FRP strengthened aluminium tubular sections subjected to web crippling

This paper presents a test programme on aluminium tubular structural members that have experienced web crippling failure due to localized concentrated loads or reactions. A series of tests was performed on fibre-reinforced polymer (FRP) strengthening of aluminium tubular structural members subjected to End-Two-Flange and Interior-Two-Flange loading conditions. A total of 58 web crippling tests was conducted. The investigation was mainly focused on the effects of different adhesive and FRP for strengthening of aluminium tubular sections against web crippling. The influence of different widths of FRP plate strengthening against web crippling has been also investigated in this study. The test specimens consisted of 6061-T6 heat-treated aluminium alloy square and rectangular hollow sections. Six different adhesives and six different FRPs were considered in this study. The properties of adhesive and FRP have significant influence on the effectiveness of the strengthening. Most of the strengthened specimens were failed by debonding of FRP plates from the aluminium tubes. There are mainly four different failure modes observed in the tests, namely the adhesion, cohesion, combination of adhesion and cohesion, and interlaminar failure of FRP plate. The failure loads, failure modes, and the load-web deformation behaviour of the aluminium sections are presented in this study. It was found that the web crippling capacity of aluminium tubular sections is significantly increased due to FRP strengthening, especially for those sections with large value of web slenderness ratio.     

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.     

Design rules for web crippling of CFRP strengthened aluminium rectangular hollow sections

Web crippling failure (web buckling and web yielding) is critical for thin-walled members when subjected to concentrated load. Carbon fibre reinforced polymer (CFRP) is attracting increasing research interest as a strengthening material for metallic structural members. Improved web crippling capacity of aluminium rectangular hollow sections has been achieved with CFRP being attached to the exterior and/or interior of the webs from a series of tests conducted by the authors. This paper focuses on developing design rules for predicting the nominal crippling strength of CFRP strengthened sharp-corner aluminium tubular sections: rectangular hollow section (RHS) and square hollow section (SHS), under end bearing load. The existing design rules for bare sections without CFRP strengthening are firstly reviewed and assessed, including design rules for both cold-formed steel structural members (Australian/New Zealand standard (AS 4100-1998) and North American Specification) and aluminium structures (Australian/New Zealand standard (AS 1664-1997) and American aluminium design manual). They are modified to take account of the improved capacity due to CFRP strengthening. The proposed design rules are calibrated against test results.     

Strengthening of ferritic stainless steel tubular structural members using FRP subjected to Two-Flange-Loading

Cold-formed stainless steel tubular structural members which may experience web crippling failure due to localise concentrated loads or reactions are investigated. A series of tests on fibre-reinforced polymer (FRP) strengthening of cold-formed stainless steel tubular structural members subjected to End-Two-Flange and Interior-Two-Flange loading conditions is presented. The strengthening only applied to a localise area of the members under concentrated load. A total of 58 web crippling tests were conducted. The investigation mainly focused on the effects of different surface treatment, different adhesive, and FRP for strengthening of stainless steel tubular sections against web crippling. The behaviour of stainless steel members strengthened by different widths of FRP plate against web crippling has been also investigated in this study. The test specimens consisted of ferritic stainless steel EN 1.4003 square and rectangular hollow sections. Two different surface treatments were considered. Furthermore, six different adhesives and six different FRPs were also considered in this study. The properties of adhesive and FRP as well as the bonding between the FRP and stainless steel tube have significant influence on the effectiveness of the strengthening. Most of the strengthened specimens were failed by debonding of FRP plates form the stainless steel tubes. Six different failure modes were observed in the tests, namely the adhesion, cohesion, combination of adhesion and cohesion, interlaminar failure of FRP plate, FRP delaminating failure and web crippling failure. The failure loads, failure modes, and the load-web deformation behaviour of the ferritic stainless steel sections are presented in this study. It was found that the web crippling capacity of ferritic stainless steel tubular sections may increase up to 51% using FRP strengthening.     

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.     

Study of web crippling in ferritic stainless steel cold formed sections

Cold-formed stainless steel members are widely used due to their high corrosion resistance and high resistance-to-weight ratio but their susceptibility to buckle implies that instability phenomena such as web crippling, where the web locally buckles due to concentrated transverse forces, must be considered. On the other hand, the emergent ferritic stainless steel has very low nickel content and therefore, they are cheaper and relatively price stable compared to austenitics and duplex. Their promising future has aimed to develop efficient design guidance and as a result, a new unified web crippling resistance expression based on numerical simulations and thereafter validated with experimental results has been proposed.     

Analysis and design of cold-formed steel channels subjected to combined bending and web crippling

The channel failures due to combined bending and web crippling may occur at the highly concentrated interior loading when there is no load stiffener in cold-formed thin-walled steel beams. This paper presents accurate finite element models to predict the behavior and ultimate strengths of cold-formed steel channels subjected to pure bending as well as combined bending and web crippling. Both geometric and material nonlinearities are considered in the finite element analysis. The nonlinear finite element models are verified against experimental results of cold-formed steel channels subjected to pure bending as well as combined bending and web crippling. The finite element analytical results show a good agreement with the experimental results in terms of the ultimate loads and moments, failure modes and web load-deformation curves thus validating the accuracy of the finite element models. The verified finite element models are then used for an extensive parametric study of different channel dimensions. The channel strengths predicted from the parametric study are compared with the design strengths calculated from the North American Specification for cold-formed steel structures. It is shown that the design rules in the North American Specification are generally conservative for channel sections with unstiffened flanges having the web slenderness ranged from 7.8 to 108.5 subjected to combined bending and web crippling. It is demonstrated that the nonlinear finite element analysis by using the verified finite element models against test results is an effective way to predict the ultimate strengths of cold-formed thin-walled steel members.     

Behaviors of concrete-filled steel tubular members subjected to combined loading

The present study is an investigation on the behaviors of concrete-filled thin-walled steel tubular members subjected to combined loading, such as compression and torsion, bending and torsion, compression, bending and torsion. ABAQUS software is used in this paper for the finite element analysis (FEA). A comparison of results calculated using this modeling shows generally good agreement with test results. The FEA modeling is then used to investigate the influence of important parameters that determine the ultimate strength of the composite members under combined loading, such as compression and torsion, bending and torsion, compression, bending and torsion. The parametric studies provide information for the development of formulae for calculating the ultimate strength of the composite members subjected to combined loading.     

CFRP板与钢梁粘接的疲劳性能研究

CFRP板与钢构件粘接疲劳性能的研究目前还不多,制约了该加固技术在桥梁结构中的使用.对一组CFRP板加固钢梁进行了理论分析结合疲劳试验研究,将粘接层的疲劳寿命分为无裂纹寿命和裂纹扩展寿命两部分进行考虑.试验得出在最大疲劳荷载作用下的最大界面主应力和无裂纹寿命的s一Ⅳ曲线.其疲劳极限值约为静载作用下极限界面主应力的30%,与英国规程中的规定相符.由基于最大疲劳荷载时粘接层的能量释放率的Paris Law,推导裂纹扩散寿命的预测公式,并由试验结果求得该公式中的经验系数.与试验结果比较发现,该公式能较为准确的预测裂纹扩展的规律.试验结果还显示,疲劳荷载幅度对疲劳寿命有一定的影响,但远小于最大疲劳荷载的影响.     

CFRP板与钢梁粘结剥离破坏的试验研究

粘结层是碳纤维增强复合材料(CFRP)板加固钢梁的最薄弱环节。通过对4根加固梁的静载试验及6根加固梁的疲劳试验研究了CFRP板加固钢梁的剥离破坏。在CFRP板底端部贴的应变片记录了试验过程中的应变变化,并结合理论分析,探讨了各项因素对板端应变的影响及CFRP板的剥离破坏过程。静载试验中记录的随外荷载变化的应变曲线以及疲劳试验中记录的随疲劳加载次数变化的应变曲线都能分为4个阶段。阶段Ⅰ为板端溢胶对板端作用的拉力的衰退阶段,板端溢胶逐渐失去作用;阶段Ⅱ为正应力衰退阶段,正应变开始减小,而剪应力继续增加,使得压应变的增加更快;阶段Ⅲ为剪应力衰退阶段,在达到压应变峰值后,剪应力和应变减小,直到应变为0;阶段Ⅳ为剥离破坏阶段。根据这一规律,利用板端的应变片可以很好地监控加固梁粘结层的性能变化及裂缝的萌生。     

冷成型不锈钢管轴心受压柱试验研究

为研究冷成型不锈钢管轴心受压柱的稳定性能,进行了国产304牌号冷成型不锈钢方管、矩形管和圆管截面,共43根轴心受压柱试验。通过对不锈钢材料、轴压短柱试件和轴压长柱试件的试验研究,得到了试件的材料力学性能和极限荷载。分析了试件的长细比、宽厚比（径厚比）对其破坏模式及变形性能的影响。结果表明：试件的宽厚比（径厚比）对其破坏模式及变形性能有较大的影响。采用GB 50018-2002《冷弯薄壁型钢结构技术规范》、欧洲不锈钢结构设计规范、美国冷成型不锈钢结构设计规范中的计算方法以及Rasmussen提出的设计方法对试验试件进行了计算,并与试验数据对比结果表明,对于圆管试件,采用三本规范计算得到的承载力均高于试验值,偏于不安全,采用Rasmussen 提出方法的计算结果与试验值较为接近;对于方矩管试件,各种方法计算结果相近,除短柱试件试验结果高于计算结果外,其余试件试验值均与计算结果吻合较好;GB 50018-2002《冷弯薄壁型钢结构技术规范》中的计算方法不能直接用于计算不锈钢管轴心受压柱承载力。     

Behaviour of concrete-filled steel tubular members subjected to shear and constant axial compression

Concrete-filled steel tubes (CFST) are used extensively in modern civil engineering. In many cases, they are often subjected to shear and constant axial compression, particularly when being used in high-rise buildings. ABAQUS Programming is used in this paper for the analysis of CFST subjected to shear and constant axial compression. A comparison of results calculated using this model shows good agreement with test results in general. The theoretical model was used to investigate the influence of important parameters that determine the ultimate shearing strength of the composite members. The parametric studies provide information for the development of formulae to calculate the ultimate strength of CFST members subjected to shear and constant axial compression.     

Simplified design approach for cold-formed stainless steel compression members subjected to flexural buckling

The design criteria of stainless steel compression member are more complicated than those of carbon steels due to the nonlinear stress strain behavior of the material. In general, the tangent modulus theory is used for the design of cold-formed stainless steel columns. The modified Ramberg–Osgood equation given in the ASCE Standard can be used to determine the tangent modulus at specified level of stresses. However, it is often tedious and time-consuming to determine the column buckling stress because several iterations are usually needed in the calculation. This paper presents new formulations to simplify the determination of flexural buckling stress without iterative process. Taylor series expansion theory is utilized in the study for numerical approximations. The proposed design formulas are presented herein and can be alternatively used to calculate the flexural buckling stress for austenitic type of cold-formed stainless steel columns. It is shown that the column strengths determined by using the proposed design formulas have good agreement with those calculated by using the ASCE Standard Specification. A design example is also included in the paper for cold-formed stainless steel column designed by using the ASCE Standard equations and the proposed design formulas.     

Bond characteristics between ultra high modulus CFRP laminates and steel

The use of high modulus CFRP laminates in strengthening steel members has the advantage of increasing the strength and stiffness of such members. In this paper, the bond characteristics between ultra high modulus (UHM) CFRP laminates with a modulus of 460 GPa and steel were studied. A series of experiments with double strap steel joints bonded with UHM CFRP laminates were conducted. Experimental results presented in this paper include failure modes, bond strength, effective bond length, CFRP strain distribution, adhesive layer shear stress distribution and bond slip relationship. Comparison was made with previous research on CFRP sheet–steel and normal modulus CFRP laminate–steel systems and different aspects of bond characteristics were discussed. Theoretical models were employed for the prediction of the specimen bond strength and effective bond length, and their applicability for UHM CFRP–steel joints was verified by comparisons with experimental results. Nonlinear finite element analysis was carried out to simulate the experimental specimens. The FEA results agreed well with those from experiments.     

Thermal stress measurement in steel plate strengthened by CFRP and aluminum alloy plates

In steel members strengthened by carbon-fiber reinforced polymer (CFRP) plates, the thermal stresses are introduced in the steel members, the CFRP plates and the adhesive layers when temperature changes because the linear thermal expansion coefficients of steel and CFRP are mismatched. As so far, the authors proposed a technique to reduce the thermal stress in steel members strengthened by CFRP plates, which involves bonding aluminum alloy plates with CFRP plates. In the proposed method, the thermal stress in steel member can be reduced so that there are negligible levels of stress in steel member when the cross sectional areas of CFRP and aluminum plates are designed to correspond to the coefficient of thermal expansion of steel, even though the thermal stresses are introduced in the CFRP and aluminum plates. In this study, to confirm the maintaining the thermal stress reduction in steel member by proposed method, thermal stress measurement in steel plate strengthened by CFRP and aluminum plates was carried out about 21 months. In this research, the thermal stress introduced in the steel plate strengthened by CFRP plates was also measured. Furthermore, to assume the thermal shear and normal (peel) stresses in adhesive layers, FE analysis with plane stress element was employed. As the result, it was shown the thermal stresses in steel plate with CFRP plate were able to calculate by using composite theory and measured temperature. Furthermore, in steel plate strengthened by CFRP and aluminum plates, the thermal stress introduced in steel plate was negligible-small through the all-season. It was found the thermal stresses in steel plate with CFRP plates as well as CFRP and aluminum plates were also estimated by using composite theory and measured temperature. In the steel plate strengthened by CFRP and aluminum plates, the thermal shear and normal stresses in adhesive layer glued to steel plate become smaller than that in the conventional CFRP bonded specimen. However, the shear stress in adhesive layers between CFRP and aluminum plates in proposed method was higher than the thermal stress in adhesive layers between CFRP plates in conventional method.     

CFRP布加固H型钢梁承载力试验研究

对不同加载方式和不同加固模式下的碳纤维增强复合材料(CFRP)加固H型钢梁进行了两组比较试验,并对试验结果进行了综合分析。通过对试验结果的比较分析,给出了各种加固方式下的钢梁的极限承载力变化及CFRP布和钢梁的各项力学性能,得出了一些具有工程指导意义的结论。     

Tests of cold-formed high strength stainless steel compression members

The paper describes a test program on cold-formed high strength stainless steel compression members. The duplex stainless steel having the yield stress and tensile strength up to 750 and 850 MPa, respectively, was investigated. The material properties of the test specimens were obtained from tensile coupon and stub column tests. The test specimens were cold-rolled into square and rectangular hollow sections. The specimens were compressed between fixed ends at different column lengths. The initial overall geometric imperfections of the column specimens were measured. The strength and behaviour of cold-formed high strength stainless steel columns were investigated. The test strengths were compared with the design strengths predicted using the American, Australian/New Zealand and European specifications for cold-formed stainless steel structures. Generally, it is shown that the design strengths predicted by the three specifications are conservative for the cold-formed high strength stainless steel columns. In addition, reliability analysis was performed to evaluate the current design rules.