Strengthening of circular hollow steel tubular sections using high modulus CFRP sheets

This paper describes the behaviour of very high strength (VHS) circular steel tubes strengthened by carbon fibre reinforced polymer (CFRP) and subjected to axial tension. A series of tests were conducted with different bond lengths and number of layers. The distribution of strain through the thickness of CFRP layers and along the CFRP bond length was studied. The strain was found to generally decrease along the CFRP bond length far from the joint. The strain through the thickness of the CFRP layers was also found to decrease from the bottom to top layer. The effective bond length for high modulus CFRP was established. Finally empirical models were developed to estimate the maximum load for a given CFRP arrangement.     

FRP strengthening of lean duplex stainless steel hollow sections subjected to web crippling

This paper presents the experimental and numerical investigations of lean duplex stainless steel hollow sections subjected to web crippling. The test specimens were strengthened with different fibre-reinforced polymer (FRP). The web crippling tests were conducted under end-two-flange, interior-two-flange, end-one-flange and interior-one-flange loading conditions. A series of web crippling tests was conducted. The investigation was focused on the effects of surface treatment, web slenderness, different adhesives and FRPs for the strengthening of lean duplex stainless steel hollow sections against web crippling. The lean duplex stainless steel type EN 1.4162 was used in the investigation. Two different surface treatments, three different adhesives and six different FRPs were investigated in this study. The tests were performed on five different sizes of square and rectangular hollow sections that covered a wide range of web slenderness ratio from 8.1 to 57.3. Three different failure modes were observed in the tests of the strengthened specimens, namely the adhesion, interlaminar failure of FRP plate and combination of adhesion and interlaminar failure of FRP plate. Finite element models have been developed and verified against the test results of the specimens subjected to two-flange loading conditions.     

密实截面组合梁的竖向抗剪强度Ⅰ:受正弯矩作用的组合梁

挖掘混凝土翼板的抗剪潜力,充分利用钢-混凝土组合梁的组合抗剪作用,有利于进一步提高组合梁的经济效益.采用通用有限元程序ABAQUS 6.5,对密实截面组合梁正弯矩区的弯剪强度问题进行研究.分析结果表明,提出的有限元分析方法,特别是钢-混凝土的界面模型,可以准确预测组合梁的弯剪强度,同时对组合梁的变形刚度也可以较准确地模拟.在此基础上,利用有限元方法,对混凝土翼板截面尺寸、剪力连接程度、剪跨比等参数进行计算分析,回归得到考虑剪力连接程度影响的组合梁竖向抗剪强度计算公式.研究发现,在正弯矩区段,组合梁抗剪强度相对于钢梁腹板抗剪名义值的提高,不仅来源于混凝土翼板的抗剪作用,组合作用的贡献也很显著;采用建议的抗剪强度公式,可以不考虑简支组合梁弯矩与剪力的相互影响.     

密实截面组合梁的竖向抗剪强度Ⅱ:受负弯矩作用的组合梁

在负弯矩区段,虽存在严重的混凝土开裂,但组合梁的竖向抗剪承载力仍远大于钢梁腹板抗剪名义值.采用通用有限元程序ABAQUS 6.5,对密实截面组合梁负弯矩区的弯剪强度问题进行研究.分析结果表明,提出的有限元分析方法可以准确预测组合梁的弯剪强度,同时对组合梁的变形刚度也可以较准确地模拟.在此基础上,利用有限元方法,对剪力连接程度、混凝土强度、力比、混凝土翼板截面尺寸、剪跨长度等参数进行计算分析,回归得到组合梁负弯矩区截面考虑力比影响的竖向抗剪强度公式.研究发现,在负弯矩区段,组合梁竖向抗剪强度的提高,只来源于混凝土翼板的抗剪作用,组合作用的贡献可以忽略;采用建议的抗剪强度公式可以不考虑组合梁负弯矩区截面弯矩与剪力的相互影响.     

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。     

方管圆管混合空间钢管节点承载性能的非线性分析

以圆管为弦管,方管(包括矩形管)为支管的混合空间钢管节点,构造新颖,受力复杂,其承载力设计和计算是工程师所关注和急需解决的问题。本文采用非线性有限元方法,选用四节点等参壳元,应用自动步长增量法求解方管圆管混合空间管节点轴向受力或平面外受弯的极限承载力。程序系统在移植ADINA程序的基础上,开发了可进行T型、X型、K型和TT型管节点有限元网格剖分的前处理程序CTUBE,并编写了AutoCADADS/RX应用程序显示管节点模型受荷后的变形和应力分布。本文通过大量算例分析,回归出相关承载力计算公式,供设计人员参考。     

Parameter optimization on compressive strength of steel fiber reinforced high strength concrete

This paper illustrates parameter optimization of compressive strength of steel fiber reinforced high strength concrete (SFRHSC) by statistical design and analysis of experiments. Among several factors affecting the compressive strength, five parameters that maximize all of the responses have been chosen as the most important ones as age of testing, binder type, binder amount, curing type and steel fiber volume fraction. Taguchi analysis techniques have been used to evaluate L₂₇ (3¹³) Taguchi’s orthogonal array experimental design results. Signal to noise ratio transformation and ANOVA have been applied to the results of experiments in Taguchi analysis. The confirmation runs were conducted for the optimal parameter level combination, which is obtained from the results of the above methodologies. The maximum compressive strength has been observed as around 124 MPa. By using the optimal parameter level combination, the direct tensile strength and flexural strength tests have been conducted. The mean values at the age of 28 days are obtained as 7.5 MPa and 13 MPa respectively. In this study, it is clearly demonstrated that all main factors except steel fiber significantly contribute to the compressive strength of steel fiber reinforced high strength concrete, yet age and binder type are the most significant contributors.     

Slip factor in slip resistant joints with high strength steel

The resistance of a slip resistant steel joint depends essentially on the preload applied in the bolts and the slip factor developed between the joined plates. The slip factor depends on the surface roughness, a parameter associated to the superficial treatment. Although some indications are already provided in the codes for the design of joints, such as the Eurocode 3, some doubts remain among researchers, designers and steelwork companies. Some surface treatments are not yet well characterized and in general the values for slip factor provided in the codes were calibrated for current mild steel only. The evaluation of the influence of the surface treatment on current mild steel, on weather resistant steel and in particular on S 690 high strength steel is presented in this paper. The slip factor was obtained through experimental tests according to EN 1090 — Part 2, carried out in the University of Coimbra — Portugal. The results obtained and discussed in the scope of this paper were also compared with values reported in the literature.     

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.     

圆钢管焊接空心球节点受压破坏机理与试验研究

从球体钢材屈服强度的角度对受压破坏机理进行研究。采用弹塑性线性强化材料本构模型建立有限模型,考虑几何非线性,利用弧长法对受压球节点进行了全过程受力分析。系统研究了受压球节点的承载能力和破坏机理,节点因材料屈服形成塑性环铰而无法继续承载,材料屈服强度是影响承载力的主要因素。提出了破坏控制截面的概念,剪应力是破坏截面的控制应力。采用Q235B和Q345B材质的4组8个典型球节点试件的对比性破坏试验研究,直观了解节点的受力性能和破坏机理,验证了有限元分析的正确性。最后,利用冲切模型对受压承载力进行了理论分析,建立了承载力实用计算公式,可以直接应用于工程设计。     

钢纤维高强混凝土抗压性能试验研究

通过对高强混凝土和钢纤维高强混凝土试件的试验,研究钢纤维类型和钢纤维体积率对高强混凝土立方体抗压性能的影响。结果表明:钢纤维类型和体积率对高强混凝土立方体抗压强度均有一定的影响,钢纤维高强混凝土立方体抗压强度的尺寸效应与普通强度钢纤维混凝土基本相同。     

Compressive strength of circular hollow reinforced concrete confined by an internal steel tube

When concrete is confined, its strength is enhanced by confining stress. Therefore many researchers have studied the confinement and its effect in a reinforced concrete (RC) member. However, their research has focused on making confining pressure from the outside of the member. In a hollow RC member, it is impossible to confine its concrete triaxially because it has a hollow section. The concrete in a hollow RC member is usually in the state of biaxially confinement, and it shows lower strength than the concrete in a solid RC member, which is confined triaxially. In this study, the compressive strength of the concrete in a hollow RC column with an internal steel tube was investigated. The internal tube makes the concrete confined triaxially with internal confinement. To show the existence and the effect of the internal confinement, an experiment was performed. Fourteen specimens were tested. The test results showed that the internal confinement existed and made the concrete in the state of triaxial confinement.     

Buckling strength of the cylindrical shell and tank subjected to axially compressive loads

This paper aims to develop practical design equations and charts estimating the buckling strength of the cylindrical shell and tank subjected to axially compressive loads. Both geometrically perfect and imperfect shells and tanks are studied. Numerical analysis is used to evaluate buckling strength. The modeling method, appropriate element type and necessary number of elements to use in numerical analysis are recommended. According to the results of the parametric study of the perfect shell, the buckling strength decreases significantly as the diameter-to-thickness ratio increases, while it decreases slightly as the height-to-diameter ratio increases. These results are different from those in the case of columns. The buckling strength of the perfect tank placed on an extremely soft foundation and a stiff foundation increases by up to 1.6% and 5.6%, respectively, compared with that of the perfect shell. The buckling strength of the shell and tank decreases significantly as the amplitude of initial geometric imperfection increases. Convenient and sufficiently accurate design equations and charts used for estimating buckling strength are provided.     

Contribution of cocktail of polypropylene and steel fibres in improving the behaviour of high strength concrete subjected to high temperature

The aim of the study is to improve the understanding of the influence of a polypropylene and steel fibres cocktail on the behaviour of high strength concretes subjected to high temperature. Concrete mixes were studied by adding polypropylene fibres, steel fibres and cocktail of fibres. The concrete specimens were subjected to various heating–cooling cycles. The initial and residual mechanical properties, the porosity and the mass loss of the studied concrete mixes were investigated. Different concretes compositions with various amounts of polypropylene and/or steel fibres were tested. Experimental results show the significant improvement of the residual mechanical properties of concretes containing the cocktail of fibres compared to concretes without fibres.     

双肋焊接空心球受压极限承载力有限元分析

依据弹塑性模型和Von—Mises屈服准则对不同形式加劲肋的空心焊接球，进行了非线性有限元分析，研究了节点参数变化对其受压极限承载力的影响，通过大量的计算分析提出了一些有益的结论。     

The assessment of residual stresses in welded high strength steel box sections

Much work on the investigation of the magnitude and distribution of residual stresses in mild carbon steel sections have been made previously. However, limited efforts have been put on residual stress measurements of high strength steel sections. The differences of stress–strain curves and high-temperature material properties between the high strength steel and mild carbon steel demands a necessary study of the residual stresses in high strength steel welded sections. In the present study, three box columns fabricated from Q460 steel plates of 11 mm in thickness with different details were used for the examination. Both sectioning and hole-drilling methods are adopted for the measurement. The measured residual stress distributions of three different box sections are presented, and the corresponding simplified residual stress pattern is proposed. By comparing with the residual stress patterns for mild carbon steel, it is found that the box section fabricated from HSS plates has the lower compressive residual stress ratio. The differences in the measurement by using sectioning and hole-drilling methods are also compared.     

自密实微膨胀高强钢管混凝土的施工工艺

腊八斤特大桥、黑石沟特大桥为大跨连续刚构钢管混凝土组合高墩混凝土工程,其主墩最高墩高分别为182.5 m和157 m,在主桥墩钢管混凝土中,核心混凝土采用C80、C60高抛自密实微膨胀高强钢管混凝土.采用高位抛落免振捣法进行施工,着重介绍主墩钢管混凝土的施工工艺.     

Strength assessment of steel plates subjected to compressive load and dent deformation

This work presents an assessment of the ultimate compressive strength of damaged steel plates with a local dent. Extensive non-linear finite element analyses are performed, where three groups of analyses are considered. In the first group, the effect of the dent orientation (longitudinal and transverse) is studied for three plate thicknesses. For the second group, the direction of the dent depth (upward and downward) with respect to the global initial imperfection is explored. In the third group, the variation of the dent size for several plate thicknesses and its influence on ultimate compressive strength is investigated. The post-collapse behaviours are discussed and the inflection plate slenderness with and without dent is observed, at which the behaviour of the plate changed. A certain dent breadth to the plate breadth ratio is established, revealing the different plate response. Based on the performed analyses, a generalised expression of the ultimate strength reduction factor due to dent is developed.     

Effect of flattening circular hollow sections in truss and dome structures

Circular hollow steel sections are normally specified in truss and dome structures to take advantage of their efficiency in compression and the ease in which they can be curved to match the various radii of different domes. However, it is not easy to connect these members together or to other members. In order to connect them together or to other members the ends are sometimes squashed or flattened. Although the process of flattening does not reduce the area of the section, it does reduce the flexural stiffness of the section. The aim of this paper is to study the behaviour of circular hollow members with flattened edges, in compression. Variables in the tests include the diameter, thickness and length of the sections, and number of bolts in the connection. Two failure modes were observed and these are overall flexural buckling (OFB) of the member and excessive deformation of the transition zone (DTZ). The results from these tests are compared with the flexural compressive resistance formula in the South African steel code (SANS10162-1) and the European steel code (EN 1993-1-1), with a view of determining a simple formula for designing such members.     

Stub-column strength of thin-walled square and rectangular hollow sections

There are several methods for the evaluation of the ultimate load of compressed rectangular thin-walled hollow stub-columns. The aim of the present paper is to review some of these and to compare their results with experimental results obtained from tests on a series of specimens. Conclusions are drawn on the validity and on the range of practical application of these methods.