Dynamic analysis and response of semirigid frames

The dynamic response of semirigid frames is studied using a computer model. The flexibilities of the connections are modelled by rotational springs with bilinear moment-rotation relationships. Geometrical nonlinearities in the form of member (P-δ) and frame (P-Δ) instabilities are incorporated in the model through the use of stability functions in the formulation of the frame stiffness matrix. Material nonlinearity in the form of column inelasticity is accounted for by the use of the tangent modulus concept in the formulation of the column stiffness relationship. Response characteristics of semirigid frames under free vibration, forced vibration and earthquake excitation are studied. Response spectra are generated for frames subjected to forced vibrations and earthquake excitations. The presence of connection flexibility and P-delta effects tends to reduce frame stiffness, and hence increase the frames' natural periods of vibration. Semirigidity and P-delta effects also affect the magnitude of base shear and the amount of energy dissipation of semirigid frames. The extent of these effects is frequency dependent. Based on the results of this study, implications for semirigid frame design will be discussed.     

Upper and lower bounds for structural design of RC members with ductile response

In the present paper, some of the complex phenomena characterizing the flexural behaviour of reinforced concrete beams, such as hyper-strength and snap-back and snap-through instabilities, are interpreted under a unified approach based on nonlinear fracture mechanics concepts. In particular, they are analysed by means of a numerical algorithm adopting the cohesive crack model for concrete in tension and the overlapping crack model for concrete in compression. According to the latter constitutive law, a fictitious interpenetration is assumed to describe the concrete damage, analogously to the fictitious crack opening used for tension.Such an integrated cohesive-overlapping crack model is applied to assess the minimum reinforcement amount necessary to prevent unstable tensile crack propagation and to evaluate the rotational capacity of plastic hinges. The main novelty is given by the capability to predict the size-scale effects evidenced by several experimental programmes available in the literature. According to the numerical results obtained, new practical design formulae and diagrams are proposed, as well as, upper and lower bounds to the reinforcement amount, the material properties and the structural dimensions are defined in order to avoid brittle failures.     

不同FRP增强混凝土梁断裂性能试验研究

为了探讨不同种类纤维增强复合材料（FRP）增强带裂缝混凝土的断裂性能,开展了芳纶纤维增强复合材料（AFRP）、碳纤维增强复合材料（CFRP）和玻璃纤维增强复合材料（GFRP）增强带裂缝混凝土梁的三点弯曲试验,分析了其断裂性能参数.结果表明：相对于普通混凝土梁试件,FRP对带裂缝混凝土梁的阻裂加固效果更明显;CFRP增强混凝土梁的起裂荷载和失稳荷载均大于AFRP与GFRP增强混凝土梁,CFRP的阻裂增强效果最佳;AFRP增强混凝土梁和CFRP增强混凝土梁的破坏形式均为试件底部混凝土 FRP界面的剥离破坏,GFRP增强混凝土梁的破坏形式为试件底部GFRP的拉断破坏;通过对不同FRP增强混凝土梁阻裂加固机理的分析,计算得出CFRP增强混凝土梁的起裂韧度和失稳韧度最大,且CFRP价格适中,因此使用CFRP对带裂缝混凝土梁进行增强加固的性价比最优.     

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。     

FRP筋混凝土梁挠度与裂缝宽度计算模型分析

收集国内外218组纤维增强复合材料(FRP)筋混凝土梁受弯性能试验结果,重点研究了正常使用阶段和极限状态下梁挠度和裂缝宽度的发展规律,基于试验数据修正了FRP筋应变不均匀系数ψ与强度配筋影响系数的关系,建立了该类梁受弯挠度计算模型;深入分析了FRP筋混凝土梁受弯截面内力臂系数η、构件受力特征系数α_cr对构件裂缝宽度计算精度的影响,确定η,α_cr的合理取值,优化计算模型,并与规范GB 50608—2010建议模型计算值进行对比分析。结果表明:根据建立的ψ与强度配筋影响系数关系,选取η=0.88,α_cr=1.5得到的建议模型能够较好预测FRP筋混凝土梁受弯挠度和裂缝宽度,比规范计算结果吻合好,验证了建议模型的准确性和合理性。     

Deflection and crack-width prediction of concrete beams reinforced with glass FRP rods

Concrete beams reinforced with fiber reinforced polymer (FRP) bars exhibit large deflections and crack widths as compared to concrete beams reinforced with steel due to the low modulus of elasticity of FRP. Current design methods for predicting deflections at service load and crack widths developed in concrete structures reinforced with steel bars may not be used for concrete structures reinforced with FRP bars. Thus, the ACI 440 Committee has provided design guidelines for concrete beams reinforced with FRP bars. Verification of the ACI 440 methods for predicting deflections and crack widths for glass fiber reinforced polymer reinforced concrete beams are presented in this paper. In addition, improvement to the crack width equation was suggested to account for 2 layers of reinforcement. This study shows that ACI 440.1R-01 can be effectively used to predict deflections in concrete beams reinforced with FRP bars and crack width in beams with one-layer FRP bars. However, when FRP bars are placed in two layers, ACI 440.1R-01 can be used after some parameters are modified. Six full concrete beams reinforced with different GFRP reinforcement ratios were load tested and the measured deflections and crack widths were analyzed and compared with those predicted by the proposed models. The experimental results compared well with those proposed by the model.     

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.     

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.     

锈裂混凝土裂缝形态的试验研究

钢筋混凝土结构中 ,钢筋锈蚀产生体积膨胀 ,对外围混凝土产生径向压应力 ,导致混凝土开裂。在锈蚀混凝土构件的评估中 ,裂缝的分布形态和裂缝宽度是最明显和最直观的评估依据。试验室中 ,通过对混凝土试件的预留孔进行机械扩孔 ,模拟钢筋锈蚀膨胀引起混凝土开裂的全过程。机械扩孔试验用膨胀螺栓和水压机对 94个试件进行了不同条件下的扩孔 ,来模拟钢筋锈蚀在构件表面引起的应力分布及裂缝分布形态。分析混凝土保护层开裂趋势、裂缝扩展方式、裂缝分布形态与混凝土强度、钢筋直径、保护层厚度、钢筋位置、浇筑方式之间的关系 ,最后用有限元对一典型试件进行数值模拟计算。     

钢筋混凝土锈蚀损伤研究综述

钢筋锈蚀是钢筋混凝土结构耐久性降低的主要因素之一。简要说明研究钢筋混凝土锈蚀损伤的重要意义,阐述了钢筋混凝土锈蚀件损伤的研究状况。主要从混凝土中锈蚀钢筋的力学性能、钢筋锈蚀引起的混凝土损伤、锈蚀钢筋与混凝土间粘结性能和锈蚀钢筋混凝土受弯构件、受压构件的受力性能、锈蚀钢筋混凝土构件抗震性能等方面总结归纳了国内外的研究现状与成果,并分析了今后的研究方向。     

Innovative Konstruktionsmöglichkeiten mit Stahlfaserbeton: Optimierter Stahlfaserbeton zum Nachweis der Rißbreitenbegrenzung eines kombinationsbewehrten Regenrückhaltebeckens

Innovative construction possibilities with steelfibre reinforced concrete Optimized steelfibre reinforced concrete for proof of crack with limitation of a combined reinforced rain storage reservoir To optimize the performance of steelfibre reinforced concrete in combination with conventional reinforced concrete, it is necessary to test selected steelfibres in a converted concrete. The optimization of Steelfibres and concrete is the presupposition to get innovative, technical formidable solutions and economic advantages. With a strong reinforced rain storage reservoir the advantages are getting clear. The steel bar reinforcement is reduced and the durability is increased.     

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。     

玄武岩纤维加筋梁抗弯性能试验研究

制作12根混凝土简支梁进行三分点静力加载直至破坏的试验,研究不同配筋率的钢筋和玄武岩纤维BFRP加筋混凝土简支梁的性能。试验结果表明：BFRP加筋混凝土梁荷载-挠度曲线近似为直线,但在施荷后期依然表现较好的延性;开裂荷载和极限荷载并没有得到明显提高;裂缝宽度可以控制在2.5 mm以内,裂缝间距随着配筋率的增加而减小。建...     

高强钢筋高强混凝土梁静力和疲劳性能试验研究

通过9根配有新Ⅲ级钢的高强混凝土梁和4根配有Ⅱ级钢的混凝土梁的静载和等幅疲劳荷载试验,分析研究了高 强混凝土梁的变形性能和疲劳特性。试验结果表明,在高强混凝土梁中应用高强钢筋,可以使两者的性能得以充分发挥, 不仅承载力大幅度提高,而且能较好的满足正常使用极限状态的要求。高强混凝土受弯构件在疲劳荷载作用下刚度降低, 裂缝宽度增大,其变化规律和受压区混凝土应变的增加规律基本一致。疲劳荷载作用N次后构件的裂缝宽度,可根据初 始裂缝宽度和受压区混凝土应变增长系数来计算。根据试验分析,得到了高强混凝土梁在疲劳荷载作用下的截面应力、裂 缝宽度及高强钢筋S-N曲线试验回归公式,可为高强混凝土梁设计提供一定的参考。     

基于不确定性的氯离子侵蚀环境下普通混凝土结构的耐久性研究

针对氯盐侵蚀环境下普通混凝土结构的锈胀开裂,通过分析普通混凝土结构的腐蚀损伤发展特点,建立氯离子侵蚀环境下普通混凝土结构的耐久性失效准则。基于工程实例,采用不确定性分析方法,通过Monte Carlo随机模拟,对氯离子侵蚀环境下普通混凝土结构的锈胀开裂时间开展了可靠性分析研究,得出了其近似概率密度分布,通过假设检验,确定其服从对数正态分布,并给出其在不同保证率情况下锈胀开裂时间的取值。     

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.     

Torsional response of fibrous reinforced concrete members: Effect of single type of reinforcement

Studies on the torsional behaviour of reinforced concrete members in general and fiber reinforced members in particular are of interest to several researchers as torsional rigidity, torsional stiffness and torsional toughness of the members play a vital role in the analysis of structures subjected to Seismic loads and Wind loads. In the present paper an investigation of the behaviour of steel fiber reinforced concrete members having single type of reinforcement, viz., longitudinal reinforcement or transverse reinforcement, has been presented. Torsion tests on 10 reinforced SFRC members revealed that single type of reinforcement, either longitudinal or hoop reinforcement, can not improve the torsional strength of the member beyond the torsional strength of the plain member without any reinforcement. However, single type reinforcement improved the ductility of the member. Steel fibers improved the cracking torque of the members noticeably.     

玄武岩-粗聚丙烯纤维钢筋混凝土管力学性能

针对实际工程中钢筋混凝土管节管壁易产生裂纹、影响工程质量问题,鉴于玄武岩纤维与粗聚丙烯纤维能够明显改善混凝土的抗拉、抗裂等力学性能,设计制作了一组普通钢筋混凝土管节B0P0和一组混掺玄武岩-粗聚丙烯纤维钢筋混凝土管节B2P4,进行室内三点试验,对比试验过程中两组管节的开裂破坏形态和荷载 位移曲线,并利用ABAQUS软件建立钢筋混凝土管节数值分析模型,研究不同纤维配比和钢筋配置对钢筋混凝土管节受力性能的影响规律。研究结果表明:相较于B0P0,混掺玄武岩-粗聚丙烯纤维钢筋混凝土管节B2P4承载力提升了30.19%,管节表面裂缝宽度明显减小,阻裂增韧效果好;数值模型与试验结果的误差在5%以内,能够合理预测管节的破坏过程、荷载 位移曲线和极限承载能力,并通过数值模拟确定了混掺玄武岩 粗聚丙烯纤维时的纤维最佳配比和合理钢筋配置。     

Experimental and theoretical analysis of the moment-rotation behaviour of stiffened extended end-plate connections

A new theoretical model to evaluate the moment-rotation (M-?) relationship for stiffened and extended steel beam-column end-plate connections has been derived in this paper. Based on a specific definition of the end-plate connection rotation, the end-plate connection is decomposed into several components, including the panel zone, bolt, end-plate and column flange. The complete loading-deformation process of each component is then analysed. Finally the loading-deformation process for the whole connection is obtained by superimposing the behaviour of each component. In addition, 5 joint tests have been conducted to verify the proposed analytical model. By comparing it with the test results, it has been concluded that this analytical model can evaluate the rotational behaviour of end-plate connections, as well as the moment-rotation (M-?) curve and the initial rotational stiffness accurately. Furthermore, it can analyse every contribution to the joint’s rotational deformation, such as the shear deformation of the panel zone, the bolt extension, the bending deformation of the end-plate and column flange, etc. This analytical model also provides moment-shear rotation (M-?_s) and moment-gap rotation () curves, which establish a reliable foundation for analysing the detailed rotational behaviour of end-plate connections.     

Cracking resistance performance of super vertical-distance pumped SFRC

The mix ratio of steel fiber reinforced concrete (SFRC) was optimized using the principles that workability must meet the pumping demand and anti-cracking performance should be optimal. The effect of SFRC on the initial cracking load, the ultimate load and the crack width of the reinforced concrete (RC) member were analyzed in this paper. It was found that the admixture had good preservation of moisture and adhesion and the fibers distributed homogeneously in one hour out of the machine. According to the pumping results, the SFRC could be pumped vertically up to 306 m. Based on the standard computation formula of cracks, the maximum crack width of an RC member with 0.8% steel fiber (by volume) is about 32% lower than that of standard RC member. Through an experimental research on full-scale model tests for the steel and concrete composite anchorage zone on a pylon, the SFRC not only remarkably increases the crack resistance and the ultimate load, but the initial load also improves 33%a pproximately. It is also indicated that plastic shrinkage cracking of SFRC in which volume fraction of steel fibers is 0.8% can be restrained obviously and the unrestrained drying shrinkage can be diminished by about 50% at early age. The results confirmed that the SFRC can lessen the shrinkage crack of concrete and enhance markedly the direct tensile strength. Therefore, the SFRC can solve the key question of crack resistance for the anchorage zone of a bridge tower. __________ Translated from Journal of Southeast University (Natural Science Edition), 2007, 37(1): 123–127 [译自: 东南大学学报(自然科学版)]