An efficient approach for traffic load modelling of long span bridges

Traffic micro-simulation is the newly developed approach for loading calculation of long span bridges. The approach is quite precise, but computationally expensive to consider the full extent of traffic loading scenarios during a bridge lifetime. To address this shortfall, an efficient multi-scale traffic modelling approach is proposed. The proposed approach uses micro- and macro-simulation with different load model varieties (LMVs), or fidelities (levels of detail) of traffic loading in different bridge regions, to achieve optimal computation efficiency while maintaining the precision of loading calculation. Metrics of influence line (IL) characteristics, such as degree of nonlinearity, are proposed to evaluate the appropriateness of the choice of LMV, and standards of the metrics are also investigated to quantify the implementation of LMVs on bridge IL regions in the multi-scale modelling. Finally, two typical ILs are used along with random traffic modelling to study the feasibility of the proposed approach. It is shown that the multi-scale modelling approach proposed here achieves high computational efficiency and accuracy, which is significant for the massive traffic load simulation for lifetime bridge load effect analysis.     

Multi-scale fatigue damage prognosis for long-span steel bridges under vehicle loading

Fatigue damage prognosis for long-span steel bridges is of the utmost importance in bridge maintenance and management. In this study, a multi-scale fatigue damage prognosis algorithm is developed to calculate the trans-scale fatigue damage accumulation of newly-built long-span steel bridges under vehicle loading. The necessity and procedure of establishing a multi-scale finite element (FE) model of a newly-built long-span bridge for fatigue damage prognosis are first introduced. The future vehicle loading on the bridge is forecasted using the recorded weigh-in-motion (WIM) data and the agent-based traffic flow micro-simulation method. Then, the multi-scale fatigue damage prognosis algorithm is developed based on the multi-scale FE model and using the future vehicle loading. Finally, the proposed algorithm is applied to a newly-built long-span cable-stayed bridge for the time period from 2010 to 2020. The results show that the macro-scale fatigue damage accumulation and micro-scale short crack evolution of the critical components of the bridge can be simultaneously predicted and visualized. The proposed algorithm can be used as a numerical tool for fatigue damage prognosis of steel bridges where (or near where) WIM station is installed.     

长跨桥动力响应和局部破坏的多级数值分析

提出了长跨桥的结构动力响应和局部破坏的多级数值分析方法。首次研究了长跨桥有限元模型的多级数值分析的策略。基于设计图纸拓展了Tsing Ma桥的多级数值模型以及对此模型的策略。提出了动力响应和局部破坏的多级数值分析过程,连接焊缝处的局部退化过程能够解释破坏的主要作用和对结构响应的非线性影响。基于ABAQUS软件和改进程序,通过便于大型土木建筑应用的用户界面,实现了多等级计算。在将该方法应用于Tsing Ma桥之前,计算程序的精度通过两个典型实例进行验证。基于安装在Tsing Ma桥上的结构健康监测系统所获得的在线监测数据,在正常荷载下,采用拓展模型和计算程序对Tsing Ma桥的整体动力响应和桥的纵向桁架中两个典型焊接细部的局部损伤累积进行了数值分析。累积损伤的最大值出现在上部焊缝的脚部和/或斜杆与节点板相交的下弦杆处。全连接计算所得的疲劳寿命小于非全连接时的疲劳寿命,这说明在用多级数值计算方法分析结构损伤时应该考虑损伤与结构响应间明显的交互作用。     

焊接空间结构节点焊缝拉裂损伤识别

针对焊接空间结构节点易发生损伤和破坏的问题,为了识别焊接空间结构节点焊缝拉裂损伤,建立了节点焊缝拉裂损伤等效模型,考虑到焊接空间结构具有杆件和节点众多的特点,提出了损伤识别两步法。首先建立有焊缝拉裂损伤焊接球节点三维实体有限元模型,划分焊缝裂缝宽度,然后分析不同焊缝裂缝宽度与节点刚度系数之间的关系,最后将所获得的不同裂缝尺寸焊接球节点刚度系数等效为弹簧刚度,施加于结构杆件两端,成为杆端弹簧约束模型,即考虑节点焊缝拉裂损伤的等效有限元模型; 建立等效模型后,采用两步法识别节点焊缝拉裂损伤位置,先对结构分区,用小波分析方法识别结构杆件有节点焊缝拉裂损伤发生的区域,然后基于等效模型提取应变模态作为损伤标示量,在损伤区域内具体定位有节点焊缝拉裂损伤发生的杆件。结果表明:焊缝拉裂损伤等效模型能够模拟实际结构节点焊缝拉裂损伤,所建立的节点焊缝损伤等效模型易于提取损伤标示量,采用两步法能够识别出焊接空间结构节点焊缝拉裂损伤位置。     

某航站楼在爆炸作用下抗倒塌性能研究

为了避免抗倒塌性能分析时确定关键构件的盲目性,考虑相邻构件损伤的影响,评估爆损构件残余承载力对结构抗倒塌性能的影响,首先根据航站楼结构的特点与可能遭遇爆炸的危险程度,合理确定大跨度钢结构抗爆关键构件的可能部位。通过对爆炸当量、爆炸距离与结构构件损伤程度关系的研究,确定关键构件及其相邻构件的损伤情况。分别采用移除构件法与多尺度模型对破坏严重的构件进行模拟,考察航站楼结构在爆炸作用下的抗倒塌性能。分析结果表明,箱形钢柱的损伤程度主要与爆心距离和爆炸当量有关,迎爆面的变形远大于背爆面,柱顶斜撑杆的损伤程度与爆炸冲击波的入射夹角关系密切。采用移除构件法进行大跨度结构抗倒塌性能分析时,屋盖的局部变形很大,结构发生局部严重破坏,分析结果偏于保守。通过多尺度模型,可以采用壳单元较为准确地模拟构件在爆炸作用下的损伤情况。由于大跨度的竖向构件具有较大的冗余度,爆损后仍然具有一定的残余承载力,可以有效抑制变形的发展,采用多尺度模型得到的屋盖变形远小于移除构件法的分析结果。     

岩石微观胶结模型及离散元数值仿真方法初探

 基于胶结铝棒接触力学特性的系列室内微观试验,提出用于模拟岩石的微观胶结接触模型,并将该模型引入二维离散元商业软件PFC2D,模拟岩石室内单双轴压缩、直接拉伸和巴西试验,并将数值试验结果与室内试验实测结果进行对比分析,验证该数值方法在反映岩石基本力学特性方面的有效性,研究结果表明,基于水泥微观胶结模型的离散单元法能够合理地描述岩石的主要力学特性;通过胶结破坏数目与轴向应力之间的关系曲线可以识别岩石裂纹扩展的规律及各个阶段,并能够确定裂纹扩展过程中的起裂应力、裂纹损伤应力和峰值应力。     

基于Griffith强度理论的岩石裂纹起裂经验预测方法研究

 裂纹起裂强度是岩石破坏过程中的重要应力阈值,研究岩石起裂准则对于揭示其破坏机制及预测围岩工程性质有着重要意义。首先进行青砂岩试样的单轴及三轴压缩起裂试验,并基于多种应变响应分析其中的起裂机制及细观破坏特征,指出局部张拉应力集中是起裂破坏的主因,总结提出低围压条件下的张开型起裂模型及高围压条件下的滑动型起裂模型。然后基于Griffith强度理论分析压应力场中岩石缺陷端部的局部最大张拉应力,其大小随差应力 的升高而增大,同时在围压条件下受表面摩擦作用的影响较大。针对岩石细观起裂机制提出起裂预测经验准则,准则中引入起裂参数 作为围压影响系数以表征摩擦作用,从而适用于不同围压条件下的起裂破坏预测。利用3组起裂试验结果对经验准则进行验证,其准确性及实用性明显优于传统线性起裂准则。最后通过分析不同围压下岩石起裂强度与峰值强度之比 ,发现试样在围压60 MPa以下时其起裂破坏属于细观张拉破坏机制。     

A concrete–steel interface element for damage detection of reinforced concrete structures

The interface between concrete and steel in reinforced concrete governs the interaction between the two types of materials under loading. When the interface is seriously damaged, e.g. when a macro-crack is formed, de-bonding takes place with large slip, and the load-transferring capacity of the interface will drop dramatically. In this study, a damaged reinforced concrete beam finite element based on the constitutive law of the lumped model on the concrete–steel interface is developed. Scalar damage parameters characterizing changes in the interface are incorporated into the formulation of the finite element that is used in the damage identification procedure from static responses. Numerical simulations show that the method is effective to detect failure at the interface between concrete and steel bar in the reinforced concrete beam.     

Risk assessment for gas pipelines using fuzzy sets

Most gas pipelines have been buried for many years and it is hard to assess their safety. In some locations, gas pipeline breaks have resulted in severe economic and social consequences. The severity of such consequences increases with the number of people exposed. For these reasons, risk assessment for gas pipeline systems should consider both the prediction of damage occurrence and the estimation of the economic and social impacts. In this paper, a new measure denoted H. Index is introduced to assess the magnitude of gas accidents. In order to examine the correlation between the magnitude of accidents and the pipeline attributes, including the environment, fuzzy component analysis is developed. This paper presents a procedure based on linguistic variables to evaluate the main characteristics for each accident. Actual accidents data in Japan are used to illustrate the approach.     

Fracture of two three-dimensional parallel internal cracks in brittle solid under ultrasonic fracturing

Similar to hydraulic fracturing (HF), the coalescence and fracture of cracks are induced within a rock under the action of an ultrasonic field, known as ultrasonic fracturing (UF). Investigating UF is important in both hard rock drilling and oil and gas recovery. A three-dimensional internal laser-engraved crack (3D-ILC) method was introduced to prefabricate two parallel internal cracks within the samples without any damage to the surface. The samples were subjected to UF. The mechanism of UF was elucidated by analyzing the characteristics of fracture surfaces. The crack propagation path under different ultrasonic parameters was obtained by numerical simulation based on the Paris fatigue model and compared to the experimental results of UF. The results show that the 3D-ILC method is a powerful tool for UF research. Under the action of an ultrasonic field, the fracture surface shows the characteristics of beach marks and contains powder locally, indicating that the UF mechanism includes high-cycle fatigue fracture, shear and friction, and temperature load. The two internal cracks become close under UF. The numerical result obtained by the Paris fatigue model also shows the attraction of the two cracks, consistent with the test results. The 3D-ILC method provides a new tool for the experimental study of UF. Compared to the conventional numerical methods based on the analysis of stress–strain and plastic zone, numerical simulation can be a good alternative method to obtain the crack path under UF.     

Fatigue life evaluation of welded joints considering the local multiaxial stress/strain states and numerically determined residual stresses/strains

This paper puts forward a kind of calculation method of estimating fatigue crack initiation life by considering welding residual stresses as initial stresses. The numerical method could be used to quantitatively analyze the influence of residual stresses on the cumulative fatigue damage. In order to gain the distribution of weld-induced residual stresses, the FE analyses as well as measurements were carried out. Based on critical plane approach, analysis of damage parameters was performed considering both welding residual stress and biaxial loads. Subsequently, validity of the proposed numerical method is verified by analyzing fatigue test results for several typical welded joints. It is concluded that the result of the present method is closer to the experimental value than that obtained with empirical formula method in the estimation of fatigue crack initiation life of welded structures.     

Updating occurrence probability and size of defect for bored piles

To ensure performance of large-diameter bored piles, integrity tests are often conducted after pile construction. It is often impractical to inspect 100% of piles due to high costs and possible delays. Accordingly, the presence of defects may not be fully detected as only a limited number of piles can be inspected. The defect sizes may also be insufficiently predicted based on limited tests. This paper proposes a procedure to predict the occurrence probability of defect and the defect size for piles at a site based on coring examinations supplemented with engineers’ experiences. This procedure is formalized in the Bayesian framework. To update and estimate the occurrence probability and size of pile defects, the concepts of occurrence probability of defect and encounter probability are presented. Then, a method to evaluate the encounter probability is formulated. The occurrence probability of defect is updated based on results from on-site coring. After measurements on encountered defects are taken, the defect size distribution is updated using the proposed procedure. Results from numerical examples indicate that the occurrence probability of defect can be estimated using the proposed procedure in a more rational way. The inspection results from on-site coring have a significant influence on the estimated occurrence probability of defect. The uncertainty in defect size is substantially reduced after being updated with the information on defect sizes from on-site coring. Analysis results can assist the specification of pile inspection program to assure given level of quality assurance.     

考虑动态应变率效应的混凝土单轴拉伸统计损伤模型

基于准脆性材料细观损伤机制,将损伤概括为"有效受力面积的减小"和"有效受力部位弹性模量的减小"两个方面,可分别利用断裂和屈服2种损伤模式表征;认为材料破坏过程实质上为2种损伤模式累积演化的连续过程。干燥混凝土所表现出的动态本构行为本质上是由于自身的惯性效应引起材料破坏形态以及细观2种损伤累积演化过程的改变。忽略水的黏滞性影响,通过假设不同应变率情况下干燥混凝土损伤演化服从某种相似性规律,建立相应增量形式的单轴拉伸动态损伤本构模型。通过算例和试验结果比较,表明该模型可预测变应变率加载情况下材料均匀损伤阶段的本构行为,等应变率加载可作为其中的一个特例。结合算例从有效应力、名义应力、吸能能力3个角度对材料的动态损伤机制进行探讨。区分开峰值应力状态和局部破坏的临界状态,并建议后者作为本构模型的最终破坏点。此举可充分考虑材料均匀受力阶段的延性,同时避免过多考虑局部破坏阶段本构模型尺寸效应的影响。     

Dynamic stress analysis for fatigue damage prognosis of long-span bridges

For fatigue damage prognosis of a long-span steel bridge, the dynamic stress analysis of critical structural components of the bridge under the future dynamic vehicle loading is essential. This paper thus presents a framework of dynamic stress analysis for fatigue damage prognosis of long-span steel bridges under the future dynamic vehicle loading. The multi-scale finite element (FE) model of the bridge is first developed using shell/plate elements to simulate the critical structural components (local models) and using beam/truss elements to simulate the rest part of the bridge (global model). With the appropriate coupling of the global and local models, the multi-scale FE model can accurately capture simultaneously not only the global behavior in terms of displacement and acceleration but also the local behavior in terms of stress and strain. A vehicle traffic load model is then developed for forecasting the future vehicle loading based on the recorded weigh-in-motion (WIM) data and using the agent-based traffic flow microsimulation. The forecasted future vehicle loading is finally applied on the multi-scale model of a real long-span cable-stayed bridge for dynamic stress analysis and fatigue damage prognosis. The obtained results show that the proposed framework is effective and accurate for dynamic stress analysis and fatigue damage prognosis.     

旧水泥混凝土路面沥青加铺层疲劳断裂分析

在道路的使用过程中,沥青加铺层一直经历着交通荷载及温度变化的循环作用,沥青加铺层结构中的裂缝扩展呈现为疲劳断裂特征.沥青加铺层的使用寿命（从开始承载直至破坏所经历的车辆荷载及温度循环次数或作用时间）可分为疲劳裂缝形成寿命和疲劳裂缝扩展寿命两部分.疲劳裂缝形成寿命为由微观缺陷发展到宏观可检裂缝所对应的寿命,目前仍由传统疲劳理论的方法确定;而疲劳裂缝扩展寿命则为由宏观可检裂缝扩展到临界裂缝而发生破坏这段区间的寿命,用疲劳断裂力学方法确定.论文着重对沥青混合料与沥青路面的疲劳断裂进行分析.     

埋地管道的裂纹扩展分析

该文采用有限元软件 ADINA,模拟具有初始裂纹等缺陷的管道破坏情况,同时建立管道的三维 Rupture模型,结合数值分析结果得出不同的结论,建立实体模型,在裂纹路径及初始缺陷都已知的情况下采用软件中的Frac-ture模块分析裂纹扩展问题。在已知初始裂纹的前提下,利用回路J 积分的动态守恒探讨二维模型的平面应变特性,通过实验结果证实了裂纹开裂以及扩展路径与理想的弹塑性材料破坏时比较吻合,也证明了理想弹塑性材料屈服理论符合实际工程需要。     

基于多尺度小波分析的在线结构损伤诊断方法

将多尺度小波分析引入到在线结构损伤诊断上来,充分利用其在处理非稳态信号上的优势,从在线监测过程中得到的时程曲线中提取结构的损伤信息。通过编制刚度随时间变化的损伤模拟程序,得到损伤结构的时程反应,提出了相应的损伤指标,从而做到对于结构损伤的确认以及时间和空间上的定位,最后通过数值算例验证了该方法在处理在线结构损伤诊断上的可行性。     

裂纹闭合应力及其用于评价岩石微裂纹损伤

 岩石压缩破坏过程存在几个重要应力阀值,即裂纹闭合应力、启裂应力、损伤应力及峰值应力。主要研究裂纹闭合应力确定方法,并讨论用其评价岩石微裂纹损伤的可行性。首先,对确定裂纹闭合应力的方法进行回顾与总结,并提出一种基于轴向应变响应确定裂纹闭合应力的新方法。该方法易于编程操作,可去除人为因素的影响,使得裂纹闭合应力的确定更加客观。基于多组岩石单轴和三轴压缩试验结果验证发现,该方法在单轴和三轴压缩下与其他方法确定的裂纹闭合应力较接近,验证了该方法的合理正确性。最后,采用裂纹闭合应力评价瑞典Forsmark和Oskarshamn地区3组岩石的取样损伤,通过将裂纹闭合应力与取样深度关联可以发现,取样深度较小时,裂纹闭合应力随取样深度线性增加;而取样深度较大时,裂纹闭合应力基本保持不变。裂纹闭合应力与主地应力差也表现出良好的线性对应关系,表明主应力差可能为影响岩样内部微裂纹发育的主要因素。     

层状岩体渗透特性多尺度演化模型研究

考虑层状岩体内部微裂纹、层面等不同尺度结构特征,采用两步均匀化方法,建立了层状岩体细观损伤模型与渗透特性多尺度演化模型。两步均匀化过程分别考虑了层状岩体内部细观结构与宏观结构对渗透特性的影响。层状岩体渗透特性演化模型可同时考虑微裂纹损伤扩展、滑移剪胀,层理面滑移磨损、剪胀演化特性以及微裂纹与层理面相互作用等特征,较好地反映了内部不同尺度微结构变化对其渗透特性演化的影响以及渗透特性的各向异性特征。基于该模型,通过数值模拟层状岩体中地下洞室开挖扰动过程,研究了层面倾角以及岩体强度各向异性特征对洞周围岩损伤与渗透特性演化规律的影响。研究成果对于深部层状岩体水–力耦合特性研究具有一定参考意义。     

A FEniCS implementation of the phase field method for quasi-static brittle fracture

In the recent years, the phase field method for simulating fracture problems has received considerable attention. This is due to the salient features of the method: 1) it can be incorporated into any conventional finite element software; 2) has a scalar damage variable is used to represent the discontinuous surface implicitly and 3) the crack initiation and subsequent propagation and branching are treated with less complexity. Within this framework, the linear momentum equations are coupled with the diffusion type equation, which describes the evolution of the damage variable. The coupled nonlinear system of partial differential equations are solved in a ‘staggered’ approach. The present work discusses the implementation of the phase field method for brittle fracture within the open-source finite element software, FEniCS. The FEniCS provides a framework for the automated solutions of the partial differential equations. The details of the implementation which forms the core of the analysis are presented. The implementation is validated by solving a few benchmark problems and comparing the results with the open literature.