基于XFEM的砼重力坝强震破坏模式及抗震安全评价

扩展有限元法（XFEM）通过在相关节点的影响域上富集非连续位移模式,使得对非连续位移场的表征独立于单元边界,可以有效描述混凝土中的裂纹扩展。采用XFEM对Koyna重力坝地震破坏过程进行了分析,结果表明大坝开裂破坏分布与Koyna大坝实际破坏情况及文献中的模型试验结果相符,验证了计算模型的有效性。将其应用到国内某混凝土重力坝,得到了强震作用下的动力响应及破坏模式,并据此评价大坝的抗震安全性及抗震超载潜能。研究表明,强震作用下,大坝首先在下游折坡出现裂缝并扩展至上游面形成贯穿性裂缝,同时在坝踵、上游坡度变化处也容易出现开裂破坏;该重力坝在设计地震作用下,尚有1.59~1.76倍的超载裕度,其抗震安全性能满足设计要求。     

混凝土重力坝抗震配筋加固数值模拟分析

针对地震作用下重力坝损伤开裂直接影响大坝安全问题,以Koyna水坝为研究对象,提出两种不同抗震配筋形式,利用混凝土塑性损伤开裂模型计算不同配筋方案混凝土重力坝地震响应。采用钢筋混凝土嵌入式模型考虑钢筋加强作用,据断裂能守恒原理调整混凝土软化段曲线,获得强化后混凝土本构。数值分析结果验证在坝体薄弱部位进行抗震配筋的必要性与实用性。     

考虑多指标因素的重力坝抗震安全评价方法研究

采用振型分解反应谱法和时程法,得到了某碾压混凝土重力坝的线性、非线性地震响应。针对重力坝的结构特点,提出从坝体拉应力区分布、抗滑稳定性、材料强度、塑性区扩展及坝体需求能力比与超应力累积持时等方面综合评价大坝的抗震安全性能,为大坝的防震抗震工作提供参考。根据综合评价结果,初步认为在新的地震动校核参数作用下,该大坝结构可能受到低至中等破坏局部开裂,结果符合"设计可修复,校核不溃坝"这一抗震复核目标。     

基于动态黏结滑移性能的钢筋混凝土分离式模型研究

基于钢筋混凝土黏结滑移机理,搭建能够描述循环荷载作用下钢筋混凝土之间黏结性能变化过程的动态黏结-滑移本构关系,并结合单弹簧联结单元法建立钢筋混凝土分离式模型。模型避免了人为选择法向刚度困难的问题,并通过局部坐标系求解钢筋单元解决工程配筋布置问题,保证了复杂受力条件下的计算精度与计算效率。结合混凝土损伤模型进行经典算例分析,展示了模型对于结构中钢筋与混凝土局部变形与作用力关系的模拟,并对比分析了地震荷载作用下Koyna重力坝表层配筋对大坝整体动力响应及最终破坏模式的影响。研究表明,当坝体表层易损区域布置抗震钢筋之后,在钢筋与混凝土的协同作用下能够起到一定程度的限裂作用,提高坝体的抗震性能。算例的仿真结果均能够反映工程界的普遍认知,同时也验证了该模型的正确性。     

基于变形离散元法的重力坝地震开裂分析

考虑脆性材料Ⅰ型和Ⅱ型复合断裂情况,基于渐进破坏理论建立了混凝土损伤破坏模型,提出一种损伤开裂模型与变形离散单元法耦合的分析方法。模拟了Koyna坝在强烈地震作用下的渐进破坏过程,通过与室内模型试验及其他方法所得结果的对比,验证了该方法的可靠性和正确性。研究表明：坝体初始裂缝是由地震拉应力集中所致,坝体开裂后,裂缝周围应力重新分布,但缝端仍存在拉应力集中现象,导致裂缝不断扩张;裂缝发展可分4个阶段;裂缝贯通后,坝头与底部剧烈错动,坝体动能减小,摩擦功急剧增加。     

水工混凝土弥散型裂缝数值模型中开裂判据的研究

重大水利工程中,混凝土结构多处于高水压环境,坝体或多或少存在着裂缝,然而大部分开裂的混凝土坝仍能够安全运行,从而引申出是否可以允许混凝土结构出现裂缝,多大的裂缝才不至于引起破坏,以及用怎样一个合理的指标来对混凝土开裂程度进行描述评判的问题。该文以等效塑性应变表征混凝土弥散型裂缝模型数值模拟中的开裂破坏,通过混凝土单轴拉伸数值模拟,并根据试验资料及相关文献,建立了等效塑性应变与主拉应变、裂缝宽度之间的关系,得到对应于“有害裂缝”的等效塑性应变值。将方法和成果运用到混凝土重力坝的开裂研究中,具有较好的实际工程意义。     

考虑多种因素影响的重力坝地震响应分析

综合考虑坝体混凝土和坝基岩体的材料非线性、无限地基辐射阻尼等因素,对Koyna重力坝进行了地震响应分析,解释了实际震害.用传统方法对Koyna坝进行了地震响应分析,可以看出,坝体混凝土采用线弹性模型不能如实反映可能出现的裂缝分布,其结果只能对起裂位置进行一定的预测;考虑材料初始抗拉强度和抗剪强度的预设缝方法预测的开裂范围较大,考虑到实际工程都需要一定的安全储备,因此预设缝方法是合理可行的.     

单排配筋L形截面剪力墙振动台试验研究

为了解配筋形式对单排配筋L形截面混凝土剪力墙在非工程轴方向的抗震能力影响,进行了2个单排配筋L形截面剪力墙模型的振动台试验,1个为普通单排配筋剪力墙,1个为带斜筋单排配筋剪力墙,2个剪力墙模型的总配筋量相同。通过在L形截面剪力墙的非工程轴方向输入El-Centro地震波,测试比较2个剪力墙模型在弹性、开裂及破坏阶段的动力特性、地震反应与破坏特征,分析不同配筋形式下L形截面混凝土剪力墙在非工程轴方向的抗震能力,结果表明,带斜筋与不带斜筋的单排配筋L形截面混凝土剪力墙在非工程轴方向均有良好的抗震性能,能够满足低多层建筑结构的抗震设计要求。     

直接基于位移的预应力混凝土框架结构抗震设计方法

通过对传统直接基于位移的抗震设计方法中等效单自由度体系的等效刚度计算方法的改进,减少了影响等效刚度计算结果准确性的不确定因素,并将其应用于直接基于位移的预应力混凝土框架结构的抗震设计,提高了设计方法的有效性。该文根据是否考虑裂缝控制和强柱弱梁要求,采用上述设计方法设计了四种不同配筋的预应力混凝土框架,其静力弹塑性分析结果表明性能点结构位移均满足性态目标的要求,设计方法是有效的。同时,预应力框架结构的裂缝控制、预应力度、柱的配筋方式、强柱弱梁及其他抗震构造要求等将造成在设计需求地震作用下框架结构的承载力严重超强,并导致性能点的位移不同程度小于目标位移。     

钢管混凝土组合桁梁-格构墩轻型桥梁振动台阵试验研究

为了研究钢管混凝土组合桁梁-格构墩轻型桥梁的抗震性能,以干海子大桥为原型,设计制作了几何缩尺比例1∶8的两跨钢管混凝土组合桁梁-格构墩试验模型,利用福州大学地震模拟振动台台阵系统,采用实桥的设计地震波,进行了该轻型桥梁的基本动力特性试验、抗震性能试验及破坏特性研究。结果表明,模型实测频率和位移与实桥满足相似比关系;在横向或纵向地震作用下,桥墩格构式区域加速度放大效应明显,减小了主梁混凝土顶板的加速度响应;相同强度下,横桥向地震作用桥墩应变大于纵桥向地震作用,同时可以不考虑纵、横向地震力共同作用。在设计地震动作用下,墩顶位移满足位移限值的规定;模型未出现开裂和破坏现象,表明钢管混凝土组合桁梁-格构墩轻型桥梁具有良好的抗震性能。     

Analysis of hydraulic fracturing in concrete dam considering fluid–structure interaction using XFEM-FVM model

High-pressure hydraulic fracturing (HF) of cracks is the crucial factor in the safety-assessment of high concrete dams, and fluid–structure interaction in HF is the key to accurately calculate the fluid pressure within cracks. In this study, a hybrid approach combining the extended finite element method (XFEM) and the finite volume method (FVM) is introduced to simulate HF in concrete dams. Through the hybrid approach XFEM-FVM, we can obtain water pressure distribution within cracks, which reflects the coupling relationship between water pressure and fracture width. This approach is verified by an example, and the numerical results agree well with the corresponding analytic solutions. Then, we use it to simulate the HF in a concrete gravity dam. Results show that, in contrast to the constant water pressure distribution within cracks, the distribution calculated by XFEM-FVM varies with time and leads to the bigger crack extension angle, contrary to growth length. It also illustrates the significance of considering fluid–structure interaction in HF progress. This study reveals that the approach is a very efficient tool for HF simulation in concrete dam.     

Seismic cracking and instability of concrete dams: Smeared crack approach

Modeling the material behavior and estimation of the cracking capacity of concrete dams under the dynamic loading is important for safety operation purposes. In the present paper, an improved 3D co-axial rotating smeared crack model is used with the ability of updating the variable shear transfer coefficient. The model is implemented in the finite element code to assess the seismic cracking of three types of concrete dams, i.e. gravity, buttress, and arch dams.Results of the crack profiles confirm importance of the shear transfer coefficient in dynamic analysis of large concrete structures. It is found that the proposed model lead to less diffused cracks in concrete dams and can reasonably matches with the results obtained from experimental tests.     

基于数据驱动的大体积结构裂缝深度反演EI北大核心CSCD

裂缝是混凝土结构的主要病害,查明裂缝的深度能够为结构的耐久性和安全性评价提供可靠的信息,但同时也是混凝土结构检测的难点之一。提出了一种基于数据驱动的学习算法,通过考察波经过带缝结构传播的信号预测裂缝深度,采用扩展有限元法(Extended finite element methods,XFEM)和边界吸收层模型模拟了带缝大体积混凝土结构中的波传播过程,将接收点的观测信号和裂缝信息配对。基于人工神经网络的机器学习模型建立了基于XFEM数据集的裂缝深度预测模型。对于含未知裂缝信息的混凝土结构,通过测得的观测点信号,利用建立的机器学习模型实现裂缝深度的实时预测。通过2个数值算例验证了该算法的性能,结果表明所提出的数据驱动算法能够准确预测裂缝深度。     

用扩展有限元方法模拟混凝土的复合型开裂过程

用扩展有限元法对混凝土梁复合型开裂过程进行了数值模拟。裂纹面间的力学行为采用粘聚裂纹模型来描述,通过引入切向保留刚度考虑剪力分量的影响。开裂方向的计算采用了一种简化的最大切向应力准则。对Arrea和Ingraffea的混凝土梁复合开裂实验进行了数值模拟。计算给出了裂纹萌生、扩展的过程及破坏形态,并获得了与实验结果对比良好的荷载-裂纹开口滑移曲线。结果表明,扩展有限元法通过附加特定的位移模式,使裂纹两侧不连位移场的表达独立于网格划分,是一种能够模拟准脆性材料复合开裂问题的有效方法。     

XFEM analysis of the effects of voids,inclusions and other cracks on the dynamic stress intensity factor of a major crack

This paper is devoted to the extraction of the dynamic stress intensity factor (DSIF) for structures containing multiple discontinuities (cracks, voids and inclusions) by developing the extended finite element method (XFEM). In this method, four types of enrichment functions are used in the framework of the partition of unity to model interface discontinuity within the classical finite element method. In this procedure, elements that include a crack segment, the boundary of a void or the boundary of an inclusion are not required to conform to discontinuous edges. The DSIF is evaluated by the interaction integral. After the effectiveness of the implemented XFEM program is verified, the effects of voids, inclusions and other cracks on the DSIF of a stationary major crack are investigated by using XFEM. The results show that the dynamic effects have an influence on the path independence of the interaction integral, and these voids, inclusions and other cracks have a significant effect on the DSIF of the major crack.     

基于改进云图法的高土石坝抗震可靠度分析

高土石坝抗震可靠度研究对大坝抗震防灾和震害风险研究具有重要意义。通过考虑地震动和筑坝料参数双重随机性,建立基于地震易损性和地震峰值加速度概率密度函数的高土石坝抗震可靠度模型,为研究不同设计使用年限的高土石坝抗震可靠度提供依据。通过拉丁抽样方法选取筑坝料参数样本并与选择地震动组合成样本对,选取坝顶相对震陷率作为性能参数,提出考虑抗震设防标准的高土石坝性能水平了;采用SWANDYNE Ⅱ程序进行动力计算,并根据改进云图法得到不同地震峰值加速度下坝顶相对震陷率的地震易损性三维曲面;结合糯扎渡高土石坝不同设计年限的概率分布函数与地震易损性曲面,确定不同设计年限失效概率和抗震可靠度。分析结果表明:随着设计使用年限增加,大坝各个性能水平可靠度不断减小,对于严重破坏状态下不同设计年限可靠度均能满足《水利水电工程结构可靠性设计统一标准》规范要求,说明糯扎渡高土石坝在变形方面抗震设计是合理的。     

A Novel Hexahedral Interface Element for Nonlinear Crack Analysis

Existence of a crack in structures would lead to a sudden failure and damage. Establishing a precise analytical model for the cracked element would be a powerful tool to achieve the right answers in the analysis of the structure. The main aim of this article is to formulate a hexahedral interface element for use in nonlinear crack analysis. In this investigation, the kinematics of the discontinuous displacement field along with the virtual work principle, for a body with an internal discontinuity, is utilized. Based on the suggested interpolation functions for the discrete segments, and also the element displacement field, the element stiffness matrix is calculated. The proposed element can be used for modeling of the discrete cracks in three-dimensional problems, such as a concrete dam. Several numerical examples are analyzed for the accuracy test and a few of them are presented here. The results indicated that utilizing sufficient elements yields suitable answers.     

基于断裂力学的混凝土坝裂缝灌浆压力计算模型

根据大坝工程上对裂缝种类的分类以及经常采用的化学灌浆方法,以某在建高拱坝中出现的几种裂缝为实例,考虑了浆液流动时的沿程阻损及重力影响,建立了基于断裂力学的化学灌浆计算模型。运用权重法,通过积分计算了灌浆时3种模型下的裂缝尖端应力强度因子。在不同的大体积混凝土断裂韧度条件下,对不同缝长及3种裂缝类型进行化学灌浆的容许灌浆...     

A framework for fracture modelling based on the material forces concept with XFEM kinematics

A theoretical and computational framework which covers both linear and non‐linear fracture behaviour is presented. As a basis for the formulation, we use the material forces concept due to the close relation between on one hand the Eshelby energy–momentum tensor and on the other hand material defects like cracks and material inhomogeneities. By separating the discontinuous displacement from the continuous counterpart in line with the eXtended finite element method (XFEM), we are able to formulate the weak equilibrium in two coupled problems representing the total deformation. However, in contrast to standard XFEM, where the direct motion discontinuity is used to model the crack, we rather formulate an inverse motion discontinuity to model crack development. The resulting formulation thus couples the continuous direct motion to the inverse discontinuous motion, which may be used to simulate linear as well as non‐linear fracture in one and the same formulation. In fact, the linear fracture formulation can be retrieved from the non‐linear cohesive zone formulation simply by confining the cohesive zone to the crack tip. These features are clarified in the two numerical examples which conclude the paper. Copyright © 2005 John Wiley & Sons, Ltd.