材料单轴滞回准则对组合构件非线性分析的影响

材料单轴滞回准则的选择是采用纤维模型进行组合构件非线性分析的关键问题之一。首先给出了混凝土、钢筋和钢材的精细滞回准则和简单滞回准则,以及精细滞回准则的程序实现流程,其中混凝土材料的精细滞回准则通过对已有模型的改进可准确考虑任意可能复杂加卸载路径下各类混凝土材料的强度退化和刚度退化特征。通过对组合梁和钢管混凝土柱的抗震性能试验进行模拟,验证了精细滞回准则的模拟精度。研究了材料单轴滞回准则对组合构件整体滞回性能模拟结果的影响。分析结果表明：钢材滞回准则是影响组合构件抗震性能模拟精度的关键因素,应考虑钢材的包辛格效应,而混凝土材料的滞回准则对非线性分析结果的影响很小,采用简单滞回准则即可取得较好的模拟精度。     

Monotonic and cyclic behavior of concrete-filled steel-tube beam-columns considering local buckling effect

A mathematical model is developed to evaluate the monotonic and cyclic behavior of concrete-filled steel tube (CFST) beam-columns with rectangular cross section. The model includes the reduction in the steel compressive strength due the local buckling effect. The degradations in unloading and reloading stiffness of steel tube due to local buckling are also included. The model is based on fiber element method in which uniaxial stress–strain material laws are used for cross section components. The results obtained from the mathematical model were compared with experimental results for columns under monotonic as well as cyclic loads. It is observed that the proposed model predicts well the columns and beams nonlinear behavior compared with the experimental results.     

钢桁架约束混凝土组合柱约束机理及截面曲率分析

钢桁架(ST)约束混凝土组合柱具有良好的力学性能及抗震性能,且其截面形式能够有效提高内部钢材的利用率.基于已进行的轴向力学性能试验,对ST约束混凝土的约束机理进行详细分析,建立有效约束力计算模型、承载力计算模型以及本构模型.进行ST约束混凝土组合柱低周反复荷载试验研究,在试验研究的基础上,建立组合柱的截面弯矩-曲率分析...     

矩形钢管混凝土柱-混凝土梁搭接式节点抗震性能研究

在现有钢管混凝土柱与混凝土梁连接形式基础上,提出一种新型搭接式钢管混凝土柱-混凝土梁节点,通过低周反复荷载试验和数值模拟分析,研究了节点的破坏形态、承载能力、刚度退化、延性及耗能能力等,分析了梁端纵筋和搭接牛腿翼缘的应变发展规律。结果表明:钢管混凝土柱-混凝土梁搭接式节点表现出典型的混凝土梁端塑性铰破坏模式,符合“强节点、弱构件”的抗震设防理念; 节点的滞回曲线饱满,捏缩现象较轻,刚度退化明显,强度退化较小,具有较好的延性及耗能能力; 梁纵筋及牛腿各测点中搭接牛腿端部截面应变最大,通过搭接连接方式可实现梁端弯矩和剪力的有效传递,但搭接牛腿长度不足会使节点发生锚固破坏; 在节点设计时,应保证足够的搭接长度,以实现钢筋混凝土梁纵筋在弯剪复合受力状态下锚固性能的可靠性,充分发挥节点的抗震耗能能力; 有限元计算结果与试验结果吻合良好,该模型能够准确反映节点在实际受力情况下的力学特性,所得结论可为此类新型搭接式节点工程应用提供技术支撑。     

钢纤维高强混凝土在单调和重复荷载作用下轴压应力-应变曲线试验研究

将钢纤维掺入高强混凝土中,可以改善高强混凝土的脆性,进而可提高结构构件的延性。为研究钢纤维高强混凝土抗压性能,分别对强度等级为C60和C80,钢纤维体积率为0%、1.0%、1.5%和2.0%的钢纤维高强混凝土进行立方体抗压强度试验以及单调和重复荷载作用下的轴心抗压全过程试验。试验结果表明：随着钢纤维体积率的增大,混凝土强度基本没有改变,但相应于峰值应力的应变略有增加,而弹性模量稍有下降。在已有研究的基础上,提出了适用的钢纤维高强混凝土轴压应力-应变曲线数学表达式和受重复荷载作用的钢纤维高强混凝土的卸载曲线和再加载曲线表达式。所提出计算式得到的结果与试验结果吻合较好,可为钢纤维高强混凝土结构的设计和非线性分析提供理论基础。     

Strength and post‐yield behavior of T‐section steel encased by structural concrete

In this study, the seismic performance of unsymmetrical steel–concrete composite precast beams with T‐shaped steel section were numerically explored and validated by their earlier experimental investigation. This design is based on the proposed calibrated finite element model in which key damage parameters for the evaluation of the nonlinear, post‐yield behavior of the precast composite steel beams were identified. The proposed nonlinear finite‐element‐based numerical model uses various parameters, including the dilatation angle and concrete‐damaged plasticity, to simulate the nonlinear behavior of unsymmetrical composite precast beams with T‐section steel. Greater seismic capacity with greater ductility, contributing to a maximized structural capacity within the composite precast beams was introduced by the effective use of the 2 materials, steel and concrete, and shown by the nonlinear hysteretic investigation of unsymmetrical steel–concrete composite precast beams that was validated experimentally. The post‐yield structural capacity found via the numerical analysis agrees with experimental results when the concrete‐damaged plasticity of the plastic‐damaged seismic model for concrete and the von Mises criteria of the steel section were introduced into the finite element model. Practical design parameters and recommendations were eventually suggested by examining the influence of precast composite beams with unsymmetrical steel sections on the concrete degradations and damage evolution.     

Analytical model for predicting axial capacity and behavior of concrete encased steel composite stub columns

The axial compressive capacity and force-deformation behavior of concrete encased steel stub columns were analytically investigated. An analytical model was developed for predicting the force-deformation response for composite stub columns with various structural steel sections and volumetric lateral reinforcement. Constitutive relationships were established for materials used in the composite cross section, which included unconfined concrete, partially and highly confined concrete, structural steel section, and longitudinal reinforcing bar. The axial capacity of composite stub columns can be determined from strengths contributed from each material component following the stress-strain relationship. Analytical results show that the axial load-carrying capacity and force-deformation behavior measured in the experiments can be accurately predicted. In addition to the lateral reinforcement, the structural steel section can provide a confinement effect on the concrete and enhance the axial capacity and post-peak strength.     

Nonlinear behaviour of unprotected composite slim floor steel beams exposed to different fire conditions

An efficient nonlinear 3D finite element model has been developed to investigate the structural performance of composite slim floor steel beams with deep profiled steel decking under fire conditions. The composite steel beams were unprotected simply supported with different cross-sectional dimensions, structural steel sections, load ratios during fire and were subjected to different fire scenarios. The nonlinear material properties of steel, composite slim concrete floor and reinforcement bars were incorporated in the model at ambient and elevated temperatures. The interface between the structural steel section and composite slim concrete floor was also considered, allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the composite beam. Furthermore the thermal properties of the interface were included in the finite element analysis. The finite element model has been validated against published fire tests on unprotected composite slim floor steel beams. The time–temperature relationships, deformed shapes at failure, time–vertical displacement relationships, failure modes and fire resistances of the composite steel beams were evaluated by the finite element model. Comparisons between predicted behaviour and that recorded in fire tests have shown that the finite element model can accurately predict the behaviour of the composite steel beams under fire conditions. Furthermore, the variables that influence the fire resistance and behaviour of the unprotected composite slim floor steel beams, comprising different load ratios during fire, cross-section geometries, beam length and fire scenarios, were investigated in parametric studies. It is shown that the failure of the composite beams under fire conditions occurred for the standard fire curve, but did not occur for the natural fires. The use of high strength structural steel considerably limited the vertical displacements after fire exposure. It is also shown that presence of additional top reinforcement mesh is necessary for composite beams exposed to short hot natural fires. The fire resistances of the composite beams obtained from the finite element analyses were compared with the design values obtained from the Eurocode 4 for composite beams at elevated temperatures. It is shown that the EC4 predictions are generally conservative for the design of composite slim floor steel beams heated using different fire scenarios.     

Creep behaviour modelling of a composite steel-concrete section

Long term behavior modelling of steel-concrete composite beams requires the use of the time variable and the taking into account of all the sustained stress history of the concrete slab constituting the cross section. In this context, a research contribution of the time-dependent behavior of composite sections under the creep influence of a concrete slab is presented in the present work. The concrete creep influence upon the composite section strength is taken into account as part of the concrete linear viscoelasticity. Thus, by considering the assumption of a complete composite connection made up of a steel beam and concrete slab, with no slip at the interface, and owing to the irreversible law of concrete, we obtain an analytical model, based upon a system of two differential linear equations governing the long term behavior of composite beam cross sections in bending. Results obtained from the application of the suggested model to a steel-concrete composite beam are satisfactory.     

Nonlinear analysis of short concrete-filled steel tubular beam-columns under axial load and biaxial bending

This paper presents a nonlinear fiber element analysis method for determining the axial load-moment strength interaction diagrams for short concrete-filled steel tubular (CFST) beam-columns under axial load and biaxial bending. Nonlinear constitutive models for confined concrete and structural steel are considered in the fiber element analysis. Efficient secant algorithms are developed to iterate the depth and orientation of the neutral axis in a composite section to satisfy equilibrium conditions. The accuracy of the fiber element analysis program is verified by comparisons of fiber analysis results with experimental data and existing solutions. The fiber element analysis program developed is employed to study the effects of steel ratios, concrete compressive strengths and steel yield strengths on axial load-moment interaction diagrams and the C-ratio of CFST beam-columns. The proposed fiber element analysis technique is shown to be efficient and accurate and can be used directly in the design of CFST beam-columns and implemented in advanced analysis programs for the nonlinear analysis of composite columns and frames.     

Eccentrically loaded concrete encased steel composite columns

This paper presents a nonlinear 3-D finite element model for eccentrically loaded concrete encased steel composite columns. The columns were pin-ended subjected to an eccentric load acting along the major axis, with eccentricity varied from 0.125 to 0.375 of the overall depth (D) of the column sections. The model accounted for the inelastic behaviour of steel, concrete, longitudinal and transverse reinforcement bars as well as the effect of concrete confinement of the concrete encased steel composite columns. The interface between the steel section and concrete, the longitudinal and transverse reinforcement bars, and the reinforcement bars and concrete were also considered allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the column. The initial overall geometric imperfection was carefully incorporated in the model. The finite element model has been validated against existing test results. The concrete strengths varied from normal to high strength (30–110 MPa). The steel section yield stresses also varied from normal to high strength (275–690 MPa). Furthermore, the variables that influence the eccentrically loaded composite column behaviour and strength comprising different eccentricities, different column dimensions, different structural steel sizes, different concrete strengths, and different structural steel yield stresses were investigated in a parametric study. Generally, it is shown that the effect on the composite column strength owing to the increase in structural steel yield stress is significant for eccentrically loaded columns with small eccentricity of 0.125D. On the other hand, for columns with higher eccentricity 0.375D, the effect on the composite column strength due to the increase in structural steel yield stress is significant for columns with concrete strengths lower than 70 MPa. The strength of composite columns obtained from the finite element analysis were compared with the design strengths calculated using the Eurocode 4 for composite columns. Generally, it is shown that the EC4 accurately predicted the eccentrically loaded composite columns, while overestimated the moment.     

钢-混组合连续梁桥墩顶应力计算研究

基于有限元分析,对某4跨钢-混组合连续梁桥进行混凝土面板与钢梁的3个不同建模形式进行分析,对比规范要求的不同建模形式下墩顶负弯矩区混凝土面板钢筋应力与钢梁应力的变化关系.研究结果表明:将开裂区钢筋等效为混凝土,与钢梁形成组合截面的建模形式计算得到的钢筋换算应力与钢梁应力均适中,所以推荐采用该种建模方式进行计算;此外在偏...     

考虑楼板空间组合效应的组合框架体系纤维模型

将正常使用极限状态和承载能力极限状态有效宽度公式与传统纤维模型相结合,通过修正楼板钢筋和混凝土材料的单轴本构关系,实现了一种改进的考虑楼板空间组合效应的纤维模型,利用该模型可以在组合框架体系非线性分析中采用高效的全杆系单元合理地考虑复杂楼板空间组合效应的影响。基于考虑楼板空间组合效应的纤维模型,开发了一套较为完整的组合结构非线性有限元分析程序COMPONA-MARC (Version 1.0)。最后,将开发程序应用于多个组合框架抗震试验的模拟,结果表明了本文模型可较为准确地模拟组合框架弹塑性全过程的楼板空间组合效应。     

基于柔度法的纤维梁柱单元及其参数分析

基于柔度法和纤维模型的基本思想,应用Opensees软件,对梁柱单元应用于钢管混凝土柱的建模方法进行研究,在此基础上,对单元参数进行分析,探讨材料本构关系、纤维截面网格划分、积分点数量等参数对模拟精度及计算效率的影响。通过与试验对比,结果表明,受约束混凝土本构关系的下降段和钢材本构关系的强化段是影响单元准确性的主要因素,而网格划分和积分点数量的影响则主要体现在计算效率方面。研究将为基于柔度法的纤维梁柱单元在组合结构中的应用提供依据。     

钢筋混凝土冻融损伤黏结滑移本构模型及其在构件中的应用

钢筋混凝土(RC)构件在冻融循环作用下易产生黏结性能的退化,以冻融损伤RC构件为对象,研究其纵筋的黏结滑移行为。通过分析冻融作用对混凝土与钢筋界面黏结行为的影响,将黏结性能的退化归因于混凝土强度退化和保护层约束效应退化,并以钢筋混凝土黏结滑移本构和冻融黏结滑移试验结果为基础,建立了适用于冻融损伤混凝土的黏结滑移本构模型。基于已有拉拔试验数据,对所提模型的精度和可靠性进行验证。结合所提模型及微元算法进一步得到冻融损伤纵筋滑移模型,采用非线性梁柱单元与零长度截面单元串联的组合方式,将该纵筋滑移模型应用于零长度截面中的钢筋纤维本构,建立了可考虑黏结滑移行为的冻融损伤RC纤维模型。根据6榀冻融损伤RC框架柱拟静力试验结果对该纤维模型的准确性进行验证,并采用仅考虑冻融损伤混凝土强度退化的纤维模型进行辅助验证。结果表明：相比已有黏结滑移本构,所提模型可更为准确地反映冻融损伤混凝土与钢筋界面的黏结滑移行为全过程;相比纤维模型,所提冻融损伤RC纤维模型的标准化滞回力误差和滞回耗能误差分别降低了8.7%和37.0%,且可更为准确地计算冻融损伤RC框架柱的滞回曲线及耗能能力。     

考虑钢筋与混凝土相对滑移的修正钢筋本构模型

为了研究钢筋滑移对钢筋混凝土柱抗震性能的影响,在材料层次上建立了考虑钢筋与混凝土相对滑移的修正钢筋本构模型。采用等效三段阶梯函数描述黏结应力沿黏结长度的非线性分布规律,结合钢筋双线性应力-应变关系,推导了锚固钢筋滑移和平均滑移应变的理论计算式。锚固钢筋的总应变定义为理想黏结的钢筋应变与平均滑移应变之和。分别通过屈服和极限应力对应的钢筋总应变修正弹性模量和硬化模量,进而得到间接考虑钢筋滑移的简化分段线性应力-应变关系。非锚固钢筋的滑移采用降低屈服应力和屈服应变的方法加以考虑。对于钢筋的滞回规则,采用Sakayi等修正的Menegotto-Pinto模型。将本构模型通过子程序嵌入ABAQUS软件中,基于纤维梁单元,对往复荷载作用下钢筋混凝土柱的非线性响应进行了数值模拟,并与试验结果对比,验证了该模型的有效性。结果表明,所建议的钢筋本构模型,数学表达形式简单且计算效率高,能够模拟地震作用下钢筋混凝土柱的承载力和变形特性,达到满足工程需要的精度。     

双钢板-混凝土组合剪力墙变形能力分析

基于纤维法和精细材料本构模型,编制了双钢板-混凝土组合剪力墙的截面分析程序,并通过已有试验验证了分析结果的准确性。通过对6379个不同参数的双钢板-混凝土组合剪力墙的计算分析,得到了影响剪力墙截面变形能力的主要因素有考虑钢板作用的轴压比、混凝土强度、墙身的材料配比和暗柱混凝土的约束效应。研究结果表明：回归分析得到的双钢板-混凝土组合剪力墙截面极限曲率的计算公式满足精度要求并偏于安全;在此基础上,提出了双钢板-混凝土组合剪力墙基于位移的设计方法。同时,根据国内规范的设计方法,给出了双钢板-混凝土组合剪力墙的轴压比限值：对于C50以下的混凝土,双钢板-混凝土组合剪力墙的设计轴压比的限值为0.45;对于C50以上的混凝土,设计轴压比的限值可建议按n_2d,lim=-0.004f_cu+0.625计算。     

考虑应变率效应的钢筋混凝土动态纤维梁单元模型

为准确描述钢筋混凝土结构的非线性地震灾变过程，建立了一种基于显式算法可考虑地震作用应变率效应的钢筋混凝土动态纤维梁单元模型，并以材料子程序(VUMAT)的形式嵌入ABAQUS有限元分析平台中。对混凝土、钢筋以及钢筋混凝土柱动态加载试验进行了数值模拟，测试并验证该模型用于钢筋混凝土材料及构件动态性能分析的准确性和有效性。研究结果表明，所提出的钢筋混凝土动态纤维梁单元模型计算精度较高，能够准确并实时反映地震作用下应变率对材料及构件动态性能的影响，可为精确计算钢筋混凝土结构在地震作用下的非线性动力反应提供理论基础。     

Innovative hybrid reinforcement constituting conventional longitudinal steel and FRP stirrups for improved seismic strength and ductility of RC structures

The use of fiber reinforced polymer (FRP) reinforcement is becoming increasingly attractive in construction of new structures. However, the inherent linear elastic behavior of FRP materials up to rupture is considered as a major drawback under seismic attacks when significant material inelasticity is required to dissipate the input energy through hysteretic cycles. Besides, cost considerations, including FRP material and construction of pre-fabricated FRP configurations, especially for stirrups, and probable damage to epoxy coated fibers when transported to the field are noticeable issues. The current research has proposed a novel economical hybrid reinforcement scheme for the next generation of infrastructures implementing on-site fabricated FRP stirrups comprised of FRP sheets. The hybrid reinforcement consists of conventional longitudinal steel reinforcement and FRP stirrups. The key feature of the proposed hybrid reinforcement is the enhanced strength and ductility owing to the considerable confining pressure provided by the FRP stirrups to the longitudinal steel reinforcement and core concrete. Reinforced concrete beam specimens and beamcolumn joint specimens were tested implementing the proposed hybrid reinforcement. The proposed hybrid reinforcement, when compared with conventional steel stirrups, is found to have higher strength, stiffness, and energy dissipation. Design methods, structural behavior, and applicability of the proposed hybrid reinforcement are discussed in detail in this paper.     

基于纤维梁模型的火灾下多层混凝土框架非线性分析

为分析和模拟多层混凝土框架结构在火灾下的反应规律及其破坏过程,基于建筑结构分析中常用的纤维梁单元,建立了钢筋混凝土梁、柱构件的火灾破坏数值模型。模型将构件截面划分成多个纤维,可以考虑构件截面的不均匀温度场分布以及材料非线性和几何非线性问题。对单层单跨混凝土框架进行火灾反应分析,并与试验结果进行比较,验证了此数值模型的准确性。通过对多层框架进行火灾反应模拟,比较不同火灾场景的模拟结果,分析其反应规律以及破坏过程。结果表明,纤维梁单元模型可以较好地模拟钢筋混凝土结构的受火破坏过程,并且火灾发生的位置不同,结构的破坏机制也不同,一定条件下蔓延的火灾比不蔓延的火灾对多层混凝土框架结构的破坏性更大。分析结果可以为实际结构的防火设计提供参考。