A new model for the analysis of composite steel – concrete slab and beam structures with deformable connection

 In this paper a solution to the bending problem of reinforced concrete slabs stiffened by steel beams with deformable connection including creep and shrinkage effect is presented. The adopted model takes into account the resulting inplane forces and deformations of the plate as well as the axial forces and deformations of the beam, due to combined response of the system. The analysis consists in isolating the beams from the plate by sections parallel to the lower outer surface of the plate. The forces at the interface producing lateral deflection and inplane deformation to the plate and lateral deflection and axial deformation to the beams are related with the interface slip through the shear connector stiffness. Any distribution of connectors along the interface and any linear or non-linear load–slip relationship or partial shear connection for the shear connectors can be handled. The creep and shrinkage effect relative with the time of the casting and the time of the loading of the plate is taken into account. The solution of the arising plate and beam problems, which are nonlinearly coupled, is achieved using the analog equation method (AEM). The adopted model compared with those ignoring the inplane forces and deformations, approaches more reliable the actual response of the plate–beams system. Moreover, it permits the evaluation of the shear forces at the interfaces, the knowledge of which is very important in the design of composite steel–concrete structures. Received: 21 June 2002 / Accepted: 5 March 2003     

Layer model for long-term deflection analysis of?cracked?reinforced concrete bending members

A numerical technique has been proposed for the long-term deformation analysis of reinforced concrete members subjected to a bending moment. The technique based on the layer approach in a simple and rational way deals with such complex issues as concrete cracking and tension-stiffening as well as creep and shrinkage. The approach uses the material stress-strain relationships for compressive concrete, cracked tensile concrete and steel. Such effects as linear and nonlinear creep, cracking, tension-stiffening as well as the reduction in concrete tension strength due to sustained loading have been taken into account. The shrinkage effect has been modeled by means of adequate actions of axial force and bending moment. A statistical deflection calculation analysis has been carried out for 322 experimental reinforced concrete beams reported in the literature. The comparative analysis of the experimental and the modeling results has shown that the proposed technique has well captured the time-deflection behavior of reinforced concrete flexural members. The results of the predictions by ACI 318 and Eurocode 2 design codes have been also discussed.     

Modelling creep and shrinkage of concrete by means of effective stresses

A novel model of mechanical performance of concrete at early ages and beyond, and in particular, evolution of its strength properties (aging) and deformations (shrinkage and creep strains), described in terms of effective stress is briefly presented. This model reproduces such? phenomena known from experiments like drying creep or some additional strains, as compared to pure shrinkage, which appear during autogenous deformations of a maturing, sealed concrete sample. Creep is described by means of the modified microprestress-solidification theory with some modifications to take into account the effects of temperature and relative humidity on concrete aging. Shrinkage strains are modelled by using effective stresses giving a good agreement with experimental data also for low values of relative humidity. Results of four numerical examples based on the real experimental tests are solved to validate the model. They demonstrate its possibilities to analyze both autogenous deformations in maturing concrete, and creep and shrinkage phenomena, including drying creep, in concrete elements of different age, sealed or drying, exposed to external load or without any load.     

Time-dependent behavior of RC beams strengthened with externally bonded FRP plates: interfacial stresses analysis

External bonding of fibre reinforced polymer (FRP) composites has becomes a popular technique for strengthening concrete structures all over the world. An important failure mode of such strengthened members is the debonding of the FRP plate from the concrete due to high interfacial stresses near the plate ends. For correctly installed FRP plate, failure will occur within the concrete. Accurate predictions of the interfacial stresses are prerequisite for designing against debonding failures. In particular, the interfacial stresses between a beam and soffit plate within the linear elastic range have been addressed by numerous analytical investigations. In this study, the time-dependent behavior of RC beams bonded with thin composite plate was investigated theoretically by including the effect of the adherend shear deformations. The time effects considered here are those that arise from shrinkage and creep deformations of the concrete. This paper presents an analytical model for the interfacial stresses between RC beam and a thin FRP plate bonded to its soffit. The influence of creep and shrinkage effect relative to the time of the casting and the time of the loading of the beams is taken into account. Numerical results from the present analysis are presented to illustrate the significance of time-dependent of adhesive stresses.     

基于徐变损伤理论的早龄期大体积混凝土化学-热-力多场耦合模型研究

受水化放热的影响,大体积混凝土在早龄期阶段涉及多个物理场作用,极易发生损伤、开裂等不利行为,会对结构服役期内的耐久性和安全性产生严重的影响。针对此问题,该文基于经典损伤理论框架,发展了一类适用于早龄期大体积混凝土的化学-热-力多场耦合模型,综合地反映了早龄期混凝土的开裂、徐变、温度变形、自收缩变形和龄期效应。通过将水化反应方程与热传导方程联立建立了化学-热场耦合作用模型。进而,基于弹塑性损伤理论框架搭建本构关系,引入考虑损伤影响的微观应力-固化理论以刻画混凝土的线性徐变和非线性徐变,根据温度和水化度的变化求解热膨胀变形和自收缩变形,并考虑了随龄期变化的混凝土力学性能的影响。结合相应的显式求解算法,将上述多场耦合模型应用于Maridal涵洞早龄期力学行为的模拟分析,并探究了混凝土徐变变形的影响。计算结果表明：该文模型可以实现对早龄期大体积混凝土开裂过程的准确模拟,对早龄期混凝土受力性能和开裂行为的研究具有一定的参考意义。     

Pickett effect and creep in flexure of steel‐fiber reinforced concrete

Abstract

Creep and shrinkage are of great concern in the design of steel fiber reinforced concrete structures. This is especially true for a prestressed flex‐ural member with thin section. The test results of creep of steel‐fiber rein‐foced concrete in flexure are presented. The concrete beams made with various fiber volume contents were tested in flexure under drying or standard moist conditions. The Pickett effect in steel‐fiber reinforced concrete was investigated. This research shows that fibers can effectively restrain the bending creep of concrete. The Pickett effect can be reduced with the addition of fibers to plain concrete beam subjected to fiexural loading.     

Bending and creep buckling response of viscoelastic functionally graded beam-columns

The viscoelastic creep response of flexural beams and beam-columns made with functionally graded materials is numerically investigated. The paper highlights the challenges associated with the modeling and analysis of such structures, and presents a nonlinear theoretical model for their bending and creep buckling analysis. The model accounts for the viscoelasticity of the materials using differential-type constitutive relations that are based on the linear Boltzmann’s principle of superposition. The model is general in terms of its ability to deal with any material volume faction distribution through the depth of the beam, and with different linear viscoelastic laws, boundary conditions, and loading schemes. The governing equations are solved through time stepping numerical integration, which yields an exponential algorithm following the expansion of the relaxation function into a Dirichlet series. A numerical study that examines the capabilities of the model and quantifies the creep response of functionally graded beam-columns is presented, with special focus on the stresses and strains redistribution over time and on the creep buckling response. The results show that the creep response of such structures can be strongly nonlinear due to the variation of the viscoelastic properties through the depth, along with unique phenomena that are not observed in homogenous structures.     

钢-混凝土组合梁收缩徐变分析的有限元方法

基于按龄期调整的有效模量法,提出了部分剪力连接钢-混凝土组合梁在长期荷载作用下收缩徐变分析的简化有限元模型,并通过建立特殊的剪力连接件单元刚度矩阵和利用Newton-Raphson迭代方法考虑滑移效应,同时考虑了负弯矩区混凝土板开裂对组合梁刚度和强度的影响。利用该模型计算了连续组合梁在长期荷载作用下的挠度、应力、滑移量,计算结果与已有的理论计算结果和实验结果吻合,证明本模型用于分析钢-混凝土组合梁收缩徐变是可靠的。     

钢筋混凝土梁长期变形的计算

基于受弯等效原则对混凝土徐变和收缩引起的截面曲率增大系数进行了分析,进而得到了钢筋混凝土梁的附加变形增大系数,计算公式形式简单、概念清楚、适用于以受弯荷载为主的钢筋混凝土梁的长期变形计算。与该文中3根试验梁和文献中31根试验梁的试验结果对比分析表明,建议方法计算结果与试验结果吻合较好,且优于文献其它方法。     

Interface stress redistribution in FRP-strengthened reinforced concrete beams using a three-parameter viscoelastic foundation model

External bonding of FRP plates or sheets has become a popular method for strengthening reinforced concrete structures. Stresses along the FRP–concrete interface are critical to the effectiveness of this technique because high stress concentration along the FRP–concrete interface can lead to the FRP debonding from the concrete beam. In this study, a novel analytical solution has been developed to predict the interface stress redistribution of FRP-strengthened reinforced concrete beams induced by the viscoelastic adhesive layer. Both the FRP plate and the RC beam are modeled as Timoshenko’s beams, connected through the adhesive layer. The adhesive layer is modeled as a three-parameter viscoelastic foundation (3PVF) using Standard Linear Solid model. The 3PVF model satisfies the equilibrium equation of the adhesive layer and the zero shear-stress boundary condition at the free edge. Closed-form expressions of the time-dependent interface stresses and deflection of the beam are obtained using Laplace transform. Finite element analysis is also conducted to verify the analytical solution using a subroutine UMAT based on the Standard Linear Solid model. Numerical results suggest that the stress concentrations within the FRP–concrete interface relax with time. The axial force in the FRP plate also reduces with time due to the creep of the adhesive layer. However, this relaxation is limited to a small zone close to the end of the FPR plate.     

预应力混凝土桥梁徐变分析的全量形式自动递进法

首先将预应力混凝土桥梁中考虑收缩、徐变影响的任意时刻混凝土应力、应变关系在持荷时段内写成代数形式,引入内力平衡方程及变形协调条件后,提出了计入截面上钢筋位置、配筋率、预应力钢筋松弛、混凝土弹性模量随时间变化等影响的徐变效应分析的全量形式自动递进法,并建立了计算式,适用于任何形式的收缩、徐变特性表达式;基于建立的全量形式公式,可方便地求解任意时刻混凝土、钢筋的应力与应变和梁体竖向变形。理论分析与试验结果比较表明,公式可方便地控制计算精度,直至给出满意的结果。将计算式编程后极易与目前桥梁设计中常用的杆系有限元软件接口,进行可靠的徐变分析。     

碳纤维布加固复合受力钢筋混凝土箱梁抗扭性能的非线性有限元分析

采用有限元方法对碳纤维布加固钢筋混凝土构件进行非线性分析,是对有限的试验研究的有效补充和进一步深入探讨。根据4根碳纤维布加固钢筋混凝土箱梁的试验研究结果,建立了合理的三维有限元模型,对碳纤维布加固钢筋混凝土箱梁在弯剪扭复合受力下的抗扭性能进行了非线性有限元分析。计算得到的扭矩-扭转角关系曲线、钢筋和碳纤维布的应变曲线以及界面粘接单元的恢复力曲线等与试验结果吻合较好,可以较好地模拟碳纤维布加固箱梁的受扭性能。进一步通过对7根数值梁的计算结果分析,提出碳纤维布加固钢筋混凝土箱梁在复合受力下的剪扭相关性符合直线方程。     

Early-age stress analysis of a concrete diaphragm wall through tensile creep modeling

This paper reports a recent large-scale experimental investigation on early-age stress evolution in a deep underground concrete diaphragm wall. To evaluate the early-age stress induced by hydration temperature rise, autogeneous shrinkage and reinforcement restraint, both laboratory tests and in situ large-scale model wall test are performed. The laboratory tests include concrete adiabatic temperature rise, autogeneous shrinkage and restraint test. The in situ model wall simulates continuous and sliding design options for the external and inner layers with thermal and strain sensors installed in the inner layer. The restraint test results are interpreted via tensile creep modeling and an algorithm is conceived to calibrate the concrete tensile creep law. With the identified creep law, a thermomechanical analysis is performed on the model wall to calculate the concrete temperature and stress evolution at early age. The identified tensile creep law is furthermore validated by the numerical results and in situ measurements. Furthermore, the early-age stress analysis is performed on the full-scale diaphragm wall. Comments on the concrete tensile creep law and the diaphragm wall design option are given in the end.     

Numerical model for composite structures with experimental confirmation

The paper briefly presents a numerical model for the simulation of composite structures. The main structure is modeled with two‐dimensional plane finite elements. The composite surface is modeled with two‐dimensional interface elements for the continuous connection simulation and modified beam elements for the discrete connection simulation. The applied material model’s primary purpose is the simulation of reinforced concrete structures. It includes the most important nonlinear effects of reinforced concrete behavior: yielding in compression and opening and propagation of cracks in tension, with tensile and shear stiffness of cracked concrete, as well as the nonlinear behavior of reinforced steel. It also includes nonlinear behavior of the composite surface and the connection elements. The model was confirmed in experimental tests of composite concrete Omnia slabs, which are in common usage. The achieved test results were compared with the results obtained through the developed numerical model.     

粘贴钢板加固钢筋混凝土梁的分离式有限元模型

提出了外粘钢板加固受弯钢筋混凝土梁的非线性有限元模型。该模型中采用了一种特殊的、具有剥离破坏功能的界面单元来模拟混凝土梁和外粘钢板之间的粘结层,这种剥离破坏主要发生在粘贴钢板端部区域和弯曲、剪切裂缝附近。影响这种剥离破坏的主要因素有两个:一是粘贴钢板的端部与加固梁支座距离;二是粘贴钢板的厚度。传统的梁理论不能描述这种加固梁破坏模式,采用有限元方法能全方位地描述这种加固梁的各种性状和破坏模式。数值计算结果与粘贴不同厚度钢板加固梁的试验结果相吻合。     

无粘结预应力高性能粉煤灰混凝土桥梁收缩与徐变变形试验研究

结合洛湛线的建设,以8根不同掺量的高性能粉煤灰混凝土无粘结预应力桥梁的收缩、徐变试验为基础,研究了不同掺量高性能粉煤灰混凝土在荷载长期作用下收缩、徐变性能及其上拱随时间的变化规律,探讨了温度、湿度等环境因素对不同掺量高性能粉煤灰混凝土收缩、徐变的影响。300多天的实验观测结果表明:高性能粉煤灰掺量20%～40%混凝土梁不但具有良好的工作性能和力学性能,而且长期性能良好,与同强度的未掺高性能粉煤灰的梁相比,其后期强度和抗压弹性模量增大,收缩徐变减小,具有良好的社会和经济效益。     

Thermohygrometric and mechanical behaviour of concrete using damage models

A numerical formulation to analyse thermohygrometrically and mechanically coupled fields in concrete is presented. One of the main applications concerns the creep and shrinkage of concrete, wherein the transient hygrothermal effects are related to the mechanical behaviour and take account of geometrical non-linearities and anisotropic damage coupled with viscosity. The stress-strain relationship is represented by an integral equation. This procedure is implemented in a finite element code in order to analyse 2-D and 3-D structures of any shape, and the results regarding the numerical simulation of the creep and shrinkage in a concrete cylinder are presented.     

Time-dependent behaviour of widened reinforced concrete under-bridge

The paper reports on data acquired during 700 days of monitoring a widened reinforced concrete underbridge. The deck was monitored before, during and after widening and both portions of the deck, existing and new, were instrumented. Concrete strains were monitored by means of embedment and surface Vibrating Wire strain Gauges (VWG). Predicted creep, shrinkage, thermal and settlement strains were evaluated by means of a finite element analysis. The creep and shrinkage factors used were those from the simplified form of the B3 model [1]. Good agreement between calculated and measured longterm strains was observed at almost all gauge positions. From the analysis carried out, it was found that the stresses from the time dependent phenomena of creep, shrinkage and temperature, combined with those from the settlement of abutments, were greater than the magnitude of stresses obtained from the design live load.     

型钢混凝土结构ANSYS数值模拟技术研究

采用ANSYS程序对6个型钢混凝土梁试件的受力性能进行非线性有限元数值分析,对型钢混凝土结构数值模拟中混凝土和钢材材料模型定义、有限元建模、钢筋单元生成及后处理等关键技术进行系统研究。着重对型钢混凝土粘结滑移性能的数值模拟技术进行了研究。采用由ANSYS程序单元库中非线性弹簧单元combination-39组成的三维连接单元模拟型钢混凝土在不同部位及不同方向上的界面相互作用,建议了非线性弹簧单元粘结力-滑移曲线与型钢混凝土粘结滑移本构关系的转换技术,并提出了生成非线性弹簧单元的实用方法。最终形成考虑粘结滑移的型钢混凝土数值模拟技术。型钢混凝土梁数值模拟结果与试验结果吻合较好,表明所建立型钢混凝土结构ANSYS数值模拟技术合理、可行,可适用于基于ANSYS程序的型钢混凝土结构有限元数值模拟和受力性能深入研究。     

Structural response of hyperstatic concrete beams reinforced with GFRP bars: Effect of increasing concrete confinement

The design of concrete structures reinforced with glass fibre reinforced polymer (GFRP) bars is influenced by their reduced stiffness and brittleness. In hyperstatic structures, the methodology used in force analysis depends on the ductility of the structural systems, which in this case, being essentially provided by the concrete, can be potentially increased by confining concrete in critical zones. This paper presents experimental and numerical investigations about the flexural behaviour of continuous beams reinforced with GFRP bars, namely of their service and failure responses, and the effect of increasing concrete confinement in critical cross-sections. A calculation procedure to quantify the confinement effect in beams due to the reduction of the spacing between shear stirrups is first presented. The experimental investigations comprised a comparative study in which two-span concrete beams reinforced with either GFRP or steel bars were tested in bending. In the former, the effect of reducing the shear stirrups spacing was analyzed together with the under- and over-reinforcement at the central support and midspan cross-sections, respectively. The development of a crack hinge in the continuity support zone highlighted the better performance of beams under-reinforced on the top layer with GFRP bars compared to “equivalent” beams reinforced with steel, namely at the resistance level. In addition, the confinement at critical zones increased significantly the strength and ductility. The numerical investigations included the development of non-linear finite element models for all beams tested - numerical results are in good agreement with test data and seem to confirm the confinement effect observed in the experiments.