and ductility, even when the columns were subjected to high axial loads. This type of composite column is adoptable in practical engineering, particularly in regions of high seismicity.     

Neural networks for modelling ultimate pure bending of steel circular tubes

The behaviour of steel circular tubes under pure bending is complex and highly nonlinear. The literature has a number of solutions to predict the response of steel circular tubes under pure bending; however, most of these solutions are complicated and difficult to use in routine design practice. In this paper, the feasibility of using artificial neural networks (ANNs) for developing more accurate and simple-to-use models for predicting the ultimate pure bending of steel circular tubes is investigated. The data used to calibrate and validate the ANN models are obtained from the literature and comprise a series of 49 pure bending tests conducted on fabricated steel circular tubes and 55 tests carried out on cold-formed tubes. Multilayer feed-forward neural networks that are trained with the back-propagation algorithm are constructed using four design parameters (i.e. tube thickness, tube diameter, yield strength of steel and modulus of elasticity of steel) as network inputs and the ultimate pure bending as the only output. A sensitivity analysis is conducted on the ANN models to investigate the generalization ability (robustness) of the developed models, and predictions from the ANN models are compared with those obtained from most available codes and standards. To facilitate the use of the developed ANN models, they are translated into design equations suitable for spreadsheet programming or hand calculations. The results indicate that ANNs are capable of predicting the ultimate bending capacity of steel circular tubes with a high degree of accuracy, and outperform most available codes and standards.     

不同构造圆钢管混凝土的黏结性能

为提高圆钢管混凝土的黏结性能,进行了 12个大尺寸圆钢管混凝土试件的推出试验.钢管内部构造措施包括无构造、内置竖向肋板、内置钢管和拉结板、内置环向肋板等6种;采用2种水胶比的核心区高强混凝土.通过各试件的破坏特征、荷载-滑移曲线和应变,分析了不同构造和不同水胶比对圆钢管混凝土黏结性能的影响,并得到了环向肋板抗剪强度的计...     

Behavior of thin-walled circular hollow section tubes subjected to bending

Thin-walled steel circular hollow sections (CHSs) are widely used in wind turbine towers. The tower tubes are mainly subjected to bending. There have been a few experimental studies on the bending behavior of thin-walled CHS steel tubes. This paper describes a series of bending tests to examine the influence of section slenderness on the inelastic and elastic bending properties of thin-walled CHS. In addition, the influence of stiffeners welded in the steel tube is considered. Sixteen bending tests were performed up to failure on different sizes of CHS with diameter-to-thickness ratio (D/t) varying from 75 to 300. The experimental results showed that the specimens with small diameter-to-thickness ratios failed by extensive plastification on the central part of the tube. With the increase of diameter-to-thickness ratio, the local buckling phenomena became more pronounced. The stiffeners in the steel tubes increased the load carrying capacity and improved the ductility of the specimens. The experimental results were compared with current design guidelines on thin-walled steel members in AISC-LRFD, AS4100 and European Specification. It was found that the test results agreed well with the results based on AS4100 design code.     

Plastic collapse analysis of thin-walled circular tubes subjected to bending

Circular tubes have been widely used as structural members in many engineering applications. Therefore, its collapse behavior has been studied for many decades, focusing on its energy absorption characteristics and collapse mechanism. In order to predict the collapse behavior of members, one could rely on the use of finite element codes or experiments. These tools are helpful and have high accuracy but are costly and require extensive running time. Therefore, an approximate model of tubes collapse mechanism is an alternative especially for the early step of design. This paper is also aimed to develop a closed-form solution to predict the moment–rotation response of circular tube subjected to pure bending. The model was derived based on the principle of energy rate conservation. The collapse mechanism was divided into three phases. New analytical model of ovalisation plateau in phase 2 was derived to determine the ultimate moment. In phase 3, the Elchalakani et al. model [Int. J. Mech. Sci. 2002; 44:1117–1143] was developed to include the rate of energy dissipation on rolling hinge in the circumferential direction. The 3-D geometrical collapse mechanism was analyzed by adding the oblique hinge lines along the longitudinal tube within the length of the plastically deformed zone. Then, the rates of internal energy dissipation were calculated for each of the hinge lines which were defined in terms of velocity field. Inextensional deformation and perfect plastic material behavior were assumed in the derivation of deformation energy rate. In order to compare, the experiment was conducted with a number of tubes having various D/t ratios. Good agreement was found between the theoretical prediction and experimental results.     

Experimental study on shear resistance of self-stressing concrete filled circular steel tubes

This paper presents the results of shear tests on 27 self-stressing concrete filled steel tube (SSCFST) specimens and 8 conventional concrete filled steel tube (CFST) specimens. The main parameters in the tests were the self-stress and shear span to depth ratio. The experimental results indicate that the shear capacity of the CFST specimen is lower than that of the SSCFST specimen when 28-day cube strength of concrete core, shear span to depth ratio, yield strength and geometrical dimension of the steel tube are kept the same. The CFST/SSCFST specimen failed in shear when the shear span to depth ratio was smaller than 0.5 and failed in flexure when the shear span to depth ratio was 0.5 or higher. The bond between concrete core and steel tube also affected the shear capacities. Finally, an empirical equation for predicting the shear capacity of a SSCFST short beam is proposed.     

Compressive and flexural behaviour of concrete filled steel tubes after exposure to standard fire

This paper describes a series of new compression and bending tests carried out on concrete filled steel tubes (CFST) after exposure to the ISO-834 standard fire. A theoretical model that has been previously developed is used to predict the post-fire load versus deformation relationships of CFST stub columns and beams. The predicted curves of load versus deformation are in good agreement with the new test results.The previously developed theoretical model had been used to investigate the influence of a number of important parameters on the residual ultimate strength and flexural stiffness of the composite sections and the results of the parametric studies were used to develop formulas for calculating the composite section residual ultimate strength under axial compression or flexural bending and the composite section residual flexural bending stiffness. In these formulas, the ambient temperature compression resistance, bending moment capacity and initial flexural bending stiffness of the composite section should be calculated using an existing design code. In this paper, these formulas are applied to the new test data to assess the suitability of using several different design codes: AIJ-1997, AISC-LRFD-1999, BS5400-1979, DBJ13-51-2003 and EC4-1994.     

Experimental behaviour of concrete filled thin walled steel tubes with tab stiffeners

This paper presents an experimental investigation into the structural behaviour of concrete filled, thin walled, steel tubular stub column with tab stiffeners. The stiffening was attained by welding together four pieces of lipped angle, whereupon two parts of the lips were notched and folded vertically in order to form the tab stiffeners. The effects of the tab stiffeners on the bond and compressive strengths were investigated experimentally using 18 and 10 specimens respectively. It was observed that the tab stiffener does enhance both the bond strength and the axial load capacity of the concrete filled thin walled steel tubular stub column tested.     

双锥型压弯(扭)圆钢管的试验研究

双锥型变截面类构件在压弯(扭)复合荷载作用下,受力情况复杂,目前规范尚未给出一套完整的强度、稳定计算公式。本文通过双锥型圆钢管的压弯(扭)载荷试验,对该类型构件在复合荷载作用下的承载能力、破坏模态、位移-荷载曲线及应变-荷载曲线进行了分析比较。试验分析表明:试件的破坏模态表现为结构的局部屈曲,提供一个较小的坡度,便能使塑性区发生内移,以获得更大的延性;双锥型构件比等截面构件具备更好的受力性能,其承载能力的提高与锥体坡度直接相关,对于塑性铰位于交界面处的构件,其承载能力的提高可达40%左右。     

Behavior of square hollow steel tubes and steel tubes filled with concrete

This paper investigates the occurrence of local buckling in bare steel and concrete-filled tubes to study how different depth-to-thickness ratios affect the response of the steel component. The experimental set-up and results of 24 tests are presented in this paper. Specimens with values of depth-to-thickness ratios in the range of 50–125 have been considered. The presence of the concrete has been observed to affect the exhibited buckling mode and to significantly increase the buckling bearing capacity of the concrete-filled steel tubes. A numerical model has been developed using the commercial software ABAQUS and has been validated against the experimental results of this study. From a design viewpoint, it has been observed that local buckling needs to be included in the calculation of the contribution of the steel component to the bearing capacity of a concrete-filled tube when its depth-to-thickness ratio is over 50. For a slender plate, i.e., with a depth-to-thickness ratio over 120, its post-buckling behavior could be included in the calculation of the steel contribution as it evidently increases its bearing capacity. Finally, an equation for the calculation of the bearing capacity of composite sections with both stocky and slender steel elements has been proposed and validated against extensive experimental results available in the literature.     

自密实钢管混凝土的研究与应用

采用自密实混凝土作为钢管核心混凝土．针对钢管混凝土的受力特点和施工工艺．进行自密实混凝土材料的选择和配合比优化。为了确保自密实钢管混凝土的顺利施工和硬化后良好的力学、变形性能，进行了自密实钢管混凝土与普通泵速铜管混凝土拱的现场模拟施工对比试验，以及钢管混凝土短柱的轴心受压对比试验。试验结果表明．自密实混凝土作为钢管核心混凝土，其施工性能明显优于普通泵送混凝土。在施工过程中不泌水、不离析。硬化的混凝土能在各截面上均匀分布；自密实钢管混凝土短柱的组合弹性模量、极限承载力和延性以及后期承载能力方面均与普通泵送混凝土短柱相似。在此基础上．成功地将自密实混凝土应用于莆田市一钢管混凝土拱桥工程，应用结果表明，采用自密实混凝土技术，钢管混凝土拱桥施工顺利、快捷。且更好地保证了工程质量，同时混凝土材料造价与普通泵送混凝土基本持平。     

Concrete-filled circular steel tubes subjected to local bearing force: Experiments

Concrete filled steel tubular (CFST) members are subjected to local bearing forces in a large number of truss and lattice structures. Previous research has focused on rectangular CFST members under such loading condition. There is a lack of understanding on circular CFST members subjected to local bearing force. This paper intends to fill the knowledge gap in this area. A series of tests were conducted on circular CFST, unfilled circular hollow section (CHS) steel tube and plain concrete specimens loaded with local bearing force. The load was applied either perpendicularly to the member or at an angle of 45°. A deformation limit was adopted to define the ultimate strength of the specimen since the load verse deformation curve exhibits a ductile behavior. The effects of important parameters were investigated based on the test results. Finally, design formulae were developed to predict the ultimate strength of circular CFST members under local bearing forces.     

方钢管高强混凝土偏压构件的试验研究与理论分析

本文进行了8根方形截面钢管高强混凝土单向压弯构件的试验研究,研究构件在不同长细比、偏心率和含钢率下的力学性能,长细比取25.4、50.8和71.6三种,偏心率在0.140～0.288之间变化,含钢率取8.2%和14.3%两种。研究结果表明:构件的承载力随着长细比和偏心率的增加迅速降低,含钢率为8.2%的构件在达到极限承载力之前钢管出现了局部屈曲。在进行试验研究的同时,还编制了非线性分析程序BC,用来分析方钢管高强混凝土压弯构件的力学性能,理论分析结果和试验结果进行了对比,两者吻合较好,在此基础上分析了长细比、含钢率、混凝土强度和钢材屈服强度等因素对偏压构件N/Nu(η)-M/Mu(ζ)相关曲线的影响。     

Fire behaviour of concrete filled elliptical steel columns

In this work, a non-linear three-dimensional finite element model is presented in order to study the behaviour of axially loaded concrete filled elliptical hollow section (CFEHS) columns exposed to fire. This study builds on previous work carried out by the authors on concrete filled circular hollow section (CFCHS) columns both at room temperature and in fire. The numerical model is first validated at room temperature against a series of experiments on CFEHS stub columns available in the literature and subsequently extended to study the performance of slender columns at elevated temperatures. The aim of this work is to understand and represent the behaviour of axially loaded CFEHS columns in fire situations and to compare their effectiveness with that of the circular concrete filled tubular (CFT) columns. Parametric studies to explore the influence of variation in global member slenderness, load level, cross-section slenderness and section size are presented. Finally, guidance on the fire design of CFEHS columns is proposed: it is recommended to follow the guidelines of Clause 4.3.5.1 in EN 1994-1-2, but employing the flexural stiffness reduction coefficients established in the French National Annex with an equivalent EHS diameter equal to P/π, where P is the perimeter of the ellipse.     

钢管混凝土轴心受压短柱可靠度分析

基于随机有限元的可靠度研究方法对钢管混凝土轴心受压短构件进行了可靠度分析,分析结果表明：在各随机变量中,材料弹性模量对钢管混凝土轴心受压短柱可靠度指标影响最大,几何尺寸随机变量次之。     

Experimental behaviour of cold-formed steel welded tube filled with concrete made of crushed crystallized slag subjected to eccentric load

This paper presents results of tests conducted on thin welded rectangular steel stubs filled with concrete that gravel was substituted by 10 mm crushed crystallized slag stone. The studied section was made of two cold steel plates with U shape and welded with electric arc to form a steel box section. The cross-section dimensions were: 100×70×2 mm³. the main studied parameters were the stub height (200, 300, 400, 500 mm), the effect of the in filled concrete, the continued weld and the eccentric force. The tests were carried out 28 days after the date of casting. A total of 20 stubs were tested in a 50 tf machine up to failure, 4 stubs subjected to axial load compression and 16 stubs subjected to eccentric load compression along the minor and major rigidity axis. The aim of the study is to provide some evidences that the use of crushed slag could be integrated in the manufacturing of non-conventional concrete. All failure loads were predicted by using the Euro code 4 and the design method proposed by Z. Vrcelj and B. Uy. From test results, it was confirmed that the length of stubs and the eccentric load had a drastic effect on the load carrying capacity. The failure mode of composite stubs was a local buckling mode with all steel sides deformed outwards. The Euro code 4 loads predictions were generally in good agreement compared with experimental loads and on safe side. The loads results of design method proposed by Vrcelj and B. Uy were generally on safe side compared with experimental load except the columns subject to eccentric load with 400 mm and 500 mm height.     

圆钢管及钢管混凝土柱的抗火设计

分别对圆钢管、钢管混凝土、中空夹层钢管混凝土柱进行了抗火设计,并对结果进行比较分析。结果表明,在较高荷载比下柱的耐火极限不能满足实际要求,必须进行防火保护。在相同条件下,耐火极限从大到小排序为:圆钢管混凝土、中空夹层钢管混凝土、钢管柱。在一级耐火等级下,钢管混凝土柱和中空夹层钢管混凝土柱需要厚涂型钢结构防火涂料的厚度可比钢管柱分别少55%和18%以上。随着荷载比的减小或截面尺寸的增加,柱的耐火极限提高,需要的保护层厚度减小。对于钢管混凝土柱,若采用水泥砂浆保护层,其厚度是防火涂料的3倍及以上。