Axial Bearing Capacity of Short FRP Confined Concrete-filled Steel Tubular Columns

The bearing capacity of FRP confined concrete-filled steel tubular （FRP-CFST） columns under axial compression was investigated. This new type of composite column is a concrete-filled steel tube （CFST） confined with fiber-reinforced polymer （FRP） wraps. Totally 11 short column specimens were tested to failure under axial compression. The influences of the type and quantity of FRP, the thickness of steel tube and the concrete strength were studied. It was found that the bearing capacity of short FRP-CFST column was much higher than that of comparable CFST column. Furthermore, the formulas for calculating the bearing capacity of the FRP-CFST columns are proposed. The analytical calculated results agree well with the experimental results.

Seismic performance of recycled concrete-filled square steel tube columns

An experimental study on the seismic performance of recycled concrete-filled square steel tube （RCFST） columns is carried out. Six specimens were designed and tested under constant axial compression and cyclic lateral loading. Two parameters, replacement percentage of recycled coarse aggregate （RCA） and axial compression level, were considered in the test. Based on the experimental data, the hysteretic loops, skeleton curves, ductility, energy dissipation capacity and stiffness degradation of RCFST columns were analyzed. The test results indicate that the failure modes of RCFST columns are the local buckling of the steel tube at the bottom of the columns, and the hysteretic loops are full and their shapes are similar to normal CFST columns. Furthermore, the ductility coefficient of all specimens are close to 3.0, and the equivalent viscous damping coefficient corresponding to the ultimate lateral load ranges from 0.323 to 0.360, which demonstrates that RCFST columns exhibit remarkable seismic performance.

Effect of contact thermal resistance on temperature distributions of concrete-filled steel tubes in fire

To predicate the temperature distribution of concrete-filled steel tubes(CFSTs) being exposure to fire,a finite element analysis model was developed using a finite element package,ANSYS.A suggested value of contact thermal resistance was therefore proposed with the supporting of massive numbers of collected test data.Parametric analysis was conducted subsequently towards the cross-sectional temperature distribution of CFST columns in four-side fire,in which the exposure time,width of the cross section,steel ratio were taken into account with considering contact thermal resistance.It was found that contact thermal resistance has little effect on the overall temperature regulation with the exposure time,the width of cross-section or the change of steel ratio.However,great temperature dropping at the concrete adjacent to the contact interface,and gentle temperature increase at steel tube,exist if considering contact thermal resistance.The results of the study are expected to provide theoretical basis for the fire resistance behavior and design of the CFST columns being exposure to fire.

Experimental and Numerical Studies on Sea Sand Concrete Filled Stainless Steel Tube with Inner FRP Tube Subjected to Axial Compression

Since fibre-reinforced polymer(FRP) and stainless steel(SS) offer advantages of corrosion resistance and hybrid confinement, this study proposed a new type of composite column: sea sand concrete(SSC)-filled SS tubular columns with an inner FRP tube(CFSTFs) to help exploit abundant ocean resources in marine engineering. To study compressive behaviours of these novel members, eight CFSTFs and two SSC-filled SS tubular columns(CFSTs)were tested under axial compression. Their axial load-displacement curves, axial load-strain curves in SS or FRP tubes were obtained, and influences of key test parameters(the existence of glass FRP(GFRP) tubes, steel tube shapes, and GFRP tube thicknesses and diameters) were discussed. Further, specimen failure mechanism was analyzed employing the finite element method using ABAQUS software. Test results confirmed the excellent ductility and load-bearing capacity of CFSTFs. The existence of GFRP tubes inside can postpone SS tube buckling, and the content of inner FRP tubes, particularly increasing diameters, was found to improve compressive behaviours. GFRP contents helped develop the second elastic-plastic stage of the load-displacement curves. Furthermore, the bearing capacity of CFSTFs with a circular cross-section was approximately 26% higher than that with a square cross-section, and this difference narrowed with the increase in GFRP ratios.

Experimental Study on Flexural Behaviour of Circular Concrete-Filled FRP-PVC Tubular Members

An experimental investigation was conducted on the flexural behavior of FRP-PVC confined concrete circular tubular members.A total of six specimens were prepared and tested under flexural loading.The main parameters varied in the tests were the layer of FRP and the strengthening approach of BFRP and CFRP.The failure modes,ultimate bending capacity and stress-strain relation curves were investigated in details.Furthermore,the relation model of moment(M)-curvature(φ)was studied,and on the basis of M-φ relation model,a simplified formula was presented to compute the ultimate bending moment capacity.The results show that the external confinement of concrete specimens by FRP-PVC tubes results in enhancing the ultimate bending strength and ultimate deformation,and the ultimate bending capacity increased with the FRP layers.Simultaneously,the reinforcement effect in CFRP is better than that in BFRP.The ultimate bending moment capacity values predicted by the presented formula agree well with the experimental results,which imply that the presented formula is applicable and efficient for prediction of the ultimate bending moment capacity as well.

Reliability analysis of circular concrete-filled steel tube with material and geometrical nonlinearity

Most of the previous research on concrete-filled steel tube is restricted to a deterministic approach. To gain clear insight into the random properties of circular concrete-filled steel tube, reliability analysis is carried out in the present study. To obtain the Structural nonlinear response and ultimate resistance capacity, material and geometrical nonlinear analysis of circular concrete-filled steel tube is performed with a three-dimensional degenerated beam ele- ment. Then we investigate the reliability of concrete-filled steel tube using the first-order reliability method combined with nonlinear finite element analysis. The influences of such parameters as material strength, slenderness, initial geo- metrical imperfection, etc. on the reliability of circular concrete-filled steel tube column are studied. It can be con- cluded that inevitable random fluctuation of those parameters has significant influence on structural reliability, and that stochastic or reliability methods can provide a more rational and subjective evaluation on the safety of CFT structures than a deterministic approach.

PVC-FRP管混凝土柱力学性能研究进展

PVC-FRP管混凝土柱作为一种新型组合结构形式,结合了FRP材料高强、耐腐蚀和PVC管质量轻、延性好等优点,该结构通过PVC-FRP管对内部核心混凝土的约束效应,增强混凝土柱的力学性能。目前,国内外研究人员对不同配筋率、FRP种类、PVC管壁厚等设计参数下PVC-FRP管混凝土柱的力学性能、抗震性能及耐久性能指标的变化规律进行了大量试验研究和理论分析。其中,基于现有的FRP约束混凝土本构关系模型,推导出了适用于PVC-FRP管混凝土柱的本构关系模型。为进一步明晰PVC-FRP管混凝土柱服役时的工作机理,针对近年来PVC-FRP管混凝土柱的研究现状进行综述,并对今后研究方向提出了建议。

Ductility and Ultimate Capacity of Concrete-Filled Lattice Rectangular Steel Tube Columns

A kind of concrete-filled lattice rectangular steel tube(CFLRST)column was put forward.The numerical simulation was modeled to analyze the mechanical characteristic of CFLRST column.By comparing the load-deformation curves from the test results,the rationality and reliability of the finite element model has been confirmed,moreover,the change of the section stiffness and stress in the forcing process and the ultimate bearing capacity of the column were analyzed.Based on the model,the comparison of ultimate bearing capacity and ductility between CFLRST column and reinforced concrete(RC)column were also analyzed.The results of the finite element analysis show that the loading process of CFLRST column consists of elastic stage,yield stage and failure stage.The failure modes are mainly strength failure and failure of elastoplastic instability.CFLRST column has higher bearing capacities in comparison with reinforced concrete columns with the same steel ratio.In addition,the stiffness degeneration of CFLRST column is slower than RC column and CFLRST column has good ductility.

Prediction of Axial Capacity of Concrete-Filled Square Steel Tubes Using Neural Networks

The application of artificial neural network to predict the ultimate bearing capacity of CFST （ concrete-filled square steel tubes） short columns under axial loading is explored. Input parameters consiste of concrete compressive strength, yield strength of steel tube, confinement index, sectional dimension and width-to-thickness ratio. The ultimate bearing capacity is the only output parameter. A multilayer feedforward neural network is used to describe the nonlinear relationships between the input and output variables. Fifty-five experimental data of CFST short columns under axial loading are used to train and test the neural network. A comparison between the neural network model and three parameter models shows that the neural network model possesses good accuracy and could be a practical method for predicting the ultimate strength of axially loaded CFST short columns.

Behavior of High Strength Concrete Filled Square Steel Tube Stub Columns with Inner CFRP Tube Under Biaxial Eccentric Compression

This paper studies the contribution of CFRP(carbon fiber-reinforced polymer)to the mechanical behavior of high strength concrete-filled square steel tube(HCFST)under biaxial eccentric compression.The new type of composite member is composed of an inner CFRP tube and an outer steel tube with concrete filled in the two tubes.The finite element analysis was made by ABAQUS on the behavior of high strength concrete filled square steel tubular columns with inner CFRP circular tube subjected to bi-axial eccentric loading.The results obtained from the finite element analysis were verified with the experimental results.In addition,the load-deflection curves in the whole process were calculated and analyzed,which can be divided into three segments:Elastic phase,plastic phase,descending phase.Based on the load-deflection curves,the stresses analysis on the core concrete,CFRP tube and steel tube were conducted.The confinement effect of the CFRP tube improves the ductility of HCFST-CFRP stub column.CFRP ratio and eccentricity affect the ultimate bearing capacity of HCFST stub column.Finally,a calculation formula of ultimate bearing capacity was proposed in the paper.

芯钢管连接的PVC-CFRP管混凝土柱-RC梁边节点抗裂承载力研究

为研究芯钢管连接的PVC-CFRP管混凝土柱-RC梁边节点的抗裂性能,文中以芯钢管含钢率、节点配箍率、轴压比、梁纵筋配筋率和CFRP条带间距为设计参数,对11根节点试件进行低周反复荷载试验,观察其裂缝发展过程,分析各参数对其初裂荷载的影响。试验结果表明:节点在加载过程中先后经历了初裂、通裂、峰值和破坏阶段,初裂荷载约为极限荷载的40%~50%,随着节点核心区表面斜裂缝的出现,试件荷载-位移滞回曲线逐渐偏离线性。与普通节点相比,内置芯钢管可有效提高节点的初裂荷载,同时,随着节点配箍率和轴压比增加,节点初裂荷载有所提高,芯钢管含钢率、梁纵筋配筋率以及CFRP条带间距对节点初裂荷载影响不明显。最后,基于节点核心区的受力分析,引入节点箍筋影响系数,提出节点抗裂承载力计算公式,与试验值吻合良好,此外,与目前已有节点抗裂承载力计算模型对比,文中公式更适用于芯钢管节点抗裂设计。

钢-FRP复合约束混凝土隧道衬砌管片力学性能试验研究

提出了一种新型隧道衬砌管片--钢-FRP复合约束混凝土(Multi-Cell steel tube-Sandwich Concrete-FRP tubeCore concrete,MCSCFC)管片。MCSCFC管片由以下四个部分组成:多腔方钢管、夹层混凝土、纤维增强复合材料(FRP)管、核心混凝土。为研究均匀围压状态下MCSCFC管片与传统钢筋混凝土(Reinforced Concrete,RC)管片力学性能的差异,本文以FRP管厚度为变量,共设计制作了4个管片模型试件(3个MCSCFC管片,1个RC管片),随后对4个管片模型试件开展均匀围压状态下的力学性能试验研究,分析试件的破坏模式、荷载-位移关系以及荷载-应变响应。研究结果表明:①MCSCFC管片表现出明显的屈服后硬化行为;②相比RC管片,MCSCFC管片在厚度减薄39.4%的情况下,极限承载力提升41.8%。

FRP管约束混凝土的轴压应力-应变关系研究

目前FRP约束混凝土轴压应力-应变关系大都只考虑FRP管环向受拉。针对FRP管约束混凝土的轴心受压性能进行分析,考虑其承受压力造成约束模量降低的影响,在现有约束混凝土模型的基础上,提出一种考虑FRP管在双向受力情况下的应力-应变关系分析模型,并与试验结果进行了分析对比,分析结果与试验结果吻合较好。还依据这一模型进行了参数研究。

局部屈曲FRP增强薄壁钢管混凝土抗震性能

为了研究FRP(Fiber Reinforced Polymer)增强薄壁钢管混凝土的抗震机理,提出FRP约束钢管局部屈曲应力-应变关系并建立FRP约束钢管恢复力模型,在此基础上建立FRP增强薄壁钢管混凝土柱滞回模型,开展FRP增强薄壁钢管混凝土柱拟静力试验以验证滞回模型的合理性,同时考查FRP布置方式对柱体抗震性能的影响,利用滞回模型对FRP增强薄壁钢管混凝土的耗能机理进行分析。研究表明,薄壁钢管局部屈曲所导致的强度退化是柱体抗震性能劣化的主要原因,基于纤维力学特性合理设计FRP的增强方式可有效提升柱体的抗震性能。CFRP宜采用环向约束方式抑制薄壁钢管的局部屈曲;GFRP宜采用纵向抗弯方式提高柱体大变形下的承载能力。FRP增强薄壁钢管混凝土的耗能主要由钢管承担,在本文研究参数范围内,薄壁钢管耗能占比超过80%,混凝土耗能介于10%～20%,纵向FRP耗能小于8%,对薄壁钢管实施有效约束后,其耗能可提高40%以上。

FRP约束钢管混凝土轴压构件力学性能研究

进行了9个钢管混凝土轴心受压构件的试验研究,对其中的6个构件包裹了纤维增强复合材料(FRP)。试验参数为钢管截面形状、截面尺寸和包裹FRP的层数。试验结果表明,FRP约束圆钢管混凝土可以较好地发挥FRP约束混凝土和钢管混凝土的双重优点,构件在具有较高承载力的同时还具有较好的延性。最后,提供了FRP约束圆钢管混凝土轴压构件承载力的简化计算公式。

FRP套管砂浆组合加固双轴对称轴压钢构件试验研究

利用FRP(纤维增强复合材料)套管和砂浆对受压钢构件进行组合加固以增强构件的抗屈曲性能。通过对不同钢构件截面形状、长细比和端部横向纤维布包裹层数的18根构件的轴压试验,得到加固前后构件的破坏模式、荷载-位移曲线和关键位置的应变变化。试验中构件的破坏模式由加固前的整体屈曲变为加固后的端部裸露钢构件集中变形及FRP套管从端部沿纵向劈裂破坏,加固构件的极限承载力最大提高178%,延性指标最大提高778%。加载过程中构件的不同材料之间会产生滑移现象,但核心钢构件截面基本能保持平截面假定。此方法对大长细比的纯钢受压构件具有更好的抗屈曲加固效果,加固时需注意构件应具有足够的加固段刚度和长度,同时端部应包裹足够的横向纤维布。

FRP-混凝土-钢双壁空心管柱抗震性能试验

为研究FRP-混凝土-钢双壁空心管柱的抗震性能,完成了9根双壁空心管柱试件和4根混凝土-钢空心管柱对比试件在定轴力和往复水平力作用下的试验。双壁空心管柱试件有3种破坏形态:内层钢管受拉屈服,混凝土压坏;内层钢管未受拉屈服,混凝土压坏或塑性铰区FRP管出现多条树脂受压剪切裂缝;塑性铰区FRP管压屈。试验过程中,发现:增大纤维特征值,试件的变形能力增大;加大轴压比,承载能力有所提高,但变形能力下降;空心率为0.6和0.74时,其承载能力和变形能力分别相差不大;FRP管的纤维缠绕角度为80°时,轴压力加载方式对试件承载能力和变形能力的影响不大;纤维缠绕角度为60°时,轴压力加载方式对试件的承载能力影响不大,但对变形能力有所影响。结果表明,由于FRP的约束作用,双壁空心管柱试件的承载能力大于混凝土-钢空心管柱试件,变形能力和耗能能力显著大于对比试件。

FRP管高强混凝土轴压力学性能研究

目的研究FRP管高强混凝土轴压柱的受力性能,以利于实际工程的设计.方法通过4根FRP管高强混凝土组合柱轴压试验,探讨组合柱的破坏特征及受力特点,研究FRP管纤维缠绕角度、FRP管厚度等参数对组合柱受力性能的影响.基于叠加法,分析混凝土和FRP管对承载力的贡献.结果组合柱承载力随着FRP管壁纤维缠绕角度的减小而增加,随着FRP管壁厚度增加而增加,采用叠加法研究并推导出了一组更为合理的FRP管高强混凝土组合柱轴心受压承载力计算公式.结论理论计算结果与试验结果吻合良好.

FRP管-混凝土-钢管组合短柱轴压性能试验及有限元分析

为了研究FRP管-混凝土-钢管组合短柱的轴压性能,进行了3个实心组合短柱的轴压试验.基于非线性有限元分析软件ANSYS,通过选择合适的单元类型以及材料的本构关系,对实心组合短柱进行了数值仿真分析,并与试验结果进行对比,证明了所采用的非线性有限元分析模型可以较好地模拟实心组合短柱的受力过程.结合已建立的有限元模型,开展了参数的敏感性分析,探讨了FRP管厚度、钢管厚度和强度、核心混凝土强度等参数对实心组合短柱轴压性能的影响.模拟结果表明:FRP管厚度对实心组合短柱的轴压性能有显著影响,钢管厚度和强度对实心组合短柱的影响规律较相似,对第二线性阶段斜率均无影响,其中钢管厚度对实心组合短柱的初始刚度略有影响,核心混凝土强度变化对极限承载力有较大影响.