钢纤维混凝土多轴损伤比强度准则

细聚丙烯(PP,直径小于100 μm)纤维混凝土I型断裂强度随纤维含量增加呈现先增后减的变化规律,其机理尚不明晰。而且由于纤维数量过多,现有数值方法无法精细刻画细纤维混凝土力学行为。对此通过考虑细PP纤维亲水性提升基体局部水灰比的强度正效应和纤维桥接力强度贡献低于砂浆的强度负效应,并引入纤维直径等效系数(r_f),实现介观尺度细聚丙烯纤维混凝土(PFRC)力学行为模拟。研究表明：纤维直径等效系数(r_f)建议小于10;当纤维微量添加时,细PP纤维提升基体局部水灰比,使基体强度上升,此时正效应高于负效应;随着纤维含量增加,由于纤维桥接力贡献无法补偿相应面积基体强度,负效应持续增长,PFRC宏观I型断裂强度下降。

     

Finite element analysis of partially prestressed steel fiber concrete beams in shear

This paper presents a finite element formulation for the modeling of the behavior of partially prestressed steel fiber concrete beams in shear. Based on a secant modulus approach, the formulation treats steel fiber concrete as an orthotropic material, characterized by appropriate constitutive relations in the principal compressive and tensile directions. An experimental program with the partial prestressing ratio, the shear span:effective depth ratio, and the volume fraction of steel fibers as test variables was carried out and the deflections of the beam, concrete, and steel strains were monitored and compared with the results of the finite element analysis. The finite element formulation was found to predict the deformational characteristics and the ultimate load of the test beams well. Steel fibres were observed to improve the beam stiffness after the occurrence of first shear crack and to enhance the shear strength of partially prestressed concrete beams significantly.     

Shear strength of reinforced aerated concrete beams with shear reinforcement

The paper reports on the analysis of shear strength of reinforced beams made of autoclaved aerated concrete with shear reinforcement. The test data are taken from three different investigations from three countries, in Europe and Japan, and include 61 tests. The analysis of the test data results in regression expressions, suitably modified from a formula used for ordinary concrete members, and shows good agreement with test values. Appropriate expressions are suggested for design.

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Résumé Ce rapport présente l'analyse de la résistance au cisaillement de poutres en béton cellulaire armées (AAC), avec armatures d'effort tranchant. Les résultats d'essais sont pris parmi trois recherches différentes dans trois pays, en Europe et au Japon, et ils regroupent 61 essais. La charge dans la majorité des essais consistait en deux forces symétriques (voir Fig. 1). Dans quelques cas, il y avait une charge en ligne à mi-portée; dans d'autres cas, une charge en ligne était appliquée près d'un appui. Aucun essai n'a été exécuté avec une charge uniformément répartie. Une formule du type de l'équation 1 prenant en compte, les trois variables principales a été utilisée depuis longtemps dans le cas du béton armé sans armature d'effort tranchant. A partir de son développement théorique, cette formule a été adaptée par le passé au béton cellulaire d'ou l'équation 2. Pour les poutres avec armatures d'effort tranchant, l'équation 3 représente l'addition de la résistance au cisaillement due au béton à celle due aux armatures d'effort tranchant. Les résultats d'essais sont présentés dans le Tableau 2, y compris les résistances au cisaillement prédites à partir de l'équation 3 avec les coefficients de régression calculés en utilisant le programme SYSTAT sur un ordinateur personnel. Les résultats des régressions sont donnés dans le Tableau 4 et l'expression finale dans l'équation 4. Dans l'optique du dimensionnement, des valeurs minimales de la résistance ultime au cisaillement avec un seuil de confiance de 90% sont présentées dans l'équation 5. L'analyse rapportée dans cet article fournit des expressions fiables pour la prédiction de la résistance au cisaillement de poutres en béton cellulaire armées avec armatures d'effort tranchant.

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Prestressed fiber reinforced concrete beams with reduced ratios of shear reinforcement

This paper presents an analysis of the influence of prestress and fibers on the shear behaviour of thin-walled I-section beams with reduced shear reinforcement ratio. Reduction of shear reinforcement in prestressed precast beams can make the reinforcement simpler and may increase the productivity in long line precasting beds. The use of short fibers can improve the shear strength and ductility. Nine concrete beams were built (six with prestressing forces) with three different mixtures: without fibers, with steel fibers, and with polypropylene fibers. Shear reinforcement ratios varied from 0 to 0.225% (geometric ratio). It was noted that prestressing increases cracking strength (both in bending and shear), extends the non-cracked area, and makes the compression struts less inclined. In the case of fiber reinforced concrete beams, control of cracking is more effective and consequently deflections are smaller. Ductility is also increased. Both fibers and prestressing reduce stresses in the stirrups and increase shear strength.     

预应力连续箱梁桥的基准动力有限元模型研究z

介绍了作为预应力混凝土连续梁桥的通扬运河特大桥的环境激励振动试验,并利用基     

The effect of traditional reinforcement – prestressed reinforcement ratio on the behaviour of concrete beams

Firstly, the effect of traditional reinforcement – prestressed reinforcement ratio on the behaviour of concrete beams up to failure was experimentally investigated. The beams were 10 m long and 0.5 m high, with different ratios of traditional and prestressed reinforcement. The total quantity of reinforcement in each beam was selected to provide their equal ultimate bending bearing capacity. Deflections, stresses in concrete, traditional and prestressed reinforcement, as well as concrete cracks, were monitored until the beams failure. Using the previously developed numerical model of authors of this paper for static analysis of spatial frame structures, which can simulate main nonlinear effects of their behaviour, then numerical analysis of tested beams was performed. Good agreement was obtained between the experimental and the numerical results, which confirms the possibility of practical application of the adopted numerical model. Main conclusions and recommendations for practical applications according to results of performed tests are given at the end.     

Tests of composite concrete beams with prestressed planks

This paper deals with tests on composite concrete beams with prestressed concrete planks. It describes the separate influences of differential shrinkage and creep as well as stress redistribution due to a great strain difference at the adjoining faces between the prestressed plank and added concrete. Existing formulas relating to differential shrinkage and creep are based on the actual strains to be expected, but in order for the designer to have this information, extensive tests are required. This paper contains qualitative data in this regard and describes means by which information can be satisfactorily obtained, including the effect of stress redistribution.     

Nonlinear finite element modeling of concrete deep beams with openings strengthened with externally-bonded composites

This paper aims to develop 3D nonlinear finite element (FE) models for reinforced concrete (RC) deep beams containing web openings and strengthened in shear with carbon fiber reinforced polymer (CFRP) composite sheets. The web openings interrupted the natural load path either fully or partially. The FE models adopted realistic materials constitutive laws that account for the nonlinear behavior of materials. In the FE models, solid elements for concrete, multi-layer shell elements for CFRP and link elements for steel reinforcement were used to simulate the physical models. Special interface elements were implemented in the FE models to simulate the interfacial bond behavior between the concrete and CFRP composites. A comparison between the FE results and experimental data published in the literature demonstrated the validity of the computational models in capturing the structural response for both unstrengthened and CFRP-strengthened deep beams with openings. The developed FE models can serve as a numerical platform for performance prediction of RC deep beams with openings strengthened in shear with CFRP composites.     

Stability of sulphur concrete beams with steel reinforcement

Tests are reported on the structural behaviour of unmodified and modified sulphur concrete beams with steel reinforcement and subjected to air and water curing. The sulphur concrete contained fly ash as a filler and both dicyclopentadiene and dipentene were used as modifiers. The tests were conducted at three months and one year. Unmodified concrete beams showed enhanced strength, stiffness and ductility with ageing when cured in a dry environment. Modified concrete beams showed improved behaviour compared to unmodified beams, but even when dry, they showed strength loss with ageing and their long term stability is open to question. Ageing in a wet environment has a destructive effect on the structural behaviour of all sulphur concrete beams. Loss of strength and softening of sulphur concrete lead to shear failures, and severe corrosion and cracking occur. Stability of sulphur concrete beams can be guaranteed only when they are unmodified and kept dry. In a wet regime sulphur concrete cannot have stability and durability.

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Résumé On rend compte d’essais du comportement structural de poutres de béton de soufre avec armatures d’acier, les unes traitées les autres non, conservées dans l’air et dans l’eau. Des cendres volantes ont été utilisées comme charge et les dicyclopentadiène et dipentène comme agents modificateurs. Les essais ont été conduits sur 3 mois et 1 an. On a constaté une amélioration des propriétés de résistance, de rigidité et de ductilité pour des poutres de béton non traitées dans le vieillissement en atmosphère sèche. Le comportement des poutres de béton traitées a été supérieur à celui des poutres non traitées, mais, même en milieu sec, elles ont accusé une perte de résistance avec le vieillissement et on peut s’interroger sur leur stabilité à long terme. Le vieillissement en milieu humide a un effet destructif sur le comportement structural de toutes les poutres de béton de soufre. La perte de résistance et l’adoucissement du béton soufré conduisent à la rupture par cisaillement; d’autre part une corrosion et une fissuration importantes se produisent. On ne peut garantir la stabilité des poutres de béton de soufre que lorsqu’elles sont non traitées et conservées à sec. En régime humide, le béton de soufre ne peut être ni stable ni durable.

     

Fracture stability and residual strength assessment of reinforced concrete beams

In conventional analysis and design procedures of reinforced concrete structures, the ability of concrete to resist tension is neglected. Under cyclic loading, the tension-softening behavior of concrete influences its residual strength and subsequent crack propagation. The stability and the residual strength of a cracked reinforced concrete member under fatigue loading, depends on a number of factors such as, reinforcement ratio, specimen size, grade of concrete, fracture properties, and on the tension-softening behavior of concrete. In this work, a method is proposed to assess the residual strength of reinforced concrete beams subjected to cyclic loading. The crack extension resistance based approach is used for determining the condition for unstable crack propagation. The effect of reinforcement is modeled as a closing force counteracting the effect of crack opening produced by the external moment. The effect of percentage reinforcement and specimen size on the failure of reinforced beams is studied. Finally, the residual strength of the beams are computed by including the softening behavior of concrete.     

A finite element for 3-D prestressed cablenets

A finite element method is presented for the analysis of prestressed cablenets. The method is based on representing the prestressed cablenet as a series of finite length curved elements. The large displacement formulation used, enables the evaluation of the static and dynamic response of 3-D cablenets. The development is general and the mathematical basis is explained at length. It is shown by means of comparison functions that for absolute continuity at nodes, a cubic displacement field is sufficient for the prediction of the first frequency of shallow nets. For globally deep networks the accuracy can be increased by employing a quintic displacement field for the normal component of displacement. The application of the proposed model to cable structures of shallow and also of deep global geometry, is presented. A variety of edge boundary shapes are employed in order to illustrate the versatility of the large displacement formulation. In all the example problems studied, gravity load has been taken as the initial load condition in calculating the equilibrium configuration. The stiffness and consistent mass matrices associated with the equation of motion are derived using Hamilton's variational principle.     

Nonlinear finite element analysis of reinforced concrete beams strengthened by fiber-reinforced plastics

Numerical analyses are performed using the ABAQUS finite element program to predict the ultimate loading capacity of rectangular reinforced concrete beams strengthened by fiber-reinforced plastics applied at the bottom or on both sides of these beams. Nonlinear material behavior, as it relates to steel reinforcing bars, plain concrete, and fiber-reinforced plastics is simulated using appropriate constitutive models. The influences of fiber orientation, beam length and reinforcement ratios on the ultimate strength of the beams are investigated. It has been shown that the use of fiber-reinforced plastics can significantly increase the stiffnesses as well as the ultimate strengths of reinforced concrete beams. In addition, with the same fiber-reinforced plastics layer numbers, the ultimate strengths of beams strengthened by fiber-reinforced plastics at the bottom of the beams are much higher than those strengthened by fiber-reinforced plastics on both sides of the beams.     

Analytical approach for the flexural analysis of RC beams strengthened with prestressed CFRP

The objective of this paper is to propose a simplified analytical approach to predict the flexural behavior of simply supported reinforced-concrete (RC) beams flexurally strengthened with prestressed carbon fiber reinforced polymer (CFRP) reinforcements using either externally bonded reinforcing (EBR) or near surface mounted (NSM) techniques. This design methodology also considers the ultimate flexural capacity of NSM CFRP strengthened beams when concrete cover delamination is the governing failure mode. A moment–curvature (M–χ) relationship formed by three linear branches corresponding to the precracking, postcracking, and postyielding stages is established by considering the four critical M–χ points that characterize the flexural behavior of CFRP strengthened beams. Two additional M–χ points, namely, concrete decompression and steel decompression, are also defined to assess the initial effects of the prestress force applied by the FRP reinforcement. The mid-span deflection of the beams is predicted based on the curvature approach, assuming a linear curvature variation between the critical points along the beam length. The good predictive performance of the analytical model is appraised by simulating the force–deflection response registered in experimental programs composed of RC beams strengthened with prestressed NSM CFRP reinforcements.     

A new method for modelling reinforcement and bond in finite element analyses of reinforced concrete

A new method of representing the steei in finite element analyses of reinforced concrete structures is described in which the steel and the concrete are analysed separately. The forces between the steel and concrete are used In an interative method which brings the two solutions together. The method converges very rapidly and it is shown that the computational effort is principally dependent on the mesh chosen for the concrete. All the steel in a reinforced concrete structure may be easily included without affecting the layout of the concrete mesh. Results, which are given for three practical problems, include detailed steel and bond stresses for all bars.     

A C1 finite element for flexural and torsional analysis of rectangular piezoelectric laminated/sandwich composite beams

This work deals with the development of a new C¹ finite element for analysing the bending and torsional behaviour of rectangular piezoelectric laminated/sandwich composite beams. The formulation includes transverse shear, warping due to torsion, and elastic–electric coupling effects. It also accounts for the inter-layer continuity condition at the interfaces between layers, and the boundary conditions at the upper and lower surfaces of the beam. The shear strain is represented by a cosine function of a higher order in nature and thus avoiding shear correction factors. The warping function obtained from a three-dimensional elasticity solution is incorporated in the present model. An exact integration is employed in evaluating various energy terms due to the application of field consistency approach while interpolating the transverse shear and torsional strains. The variation of the electric potential through the thickness is taken care of in the formulation based on the observation of three-dimensional solution. The performance of the laminated piezoelectric element is tested comparing with analytical results as well as with the reference solutions evaluated using three-dimensional finite element procedure. A detailed study is conducted to highlight the influence of length-to-thickness ratio on the displacements, stresses and electric potential field of piezoelectric laminated beam structures subjected to flexural and torsional loadings. Copyright © 2004 John Wiley & Sons, Ltd.     

Evaluation of size dependent design shear strength of reinforced concrete beams without web reinforcement

Analytical studies on the effect of depth of beam and several parameters on the shear strength of reinforced concrete beams are reported. A large data base available has been segregated and a nonlinear regression analysis (NLRA) has been performed for developing the refined design models for both, the cracking and the ultimate shear strengths of reinforced concrete (RC) beams without web reinforcement. The shear strength of RC beams is size dependent, which needs to be evaluated and incorporated in the appropriate size effect models. The proposed models are functions of compressive strength of concrete, percentage of flexural reinforcement and depth of beam. The structural brittleness of large size beams seems to be severe compared with highly ductile small size beams at a given quantity of flexural reinforcement. The proposed models have been validated with the existing popular models as well as with the design code provisions.     

混合配筋钢纤维增强混凝土梁受弯承载力试验及理论计算

为研究玻璃纤维增强聚合物复合材料(GFRP)筋与普通钢筋混合配筋钢纤维增强混凝土(SF/混凝土)梁的受弯性能及其受弯承载力计算方法,在考虑受拉区混凝土抗拉强度的基础上,给出混合配筋SF/混凝土梁的界限配筋率及受弯承载力计算公式;在此基础上设计制作了三种配筋方式的SF/混凝土梁,重点探讨了混合配筋率及筋材面积比(A_f/A_s)对试验梁失效模式和受弯承载力的影响;同时,借助已有相关试验结果,对比分析了混凝土强度对混合配筋SF/混凝土梁受弯性能的影响。试验和对比分析结果表明:混合配筋SF/混凝土梁正截面应变仍符合平截面假定;相同配筋形式下,混合配筋SF/混凝土梁的受弯承载力和跨中挠度随筋材面积比A_f/A_s的增加而增大;单层配筋梁的受弯承载力比双层配筋梁大;合理提高混凝土强度可在充分发挥GFRP筋抗拉作用的同时进一步提高混合配筋SF/混凝土梁的受弯承载力;采用本文给出的界限配筋率公式能有效预测混合配筋SF/混凝土梁的失效模式;梁受弯承载力建议公式的预测值与试验值吻合较好,具有良好的适用性。      

Compressive-force path and behaviour of prestressed concrete beams

A study of the performance of prestressed concrete beams designed either by conventional design methods or to a physical model proposed in compliance with the ‘compressive-force path’ concept is presented. Results obtained from testing heavily prestressed beams are reported, which show that members designed to the latter concept may be safer than their Code counterparts. The behaviour of these beams has been compared with similar prestressed concrete beams (also designed either to current code provisions or to the proposed method)— but subjected to a smaller amount of prestressing—in an effort to monitor the effect of the level of prestressing on the behaviour of members designed by different methods. In contrast to the current way of thinking, it emerges that an additional amount of prestressing, instead of increasing the shear contribution of concrete, may, in fact, limit the load-bearing capacity of the member itself.     

Shear strength of reinforced concrete beams with stirrups

This study presents alternative shear strength prediction equations for reinforced concrete (RC) beams with stirrups. The shear strength is composed of the contribution of the nominal shear strength provided by stirrups and the nominal shear strength provided by concrete. For the concrete contribution, cracking shear strength values estimated by Arslan’s equations are almost same those obtained with ACI 318 simplified equation in terms of coefficient of variation (COV). However, mean values estimated by ACI 318 tend to be more conservative comparing to the mean values obtained with Arslan’s equations. Thus, for the consideration of concrete contribution to shear strength, Arslan’s equations are used. To obtain the shear strength of RC beams, shear strength provided by stirrups is added to the concrete shear strength estimated by Arslan’s equations. Results of existing 339 beam shear tests are used to investigate how accurate proposed equation estimates the shear strength of RC beams. Furthermore, ACI 318 and TS500 provisions are also compared to the aforementioned test results. It is found that proposed equations for beams with shear span to depth ratios (a/d) between 1.5 and 2.5 are also conservative with a lower COV than ACI 318 and TS500. However, when a/d ratios exceed 2.5 (both normal and high strength concrete beams), ACI 318, TS500 and proposed equations give similar COV value.