Three-dimensional nonlinear analysis of creep in concrete filled steel tube columns

This paper proposes a based on 3D-VLE （three-dimensional nonlinear viscoelastic theory） three-parameters viscoelastic model for studying the time-dependent behaviour of concrete filled steel tube （CFT） columns. The method of 3D-VLE was developed to analyze the effects of concrete creep behavior on CFT structures. After the evaluation of the parameters in the proposed creep model, experimental measurements of two prestressed reinforced concrete beams were used to investigate the creep phenomenon of three CFT columns under long-term axial and eccentric load was investigated. The experimentally obtained time-dependent creep behaviour accorded well with the cu~＇es obtained from the proposed method. Many factors （such as ratio of long-term load to strength, slenderness ratio, steel ratio, and eccentricity ratio） were considered to obtain the regularity of influence of concrete creep on CFT structures. The analytical results can be consulted in the engineering practice and design.

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.

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.

Damage assessment for seismic response of recycled concrete filled steel tube columns

A model for evaluating structural damage of recycled aggregate concrete filled steel tube （RCFST） columns under seismic effects is proposed in this paper. The proposed model takes the lateral deformation and the effect of repeated cyclic loading into account. Available test results were collected and utilized to calibrate the parameters of the proposed model. A seismic test for six RCFST columns was also performed to validate the proposed damage assessment model. The main test parameters were the recycled coarse aggregate （RCA） replacement percentage and the bond-slip property. The test results indicated that the seismic performance of the RCFST member depends on the RCA contents and their damage index increases as the RCA replacement percentage increases. It is also indicated that the damage degree of RCFST changes with the variation of the RCA replacement percentage. Finally, comparisons between the RCA contents, lateral deformation ratio and damage degree were implemented. It is suggested that an improvement procedure should be implemented in order to compensate for the performance difference between the RCFST and normal concrete filled steel tubes （CFST）.

Performance Investigation of Square Concrete-filled Steel Tube Columns

The behaviour of square concrete-filled steel tube columns under concentrical loading was studied. More than one hundred specimens were tested to investigate the effects of thickness of steel tube on the load carrying capacity of the concrete-filled tubular columns （CFTs）. The effect of the grade of concrete and content of expansive agent were also investigated. The effect of these parameters on the confinement of the concrete core was studied as well. From the experimental study it was found that for both CFTs with different strength grade concrete core, the ultimate load carrying capacity increases with the increase in percentage of expansive agent up to 20% but it again decreases at 25% of expansive agent content. It was also shown that the failure mode of CFTs depends on the strength grade of concrete core.

Bearing capacity of composite columns reinforced by concrete filled steel tube

In this study, nine simplified short composite columns consisting of core CFST (concrete filled steel tube) of different diameters and outer reinforced concrete were constructed to study their compressive performance under axial or eccentric compression. The failure mode is characterized by the crush of the outer concrete. The bearing capacity increases at first and then decreases with further increase of the position coefficient. It can be concluded that position coefficient is an important structural parameter that has considerable influences on the ultimate bearing capacity of the composite columns. The outer concrete, steel tubes and longitudinal reinforcement are found to work in a cooperative manner under axial or eccentric compression when the position coefficient is about 0.5. An improved bearing capacity algorithm that takes the position coefficient into account has been proposed based on the experimental and simulation results and current technical specification in China. It has been proven to be precise and safe.

Analysis and seismic tests of composite shear walls with CFST columns and steel plate deep beams

A composite shear wall concept based on concrete filled steel tube （CFST） columns and steel plate （SP） deep beams is proposed and examined in this study. The new wall is composed of three different energy dissipation elements： CFST columns; SP deep beams; and reinforced concrete （RC） strips. The RC strips are intended to allow the core structural elements - the CFST columns and SP deep beams - to work as a single structure to consume energy. Six specimens of different configurations were tested under cyclic loading. The resulting data are analyzed herein. In addition, numerical simulations of the stress and damage processes for each specimen were carried out, and simulations were completed for a range of location and span-height ratio variations for the SP beams. The simulations show good agreement with the test results. The core structure exhibits a ductile yielding mechanism characteristic of strong column-weak beam structures, hysteretic curves are plump and the composite shear wall exhibits several seismic defense lines. The deformation of the shear wall specimens with encased CFST column and SP deep beam design appears to be closer to that of entire shear walls. Establishing optimal design parameters for the configuration of SP deep beams is pivotal to the best seismic behavior of the wall. The new composite shear wall is therefore suitable for use in the seismic design of building structures.

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.

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.

反复荷载下圆钢管型钢再生混凝土组合柱恢复力模型研究

为建立圆钢管型钢再生混凝土组合柱的恢复力模型,对11根圆钢管型钢再生混凝土组合柱试件进行了低周反复荷载试验研究,考虑了再生骨料取代率、配钢率及钢管径厚比等不同设计参数的影响,分析了组合柱的地震破坏形态及滞回性能。基于组合柱的力学特征及曲线形状,提出了圆钢管型钢再生混凝土组合柱骨架曲线的三折线参数模型,采用理论推导与数据拟合的方法确定了组合柱骨架曲线的模型参数。在此基础上,给出了组合柱的滞回规则和卸载规律,构建了组合柱的恢复力模型,计算滞回曲线与试验滞回曲线吻合良好,表明该恢复力模型较好地反映了反复荷载下组合柱的受力特征点及滞回性能,可为此类组合柱的推广提供技术参考。

圆钢管型钢再生混凝土组合柱水平承载力计算方法研究

通过对11个圆钢管型钢再生混凝土组合柱进行低周反复荷载试验,分析再生粗骨料取代率、型钢截面形式、轴压比、型钢配钢率及圆钢管壁厚参数对组合柱水平承载力的影响规律;观察了组合柱的破坏形态及特征,研究了圆钢管、型钢翼缘及腹板应变的发展规律,分析了组合柱的地震破坏特征。研究表明,在水平地震作用下组合柱发生典型的压弯塑性铰破坏。在此基础上,结合现有规范提出了基于叠加原理的圆钢管型钢再生混凝土组合柱水平承载力计算方法。计算结果表明,其水平承载力计算值与试验值吻合度较好,能较为准确地预测组合柱的水平承载力。

设有叠合柱的局部错层RC框架结构地震易损性分析

为研究某一设有叠合柱的局部错层钢筋混凝土(reinforced concrete,RC)框架结构在近断层和远场地震作用下的破坏概率,自定义钢管混凝土叠合柱塑性铰参数,并与试验对比验证塑性铰参数的有效性。采用SAP2000建立非线性分析模型,选取近断层和远场地震波共22条,分别以地震峰值加速度(peak ground acceleration,PGA)和最大层间位移角作为地震动强度指标和结构性能指标,基于增量动力分析(incremental dynamic analysis,IDA)结果和点估计函数对错层框架结构进行易损性分析。结果表明:对于远场地震动,结构满足“三水准”抗震设防要求,在罕遇地震下超越防止倒塌极限状态的概率仅为0.24%;结构在近断层多遇地震下仍能满足“小震不坏”的设计要求,设防地震下结构超越修复后使用极限状态的概率为53.63%,发生中等破坏的概率较大,罕遇地震下结构达到倒塌极限状态的概率为5.37%,较远场地震作用破坏更为严重。研究结果可为错层框架结构的设计和地震风险评估提供参考。

内置型钢增强钢管约束钢筋混凝土柱节点抗震性能研究

采用钢管对传统RC柱进行约束,形成的钢管约束钢筋混凝土(STRC)柱具有优越的承载能力和耗能性能。但在应用于框架节点设计时,节点区域仍采用普通RC柱节点构造,可能导致节点处没有钢管约束而成为薄弱点,节点处易发生破坏。针对钢管约束钢筋混凝土(STRC)柱节点抗震性能薄弱的现状,提出采用内置型钢短柱进行增强。基于ABAQUS工作平台对具有相同构造形式的节点试件建立有限元模型,将数值计算所得破坏形态、滞回曲线和型钢应力与试验结果进行对比,两者吻合较好,在此基础上探讨内置型钢长度和轴压比对节点抗震性能的影响。结果表明:随着内置型钢长度增加,试件的承载力、极限位移和延性均有不同程度的提升,轴压比对试件受力性能影响较小。通过进一步分析试件的破坏机制、型钢抗弯性能和耗能能力,提出满足节点抗震性能需求的内置型钢长度建议取值。

圆钢管型钢再生混凝土组合柱滞回性能非线性有限元分析

基于圆钢管型钢再生混凝土组合柱低周反复荷载试验研究,采用OpenSees软件对该组合柱进行了滞回性能数值分析,获取了组合柱的滞回曲线及抗震性能指标,并与试验结果进行了比较,验证了组合柱数值模型的合理性。在此基础上,对组合柱的滞回性能开展了参数影响分析。结果表明:再生骨料取代率从0增大到100%,组合柱的峰值荷载下降了7.78%,延性略有降低;提高再生混凝土强度、型钢强度及圆钢管强度,对于提升组合柱的承载力及刚度是有利的,但却使组合柱的脆性增大;随着轴压比的增大,组合柱的承载力呈现先上升后下降的趋势,而延性逐渐变差;型钢配钢率从5.54%增至9.99%,组合柱的峰值荷载提高了24.34%,但对于改善组合柱的延性不明显;增大钢管壁厚对组合柱的承载力和延性均是有利的。

基于机器学习的CFST柱高温剩余强度系数预测方法

目的 为提高计算精度和速度,利用机器学习模型泛化数据,以预测CFST柱的高温剩余强度系数。方法 利用生成对抗网络将搜集到的110个试验结果泛化生成407组数据,据此训练机器学习模型,并使用试验结果评估其性能,以确定最优模型;然后使用生成数据输入建立的模型预测CFST柱高温剩余强度系数,并和现有计算方法进行对比。结果 建立的随机森林模型在性能度量上表现最好,拟合优度达到0.947 7,均方误差为0.001 8,精度为94.7%;预测结果误差在±10%内的数据为83%,在±20%内的数据为100%;剩余强度系数主要影响因素依次为温度、钢材屈服强度、混凝土抗压强度和横截面积,钢管厚度影响很小。结论 提出的预测方法优于现有计算方法,具有更快的计算速度、更小的结果误差以及更强的模型可解释性,该方法可为CFST柱抗火设计提供参考。

新型波纹管连接方钢管混凝土拼接柱抗震性能试验研究

为研究一种新型波纹管连接方钢管混凝土拼接柱的抗震性能,以不同节点配筋率、不同节点锚固长度和轴压比为主要研究变量,对1个整浇柱和4个装配柱进行低周往复加载试验。研究分析了整浇柱与装配柱在破坏模式、滞回性能、位移延性等方面的抗震性能差异。结果表明:装配柱与整浇柱破坏模式类似,采用不同节点构造的装配柱与整浇柱在滞回曲线、骨架特性和刚度退化等方面差异不大,装配柱的耗能性能优于整浇柱。各装配柱极限位移角为1/16.7~1/25,满足规范设计要求。在较大轴压比下,装配柱的承载力提升,延性降低。在较大位移角下,装配柱节点抗震性能受焊缝质量的影响较大,建议实际工程中加强焊接质量管理,以保证构件具有足够的变形能力。

开槽侧板加强型连接梁柱节点子结构抗连续倒塌性能研究

为了优化侧板加强型节点的抗连续倒塌性能,提出了一种开槽侧板加强型梁柱连接节点形式。通过静力加载试验,得到了新型节点子结构的破坏模式、竖向抗力发展机制、应变分布、延性水平和动力承载力等。试验结果表明:钢梁翼缘在盖板附近首先开裂并向腹板外排螺栓中部延伸贯穿,导致竖向承载力快速下降为峰值荷载的20%左右。相比传统侧板加强型和圆弧扩盖板型梁柱节点,新型节点的峰值承载力分别提高了62.1%和19.6%,塑性弦转角分别提高了203.6%和25.8%。通过有限元分析得到了试件的断裂路径发展过程,进一步地通过参数化分析发现:扩大过焊孔构造对改善峰值后塑性弦转角及残余强度提升无明显作用;随着螺栓群远离钢梁翼缘起始断裂位置,最小残余强度比不断增大,但峰值后塑性弦转角呈先增大后减小的趋势;当螺栓群内移45mm时,峰值后塑性弦转角及最小残余强度比分别达到0.149rad和0.88,提高了38.2%和200.8%。

高温后钢管高强混凝土柱的黏结性能

为了研究高温后钢管高强混凝土柱的黏结性能,通过有限元软件ABAQUS建立模型,并与已有试验结果进行验证,吻合良好。研究表明:钢管高强混凝土柱的黏结应力随着温度的升高呈现出先增加后减小的趋势;黏结应力随着混凝土强度的增大而增大,随着径厚比的增大而减小;长径比在2.74~7.72的范围内,随着长径比的增加,黏结力逐渐增加;黏结界面破坏的速度随温度的增加而减缓;随着温度的增加耗能因子呈现先增大后减小的趋势。

铅黏弹性阻尼器增强钢管约束钢筋混凝土柱节点抗震性能研究

针对钢管约束钢筋混凝土(steel tubed reinforced concrete,STRC)柱节点抗震性能薄弱的现状,提出采用改良的铅黏弹性阻尼器进行增强。基于ABAQUS工作平台对铅黏弹性阻尼器试件建立有限元模型,将数值计算所得滞回和骨架曲线、耗能性能及疲劳性能与试验结果进行对比,两者吻合较好。在此基础上探讨了铅芯直径、铅芯布置形式及复合黏弹性体厚度比值对铅黏弹性阻尼器力学性能的影响。结果表明:相较于扇形铅黏弹性阻尼器,改良后的四边形铅黏弹性阻尼器表现出更好的力学性能;随着铅芯直径的增大,阻尼器的耗能能力指标均得到大幅提高;建议铅芯个数取2个,铅芯面积与复合黏弹性层面积比值取6%~8%,复合黏弹性体厚度比值约为0.67。建立不同布置方案的铅黏弹性阻尼器增强STRC柱节点试件模型,对其破坏形态、滞回特性和箍筋应力进行对比分析,给出合理的布置方案,为实际工程提供参考。

反复荷载下圆钢管型钢再生混凝土组合柱耗能能力及延性分析

对11个圆钢管型钢再生混凝土组合柱试件进行了低周反复荷载试验,观察了组合柱的破坏形态及特征,获取了组合柱的滞回曲线、等效粘滞阻尼系数、延性系数及侧向位移角等性能指标,分析了再生粗骨料取代率、型钢截面形式、轴压比、型钢配钢率和圆钢管径厚比等参数对组合柱耗能能力及延性的影响规律。结果表明:反复荷载作用下组合柱呈压弯塑性铰破坏模式;组合柱的荷载-位移滞回曲线饱满呈纺梭形,屈服位移后的等效粘滞阻尼系数在0.30~0.48之间,平均位移延性系数大于3.0,极限侧向位移角介于1/30~1/20之间且均大于规范限值1/50,组合柱表现出良好的耗能能力和延性。此外,组合柱的耗能能力和延性受再生粗骨料取代率的影响较大,全再生混凝土较普通混凝土耗能能力下降14.5%;增加钢管壁厚或型钢配钢率有利于提升组合柱的耗能能力和延性,而增大轴压比则对组合柱的延性和变形不利;内置型钢采用箱型截面时,组合柱耗能及延性最优,与工字型钢相比,采用箱型截面型钢的组合柱耗能能力和延性分别提升了23.5%和51.9%。