RC桥墩残余位移指标影响因素分析及贝叶斯估计

为揭示拟静力作用下钢筋混凝土(RC)桥墩残余位移指标参数影响规律,并给出变形极限状态残余位移指标贝叶斯估计。采用OpenSees软件建立RC桥墩拟静力试验构件模型,基于试验结果模拟进行模型验证。通过256个RC桥墩拟静力数值分析计算结果,研究残余位移指标统计特性及主要影响因素。在此基础上,采用线性回归分析方法,建立桥墩残余位移指标先验概率分布模型。应用贝叶斯理论,通过小样本试验数据更新,建立拟静力作用下残余位移指标后验概率分布模型。结果表明,RC桥墩残余位移指标主要影响因素为轴压比,且随轴压比的增大而减小。建立的后验概率分布模型能够准确估计残余位移指标,从而为RC桥墩抗震设计或评估中考虑残余位移随机性提供了便利。     

跨海桥梁高桩承台波浪冲击荷载概率模型

高桩承台是跨海桥梁常用的基础形式之一。但是高桩承台质量大、侧向刚度小，对波浪冲击荷载的水平分量十分敏感。在极端波浪作用下，高桩承台水平向的动力响应会明显大于静力方法算得的结构响应，严重威胁结构安全。传统波浪荷载计算方法无法充分考虑波浪冲击荷载的时变和随机特性。该文以跨海桥梁高桩承台为研究对象，开展了极端波浪冲击高桩承台尺缩模型的水槽试验。通过分析归一化后的波浪冲击荷载时程，建立了波浪冲击荷载时程模型；运用概率方法对波浪冲击荷载峰值与冲击上升段持续时间进行分析，给出了冲击荷载峰值和冲击上升段持续时间的边缘分布以及基于Copula的联合概率分布形式。     

水中桥塔波浪作用动力模型试验相似方法与模型设计方法

针对跨海桥梁桥塔结构波浪作用动力模型试验研究方法,通过分析水中结构动力模型试验相似原理,提出了桥塔结构动力模型试验的相似方法与模型设计方法。基于此,设计制作了一大型跨海斜拉桥桥塔结构的动力模型,并利用该试验模型进行了结构动力特性试验和波浪作用动力响应试验。动力特性试验结果表明:试验测得的结构主要自振频率与原型结构设计自振频率相接近,试验模型结构能较好地模拟原型结构自振特性。波浪作用下模型结构的基底受力和运动响应与原型结构相应的数值计算结果大致吻合。试验结果验证了提出的模型试验相似方法与模型设计方法的可行性,对跨海桥梁结构的水弹性模型试验研究具有应用和借鉴意义。     

风雨耦合环境结构荷载与响应的分析及试验研究

采用理论分析及试验研究方法对风雨共同作用下的结构荷载与响应进行了研究。首先在确定风雨联合作用概率模型的基础上通过对降雨作用特性的分析,比较了雨滴冲击作用模型和空气密度等效变化模型对建筑结构等效静力的作用性能;其次,基于高精度风雨耦合荷载环境模拟试验设备,进行了风雨定常气动力荷载模型高频天平测力试验,研究了简单几何形体的风雨定常荷载变化规律;然后,采用二维悬挂系统测振试验方法,研究了风雨非定常气动力荷载模型及其结构振动响应特点;最后,对缆索承重桥梁拉索风雨激振现象进行耦合荷载环境参数精细化控制试验,揭示了拉索空间姿态、拉索动力特性、来流风速和不同雨强组合等条件对拉索风雨振的影响规律。     

行波效应对深水连续刚构桥地震响应的影响

跨海或库区的大跨度桥梁在地震作用下不仅需要考虑水体与桥墩的动力相互作用,同时由于各桥墩间跨度较大应考虑地震输入的行波效应。该文采用辐射波浪理论求解桥墩地震动水压力,建立了考虑地震动输入空间效应的深水桥梁地震响应分析方法,并考虑行波效应对深水连续刚构桥进行地震响应分析。研究表明:动水压力增大了桥梁结构的地震响应,其影响程度随着输入地震波和墩梁约束条件的不同而有所差异;考虑行波效应时地震动水压力对桥梁结构动力响应的影响较一致激励而言有所差别,同时地震动水压力对桥梁地震响应的影响随着视波速的不同而变化。由此得出结论,为合理评价地震动水压力对深水长大桥梁动力响应的影响应考虑地震动输入的行波效应。     

地震作用下海冰与桥墩的间距对桥墩动力响应的影响研究

位于冰水海域的桥墩,在波浪力作用下,桥墩结构与周围海冰往往出现间隙,地震作用下海冰与结构的动力相互作用呈现复杂多样化。本文在弹性力学中的弹性波传播理论及结构动力学的基础上,考虑不同的海冰与桥墩间距,建立了地震作用下桥墩、冰与水体的流固耦合动力方程。并以一矩形桥墩为研究对象,研究了地震作用下,海冰与桥墩的间距对矩形桥墩结构的动力响应及动水压力的影响。研究表明:地震作用下,海冰与桥墩间距对桥墩顶部的最大位移和最大加速度响应的影响不显著,但是对桥墩侧面动水压力和墩底内力影响较大;当海冰与桥墩存在间距且小于1倍结构尺寸时,墩底的弯矩和剪力达到最大值。     

近海风力发电高塔波浪随机动力响应分析

基于拟层流风波生成机制建立的随机Fourier海浪模型,采用概率密度演化理论研究了近海风力发电高塔在随机波浪作用下的动力响应问题,给出了结构响应概率密度函数的时间演化过程、概率密度等值线图及其均值和标准差.其中随机波浪力由线性波浪理论和M0rison公式计算.结果表明,概率密度演化方法可以获得结构波浪动力响应的时变概率密度函数和等概率密度响应轨迹.据此计算的均值及标准差与Monte Carlo计算结果吻合较好.     

近、远场地震下深水桥墩动力响应特性对比研究

以某典型铁路深水桥梁等效单墩模型为对象,采用基于流固耦合理论的势流体计算方法,首次对近、远场地震作用下等效单墩结构振动特性及动力响应进行对比分析。结果表明,动水环境的存在会改变桥墩振动特性,随水深增加桥墩自振周期不断增大,30 m水深时第一阶周期增长率达10.4%。墩周动水压力呈抛物线型分布,近场地震下大于远场地震,二者差别随水深增加而增大。近、远场地震下桥墩结构的动力响应存在明显差别,较无水环境,近场地震下墩顶位移、墩底弯矩及剪力峰值分别增大34.5%、37.8%及51.3%;远场地震下三项指标分别增大17.0%、21.8%及40.0%,具有明显速度脉冲的近场地震下结构动力响应显著大于远场地震。具有速度脉冲的近场地震破坏能力更强,在近断层区深水桥梁抗震设计中应特别重视。     

正压冲固平台波浪和海流作用下动力响应分析

针对正压冲固平台在波浪和海流作用下动力响应问题，应用Morison方程和Stokes五阶波理论计算波浪和海流合力；基于弹性半空间理论并采用三次样条函数双向插值方法求解基础的动刚度和阻尼；在此基础上采用有限元方法计算该平台的动力响应。此外采用6倍桩径法计算了平台的动力响应，对比二者结果，说明本文研究的计算方法可靠。     

铁路重力式桥墩抗震计算简化模型研究

针对低配筋铁路重力式桥墩破坏时仅在墩底形成一条主裂缝,墩底混凝土压碎不明显,未形成塑性铰区,且动力作用下有明显摇摆现象的特征,提出了一种适用于该类桥墩的抗震计算简化模型,即在自由摇摆桥墩双弹簧模型的基础上再加上考虑纵向钢筋仅受拉的两个弹簧的四弹簧模型。依据模型桥墩试验结果确定了仅受拉弹簧刚度的参数取值;通过静力作用下和动力作用下的四弹簧模型计算值与试验值对比,验证了四弹簧抗震分析模型可用于铁路重力式桥墩的抗震简化计算。     

非规则高墩曲线桥梁振动台试验研究

据模型相似原理及多点激励理论,对缩尺比1:20的钢筋混凝土高墩曲线桥梁进行地震模拟振动台试验。研究高墩曲线桥梁结构在不同频谱地震波、不同峰值加速度及局部场地效应作用下高柔桥墩损伤模式及结构动力响应规律。结果表明,高柔桥墩损伤有明显的分布柔性特点,墩高差对桥墩裂缝开展及桥梁动力响应影响显著;不同频谱地震波作用下曲线桥梁结构动力响应规律各异;曲线桥梁结构动力响应峰值与地震波加速度峰值基本呈线性增长;局部地形效应使结构动力响应增大,导致曲线桥梁结构动力响应呈非线性增长;高墩曲线桥梁地震响应有明显空间性,首尾桥墩顶径向动力响应较大,易发生横桥向落梁破坏。     

深水桥梁墩水耦合振动试验研究与数值计算

为研究深水桥墩动力特性以及动水压力分布规律,开展了本次墩水耦合振动台试验。通过比较试验结果与ANSYS-CFX计算结果验证了试验数据的有效性。通过数值拟合得到了动水压力沿桥墩截面周边以及水深的变化规律。提出附加刚度法对有水桥墩弹性振动问题进行数值模拟,结果表明:附加刚度法计算结果与试验结果相符,且能较好的解释实验原理;桥墩基频会随着水深的增加而增大,桥墩的响应随着水深的增加而减小;附加刚度法能较好地模拟深水桥梁墩水耦合弹性振动行为。     

Rate effect of concrete strength under initial static loading

The dynamic strength enhancement of concrete under different initial static loadings is investigated in this paper. The mechanism of the influence of the inertia effect on the dynamic strength of concrete is discussed first by analyzing a single dimension of freedom system, which shows that the inertia effect only influences the dynamic loading part. The dynamic strength of concrete at initial static loading is calculated assuming a single crack model with consideration of free water viscosity using dynamic fracture mechanics. The dynamic stress intensity factor of the crack under linearly increasing loading on the basis of an initial static loading is achieved. The relationship between the dynamic strength increase factor and the static stress level is obtained. The comparison between the results by the model proposed in this paper and those by experiments in some available references indicates a good agreement.     

Influence of loading rate on concrete cone failure

Three different effects control the influence of the loading rate on structural response: creep of bulk material, rate dependency of growing microcracks and structural inertia. The first effect is important only at extremely slow loading rates whereas the second and third effects dominate at higher loading rates. In the present paper, a rate sensitive model, which is based on the energy activation theory of bond rupture, and its implementation into the microplane model for concrete are discussed. It is first demonstrated that the model realistically predicts the influence of the loading rate on the uniaxial compressive behaviour of concrete. The rate sensitive microplane model is then applied in a 3D finite element analysis of the pull-out of headed stud anchors from a concrete block. In the study, the influence of the loading rate on the pull-out capacity and on the size effect is investigated. To investigate the importance of the rate of the growing microcracks and the influence of structural inertia, static and dynamic analyses were carried out. The results show that with an increase of the loading rate the pull-out resistance increases. For moderate loading rates, the rate of the microcrack growth controls the structural response and the results of static and dynamic analysis are similar. For very higher loading rates, however, the structural inertia dominates. The influence of structural inertia increases with the increase of the embedment depth. It is shown that for moderately high-loading rates the size effect becomes stronger when the loading rate increases. However, for very high-loading rate the size effect on the nominal pull-out strength vanishes and the nominal resistance increases with an increase of the embedment depth. This is due to the effect of structural inertia.     

Finite element implementation of virtual internal bond model for simulating crack behavior

The virtual internal bond (VIB) model has been recently proposed to describe material deformation and failure under both static and dynamic loading. The model is based on the incorporation of a cohesive type law in a hyperelastic framework, and is capable of fracture simulation as a part of the constitutive formulation. However, with an implicit integration scheme, difficulties are often encountered in the finite element implementation of the VIB model due to possible negative eigenvalues of the stiffness matrix. This paper describes the implementation of an explicit integration scheme of the VIB model. Issues pertaining to the implementation, such as mesh size and shape dependence, loading rate dependence, crack initiation and growth characteristics, and solution time are examined. Both quasi-static and dynamic loading cases have been studied. The experimental validation of the VIB model has been done by calibrating the model parameters using the experimental data of Andrews and Kim [Mech. Mater. 29 (1988) 161]. The simulations using the VIB model are shown to agree well with the experimental observations.     

A probabilistic static fatigue model for transverse cracking in CFRP cross-ply laminates

This paper presents a delayed-fracture model for transverse cracking in CFRP cross-ply laminates under static fatigue loading. First, a delayed-fracture model for a crack in a brittle material was established on the basis of the slow crack growth (SCG) concept in conjunction with a probabilistic fracture model using the three-parameter Weibull distribution. Second, the above probabilistic SCG model was applied to transverse cracking in cross-ply laminates under static fatigue loading. The stress and the length of the unit element in the transverse layers were calculated with the aid of a shear-lag analysis, taking the residual stress into account. The transverse crack density was expressed as a function of applied stress and time with the parameters in the Paris law and the Weibull distribution function specified, in addition to the mechanical and geometrical properties. Unknown parameters were determined from experiment data gathered in static tensile and static fatigue tests. The reproduced transverse crack density at various applied loads agreed well with the experiment results.     

Probabilistic fatigue crack growth analysis of spot‐welded joints

This paper presents a probabilistic fatigue crack growth life prediction methodology for spot‐welded joints under variable amplitude loading history. The loading is multi‐axial and is obtained from transient response analysis of a vehicle model using finite‐element analysis. A three‐dimensional (3D) finite element model of a simplified joint with four spot welds is developed, and the static stress analysis of this joint is performed. Then the fatigue crack inside the base material sheet is modelled as a surface crack. Probabilistic crack growth model is combined with the stress analysis result to develop a probabilistic fatigue crack growth life prediction methodology for spot welds. This new method is implemented with MSC/NASTRAN and MSC/FATIGUE and is useful for the reliability assessment of spot‐welded joints against fatigue crack growth.     

Bonded particle modeling of grain size effect on tensile and compressive strengths of rock under static and dynamic loading

The effect of grain (particle) size on the strength is an interesting subject in the rock engineering. Some investigations about the impact of particle size on static strength of rock have been conducted and reported in the literature. However, this issue has not received enough attention when high loading rates are involved. In this work, by utilizing the CA3 bonded particle - finite element computer program, the combined influence of loading rate and particle size on the compressive and tensile strengths of rock is examined. The bonded particle model is used to simulate the crack initiation and failure of the rock specimen and the finite element is utilized to model the elastic bars in the Split Hopkinson Pressure Bar (SHPB) apparatus employed for the dynamic testing. Specimens with four different particle sizes were prepared. The results suggest that the particle size does not affect the rock strength under static and dynamic loading. However, the particle size modifies the nominal tensile strength of the notched Brazilian specimens. For the intact Brazilian specimens under high stress rates, the particle size contributes to the tensile strength and this contribution can be justified based on the principles of fracture mechanics. The theoretical reason for these observations is derived for a 3D bonded particle system and discussed.