Querkraftwiderstand von Stahlbetonbauteilen ohne Querkraftbewehrung

Shear Strength of Reinforced Concrete Members without Transverse Reinforcement A new theoretical model concerning the shear strength of reinforced concrete members without transverse reinforcement is presented, considering free‐bodies unilaterally bounded by a fictitious crack, inclined at π/4 to the tension chord and extending from the tension chord to the compression chord. It is assumed that the shear stress that can be transferred across a crack decreases linearly with the crack width and that the crack width at the level of the tension chord is proportional to the product of the tension chord strain at the crack and the chord distance. This results in a linear‐hyperbolic relationship between the shear strength and the (elastic) tension chord force at the crack, allowing to determine the governing crack location and other relevant quantities via equilibrium considerations. Due to potential strut or arch action for the transfer of loads applied in the vicinity of supports the crack location is somewhat restricted; it is assumed that the corresponding length is equal to twice the chord distance.     

Model for post-yield tension stiffening and rebar rupture in concrete members

A tension stiffening model is presented which enables the calculation of average tensile stresses in concrete, after yielding of reinforcement, in reinforced concrete elements subjected to uniaxial tension, shear or flexure. To determine the average tensile stress-strain relationship for concrete, a crack analysis approach is employed taking into account the bond mechanism between concrete and deformed reinforcing bars, and numerical analyses are conducted to determine the tensile behavior of reinforced concrete members including post-yield response. Analytical parametric studies are conducted to determine the influence of various parameters including concrete compressive strength and reinforcement yield strength, ultimate strength, hardening stress, and hardening strain. Analysis results obtained from the proposed model, when compared to experimental results for uniaxial members, indicate good agreement for structural behavior after yielding of reinforcement. The proposed model makes it possible to accurately calculate reinforcement stresses at crack locations and, thus, average strain conditions which result in rupture of reinforcement. This leads to more realistic predictions of the uniaxial, flexural, and shear ductility of reinforced concrete members.     

Load-slip relationship of tension reinforcement in reinforced concrete members

An accurate evaluation of the moment-rotation relationship of reinforced concrete members at both serviceability and ultimate limit states is a very important aspect as rotation has a significant contribution to the deflection of the member and also has a direct impact on the magnified moment, the ability to absorb energy and the redistribution of moments. The rotation in the un-cracked or homogenous parts of a reinforced concrete member can be determined by integrating the curvature using standard procedures. However, in the cracked or non-homogenous regions, rotations are found to have sudden or discrete changes at each crack between their crack faces. This can be quantified by the crack opening produced by the slip Δ between the reinforcement and the concrete at the crack face induced by the force in the reinforcing bar P. In this paper, closed form solutions are derived for the P-Δ relationships which are applicable to any type and shape of reinforcement. It is then shown how these closed form solutions can be conveniently used to derive the moment-rotation relationship at a crack.     

Modeling of Tension Stiffening Behavior in FRP‐Strengthened RC Members Based on Rigid Body Spring Networks

Abstract: Allowing for the tension stiffening effects resulting from the bond between steel reinforcement and surrounding concrete leads to effective deformation analysis of reinforced concrete (RC) members when using a nonlinear finite element analysis modeled on the smeared crack concept. Nowadays, externally bonded fiber reinforced polymer (FRP) composites are widely used for strengthening existing RC structures. However, it remains unclear to what extent the tension stiffening of postcracking concrete is quantitatively influenced by the addition of FRP composites, as a result of the bond between the FRP and the concrete substrate. This article presents a discrete model, which is based on rigid body spring networks (RBSN), for investigating the tension stiffening behavior of concrete in FRP‐strengthened RC tensile members. A two‐parameter fracture energy‐based model was deployed to represent the bond‐slip behavior of the FRP‐to‐concrete interface. The reliability of the RBSN model was verified through comparisons with previous test results. Further parametric analysis indicates that the tension stiffening of concrete is hardly influenced by the addition of FRP composites before the yield of steel reinforcement has occurred although concrete crack patterns and crack widths may be influenced by the bond‐slip behavior of the FRP‐to‐concrete interface.     

与有粘结相协调的无粘结预应力混凝土梁刚度及裂缝宽度计算方法

由于无粘结筋相对于其周围混凝土可发生纵向相对滑动,因此受弯构件中的无粘结筋对构件抗弯刚度的贡献及其对裂缝开展的抑制作用小于有粘结预应力筋。为了实现无粘结与有粘结预应力混凝土受弯构件刚度及裂缝宽度计算方法的协调和统一,提出了受弯构件中无粘结筋等效折减系数的概念,通过对无粘结预应力混凝土受弯构件的变形试验数据和按已有公式对设计试件的变形试算数据进行分析,得出了无粘结筋等效折减系数的建议取值(α=0.23)。用等效纵向受拉钢筋配筋率代替有粘结预应力混凝土受弯构件刚度计算公式中纵向受拉钢筋配筋率,用等效纵向受拉钢筋面积代替用于计算有粘结预应力混凝土受弯构件裂缝宽度的纵向受拉钢筋等效应力计算公式中的纵向受拉钢筋面积,就可分别得到与有粘结相协调的无粘结预应力混凝土受弯构件刚度及裂缝宽度计算公式,两公式计算结果与试验结果均吻合良好,可用于工程设计。     

混凝土轴心受拉构件最小配筋率与抗裂度的协调

最小配筋率和裂缝宽度是混凝土结构设计中必须同时满足的验算项目。通过对一个工程实例中的轴拉构件进行分析,阐述了现行混凝土设计规范中最小配筋率建议值与抗裂验算的不协调现象。通过分析其原因,提出了与抗裂度相协调的最小配筋率建议值。该建议值既保证了轴拉构件不会出现一裂就断的脆性破坏,又保证了轴拉构件不会一裂就不满足裂缝宽度验算的要求,完成了最小配筋率与抗裂度的协调。     

Accurate cover crack modelling of reinforced concrete structures subjected to non-uniform corrosion

Abstract

Concrete cover cracking caused by reinforcement corrosion is a significant durability problem of reinforced concrete (RC) structures. Extensive research has been carried out in the last few decades while most were focused on corrosion of a single reinforcing bar. Very little research has examined the whole cover cracking of RC structures due to multiple reinforcement corrosion. This article develops a numerical model to predict the structural failure of the whole cover of concrete induced by corrosion of multiple reinforcing bars. Moreover, a non-uniform corrosion model is established based on experimental results, in contrast to conventional uniform assumption. Two typical cover failure modes under the non-uniform corrosion of multiple reinforcing bars are identified and discussed. The effects of cover thickness, reinforcement spacing, fracture energy of concrete, etc., on cover cracking patterns and crack width are also investigated. The derived numerical model is verified by comparing the results with those from experiments in literature. Accurate prediction of concrete cover cracking can allow timely maintenance of existing structures and rational design for new buildings which prolongs the service life of the RC structures.     

地下高压钢筋混凝土岔管渗水开裂三维数值分析计算

根据地下高压钢筋混凝土岔管受内水压力初砌开裂后的渗流场变化特征，提出了高压管道内水外渗的渗流场与应力场耦合的三给有限元数值分析方法。根据地下高压钢筋混凝土岔管衬砌开裂破坏规律，给出了高压管道受内水压开裂的裂缝宽度估计公式和混凝土衬砌结构的配筋计算方法。通过对实际工程的分析计算，论证了高压管道内水外渗的初砌结构配筋的影响，为地下高压管道的设计提供了一种有效的分析方法。     

沉井结构裂缝宽度限值的选取分析与设计

沉井被用作地下构筑物时,截面配筋往往由裂缝宽度验算确定,按承载力极限状态计算所需配筋小于满足裂缝控制要求所需配筋,特别是刃脚根部以上1.5倍厚度的井壁。对于钢筋混凝土构件的裂缝,有各种不同的裂缝计算理论及包含各种不同变量的、各种不同形式的裂缝计算公式,保护层厚度也有不同的规定。结合具体工程,依据不同的现行规范选取裂缝宽度限值,进行裂缝宽度验算。通过分析表明:根据《混凝土结构耐久性设计规范》进行沉井设计,能在满足耐久性要求条件下减少配筋量多达25%,对类似工程具有借鉴作用。     

Nonlinear seismic analyses of a high gravity dam with and without the presence of reinforcement

Comparative studies are conducted to investigate the seismic response of a 160 m high gravity dam with and without reinforcement. The numerical analyses account for several nonlinearities such as concrete cracking, tensile plastic offset strain of concrete, stiffness recovery when a crack closes and the bond–slip effect. The numerical results show that the stiffness recovery effect has a significant influence on the seismic behavior of the dam, for example the horizontal displacement history, residual deformation and cracking patterns. Although there is a slight difference in the dynamic deformation obtained with and without strengthening, the reinforcement causes secondary cracks in the vicinity of the major one and decreases the maximum opening and extension of the major crack. The results also show that the reinforcement is beneficial in improving the seismic-resistant capacity of the gravity dam.     

Zur Mindestquerkraftbewehrung von Stahlbetonbalken

On minimum shear reinforcement amounts for reinforced concrete beams A ductile shear bearing behaviour of reinforced concrete beams is recently ensured by providing standardized amounts of minimum shear reinforcement in compliance with established detailing provisions. However, lower than minimum shear reinforcement amounts do not necessarily go along with a brittle mode of failure in all cases. Dependent on the individual layout and amount of stirrups, the contributions of other bearing capacities and the crack formation itself, the initial cracking force might be overtaken even along with further load gains and reasonable limited shear crack widths. Especially the latter one holds potential to rate the failure mode – brittle or ductile – on site. Experiments on lightly shear reinforced single span concrete beams are evaluated and rated with respect to minimum shear reinforcement amounts required. Thereby, special interest is set on the load‐deformation response obeying the critical shear crack and the structural design. Finally, the overall shear resistance as well as the stirrups' efficiency in the effective shear domain are analysed to derive suggestions for future practical application on‐site.     

Application of a visco-elastic tension-softening constitutive model to cracked and ageing concrete

The development of a non-linear visco-elastic tension-softening constitutive model for cracked and ageing concrete is outlined. It combines a micro-mechanical model for tension softening with a Kelvin chain rheological model. The ageing phenomenon is included through the concept of consolidation theory and the introduction of an ageing function—the same for all Kelvin chain units. The constitutive model is then used with the fictitious crack model for studying the creep-induced sub-critical growth of a crack in a large concrete dam. The rate of crack growth is determined in order to calculate the rupture lifetime.     

碾压混凝土拱坝诱导缝开裂的等效应力准则研究

通过多组不同削弱度的深埋椭圆裂缝、深埋矩形裂缝、穿透裂缝三种形式的碾压混凝土试件单轴拉伸试验结果,建立深埋矩形裂缝碾压混凝土试件的应力强度因子的近似解析表达式,考虑尺寸效应得到碾压混凝土材料的断裂韧度。根据碾压混凝土拱坝诱导缝构造特点,分别将矩形诱导缝简化为修正穿透裂缝模型和修正深埋椭圆裂缝模型,假定为I型开裂,考虑相邻裂缝的影响,采用虚拟裂缝模型和线弹性断裂力学相结合的方法建立裂缝失稳扩展时的亚临界扩展量和等效断裂韧度的解析表达式,应用双K断裂准则,建立碾压混凝土拱坝诱导缝开裂的等效应力准则。并将此结果应用到三维非线性应力场仿真计算中,对沙牌碾压混凝土拱坝进行非线性开裂计算分析。     

Experimental evaluation of the tension stiffening behavior of HSC thick panels

The experimental investigation presented in this paper summarizes the test results of nineteen reinforced concrete panels. This research is focused on providing a clear understating for the tension stiffening response of thick high strength concrete (HSC) and normal strength concrete panels used for containment structures for nuclear power plants and offshore structures. Major variables include the concrete’s strength, reinforcement spacing in the concrete section, thickness of the concrete cover, and applying the axial load in axial and/or biaxial directions. The contribution of the concrete’s matrix between cracks is obtained by considering the difference between the bare bar and panel response.The average tension stiffening contribution of HSC is found to be higher during the crack formation and stabilized cracking stages, compared with NSC panels. Concrete tension stiffening contribution between cracks for thick HSC panels is dependent on the spacing of the reinforcement in the concrete matrix. Applying the load in two directions causes an obvious decrease in tension stiffening response, compared with panels subjected to axial loading. A relevant constitutive model to simulate the tension stiffening response for HSC panels is recommended.     

GFRP筋活性粉末混凝土梁受力性能试验研究

为了研究GFRP筋活性粉末混凝土梁的受力性能,对8根梁进行三分点加载试验,获得了试验梁的开裂弯矩、极限弯矩以及各级荷载作用下的变形及裂缝分布与开展。试验结果表明：活性粉末混凝土试验梁纯弯区段开裂应变 (750×10-6) 约为普通混凝土梁的7倍,开裂弯矩及截面塑性系数计算应考虑纵向受拉GFRP筋的有利影响。GFRP筋活性粉末混凝土梁正截面受弯破坏形式可分为纵向受拉GFRP筋被拉断而受压边缘活性粉末混凝土未被压碎的受拉破坏,受压边缘活性粉末混凝土被压碎(5500×10-6)而纵向受拉GFRP筋未被拉断的受压破坏,以及纵向受拉GFRP筋被拉断的同时受压边缘活性粉末混凝土被压碎的界限破坏等三种。对于受压破坏可按拉区应力为0.25倍活性粉末混凝土抗拉强度来考虑拉应力对正截面受弯承载力的贡献。对于受拉破坏则基于材料应力-应变关系通过数值积分迭代计算正截面受弯承载力。刚度及裂缝宽度计算的关键是合理计算使用阶段GFRP筋的拉应力,在计算GFRP筋拉应力时所用弯矩应为外荷载弯矩减去拉区活性粉末混凝土拉应力合力对压区合力点的弯矩。图9表12参10     

Ultimate strength of large scale reinforced concrete thin shell structures

This paper presents ultimate behavior of large scale reinforced concrete shell structures: hyperbolic cooling tower shell and hyperbolic paraboloid (HP) saddle shell. Both geometrical and material nonlinearities were considered in the analysis. To investigate the influence of concrete tension cracks on the structural behavior, the smeared crack model having the capability of representing double crack and crack rotation was used in the analysis. The biaxial stress state in shells is represented by the improved work-hardening plasticity concrete model, where the ductility increase phenomenon can be depicted. The load-displacement relationship, stress fields, occurrence and propagation of cracks in concrete and steel yield patterns are presented. Due to the factors such as modification in plasticity model, stiffness contribution in the doubly cracked elements, the model predicts a more ductile behavior than the results reported in the current literature. The failure of cooling tower shell seems to occur due to local yielding of meridional reinforcement in the windward meridian. In the case of HP saddle shell, structural instability occurs due to severe tension cracks in the shell part before yielding of reinforcement could occur.     

结合实例探讨钢筋屈服后混凝土楼板裂缝宽度的数值方法

本文首先以双向板塑性绞线法为背景,通过计算极限荷载判断楼板纵筋屈服,采用数值模拟的办法研究了钢筋屈服后混凝土楼板的裂缝宽度。这种方法基于Houde黏结滑移理论,假定模型条件和加设COMBINE39三维非线性弹簧单元模拟出钢筋与混凝土之间的黏结滑移,滑移产生的单元间相对位移差即为裂缝宽度。同时以工程实测裂缝宽度为依据,检验数值模拟裂缝宽度的可行性,为探究钢筋屈服后混凝土楼板裂缝宽度提供了实际参考价值。     

Theoretical analysis of post‐peak flexural behaviour of normal‐ and high‐strength concrete beams

A new method of analysing the post‐peak flexural behaviour of reinforced concrete beams has been developed and applied to normal‐ and high‐strength concrete beams. It was revealed that at the post‐peak stage the neutral axis depth keeps on increasing, and at a certain point the strain in the tension reinforcement starts to decrease, even though the curvature is increasing monotonically. Such strain reversal in the tension reinforcement occurs in all concrete beams and has significant effects on the post‐peak behaviour and flexural ductility of concrete beams. Therefore, the stress path dependence of the tension reinforcement needs to be taken into account in the analysis. By means of a parametric study, the variation of ultimate concrete strain with tension steel ratio and the effects of various structural parameters on flexural ductility have been studied. Based on the numerical results, design values of ultimate concrete strain that are independent of tension steel ratio have been recommended and a simple formula for predicting the flexural ductility of reinforced normal‐ and high‐strength concrete beams has been developed. Copyright © 2003 John Wiley & Sons, Ltd.     

浅谈现浇板裂缝补强处理措施

根据现场裂缝的宽度，介绍了裂缝的不同补强措施，阐述了灌浆前裂缝的处理方法，论述了不同状况封缝的具体方法，总结了灌浆补强处理的要点，经过实践补强应用，证明这些方法是有效的，现浇板也满足了规范要求。     

Predicting the response of continuous RC deep beams under varying levels of differential settlement

This paper investigates the effect of differential support settlement on shear strength and behavior of continuous reinforced concrete (RC) deep beams. A total of twenty three-dimensional nonlinear finite element models were developed taking into account various constitutive laws for concrete material in compression (crushing) and tension (cracking), steel plasticity (i.e., yielding and strain hardening), bond-slip at the concrete and steel reinforcement interface as well as unique behavior of spring-like support elements. These models are first validated by comparing numerical predictions in terms of load-deflection response, crack propagation, reaction distribution, and failure mode against that of measured experimental data reported in literature. Once the developed models were successfully validated, a parametric study was designed and performed. This parametric study examined number of critical parameters such as ratio and spacing of the longitudinal and vertical reinforcement, compressive and tensile strength of concrete, as well as degree (stiffness) and location of support stiffness to induce varying levels of differential settlement. This study also aims at presenting a numerical approach using finite element simulation, supplemented with coherent assumptions, such that engineers, practitioners, and researchers can carry out simple, but yet effective and realistic analysis of RC structural members undergoing differential settlements due to variety of load actions.