Exploring the impact of chloride-induced corrosion on seismic damage limit states and residual capacity of reinforced concrete structures

A non-linear finite element (FE) framework for time-dependent capacity assessment of corroded rectangular reinforced concrete (RC) columns is developed. The proposed non-linear FE model includes the impact of corrosion on inelastic buckling and low-cycle fatigue degradation of longitudinal reinforcement. The proposed non-linear FE model is validated against a set of experimental data and then extended to evaluate the impact of corrosion on damage limit states to be used in seismic performance and evaluation of corroded structures. This is done through a parametric study on hypothetical RC columns, varied in axial force ratios, mass loss ratios, cover crack widths and confinement levels. Moreover, the application of the proposed model in seismic collapse capacity assessment of corroded structures is shown through non-linear dynamic analyses of prototype columns. Results show that, depending on the axial force ratio, corrosion changes the failure mechanism of the columns. The results of this study suggest that in seismic fragility analysis of corroded structures, the damage limit states should be considered as time-variant parameters.     

Analysis of lifetime losses of low-rise reinforced concrete buildings attacked by corrosion and earthquakes using a novel method

In the estimation of the losses caused by an earthquake for a reinforced concrete (RC) building, the effect of corrosion of the reinforcing steel incurred by environmental conditions, e.g. carbonation and chloride ions, is seldom mentioned because of the corrosion with uncertainty and time dependence. However, because the structural capacity of a corroded RC building declines over time, one must apply an appropriate method that estimates the structural capacity of an RC building in a corrosive environment. Therefore, this work integrated degradation factors into the structural properties of a corroded RC building. Additionally, by considering life-cycle earthquake events, lifetime losses resulting from earthquakes and corrosion can be derived. This work can help both owners and investors to identify lifetime losses of RC buildings due to seismic structural damage, including the corrosion effect, within a specified service life. Although the case study only addresses a selection of the most appropriate concrete cover depth for an RC building corroded by chloride ions, the proposed procedure can be utilised when making decisions about whether to prevent building deterioration based on economic considerations.     

钢筋混凝土框架结构基于时变的抗震性能研究

钢筋混凝土结构在使用过程中由于钢筋锈蚀抗震性能退化。为了研究钢筋混凝土框架结构在正常使用环境下随使用时间的增加其抗震性能的退化规律,采用通用有限元分析软件ABAQUS对锈蚀钢筋混凝土框架进行多尺度建模以提高计算精度并保证计算效率。基于锈蚀钢筋混凝土构件及单榀框架的试验数据,验证了多尺度建模方法的正确性。采用多尺度模型对4层钢筋混凝土框架结构进行了静力推覆分析及弹塑性时程分析,对比不同使用时间的钢筋混凝土框架结构的地震反应和损伤情况。结果表明：当使用时间为25a时,结构的抗震性能与新建结构接近;当使用时间超过50a时,随着使用时间的增加,结构的承载能力及刚度退化明显,在地震作用下的位移响应逐渐增大,地震损伤范围不断扩大;相比新建结构,当使用时间为50、75a和100a时,结构的抗侧承载力分别降低了6.2%、14.1%及18.1%,罕遇地震作用下的最大层间位移角分别增大了9.2%、19.8%及25.1%。     

箍筋锈蚀钢筋混凝土柱受火后抗震性能试验研究及有限元分析

为研究箍筋锈蚀钢筋混凝土柱的火灾反应及其火灾后的抗震性能,设计并制作4根尺寸及配筋相同的钢筋混凝土柱,以锈胀裂缝为损伤指标,对混凝土柱的箍筋进行电化学加速锈蚀试验,得到箍筋不同锈蚀率的混凝土柱。对RC柱进行限时受火试验,研究箍筋不同锈蚀率的混凝土柱的高温反应。对历经高温的钢筋混凝土柱进行低周水平反复荷载试验,并利用ANSYS进行有限元分析,探究受火后锈蚀钢筋混凝土柱抗震性能变化规律,分析建模方法的可靠性。研究表明：箍筋锈蚀将导致钢筋混凝土柱截面温度场畸变;锈蚀箍筋对受火后钢筋混凝土柱抗震性能影响显著,锈蚀率越高,耗能能力越低,刚度和延性退化越严重;有限元分析与试验结果吻合较好,模拟方法可行;提出了高温后箍筋锈蚀钢筋混凝土柱受剪承载力计算式,可供箍筋锈蚀火灾后钢筋混凝土柱安全性鉴定参考。     

Lifetime seismic performance assessment for chloride-corroded reinforced concrete buildings

The main aim of this paper was to build an estimating procedure based on the static nonlinear analysis (pushover analysis) such that structural engineers can evaluate the seismic performance of a deteriorating reinforced concrete (RC) building. For engineers' convenience, this paper suggests the probabilistic deterioration prediction model and the visual estimation of deterioration degree to evaluate corrosion-induced weight loss of reinforcing steel bars. In addition, flexural and shear capacity models for a corroded column or beam are proposed and verified by full-scale corroded beam specimens, allowing engineers to simulate its nonlinear mechanical behaviour. This paper incorporates these mechanical models of corroded members into the static nonlinear analysis to construct a procedure for assessing the seismic performance of a deteriorating RC building. An elementary school at Lanyu Island, Taitung, Taiwan, is used as an example and its lifetime seismic performance is identified utilising the proposed approach.     

A novel lifetime cost-benefit analysis method for seismic retrofitting of low-rise reinforced concrete buildings

This article presents a novel simplified method for assessing seismic damage to low-rise reinforced concrete (RC) buildings by using the hazard curve of response spectral acceleration. Moreover, the occurrence of an earthquake is assumed to follow a Poisson process when analysing the occurrence probability of a specified damage state in the remaining service life and expected losses induced by seismic damage. Then, a novel procedure for estimating lifetime costs and benefits of seismic retrofitting is proposed. In the case study, 16 practical design projects for seismic retrofitting of RC school buildings in Taipei are subjected to lifetime cost-benefit analysis using the proposed method. It can be found that not only lifetime cost-benefit ratios but also the financial return period for each dollar invested seismic retrofitting can be identified conveniently. Additionally, they are useful information for making decisions about whether to retrofit a building or not.     

FRP加固钢筋混凝土柱经历模拟地震作用后的残余性能试验研究

本文对8根大比例的FRP加固钢筋混凝土桥柱模型在轴压比为0.2的恒定轴压力下进行拟静力试验,再对其中5根试验柱在经历有限的地震作用后进行长期荷载作用下的轴压试验。试验表明,FRP加固柱的徐变变形远小于对比柱的徐变变形;带损伤的加固柱在长期荷载作用下的变形发展受加固柱的损伤程度和持荷大小的控制,并与FRP的弹性模量有关;在试验条件下,带损伤的加固柱长期轴向变形呈稳定态势。采用AEMM和F ind ley模型为基础,考虑损伤程度、密封状态、三向应力状态和应力重分布等因素的影响,建立了损伤加固柱的徐变计算模型,并进行了验证。分析表明,损伤对加固柱的徐变发展和徐变破坏时限有显著影响。     

Seismic fragility estimates for corroding reinforced concrete bridges

Seismic fragility of reinforced concrete (RC) bridges is defined as the conditional probability that the seismic demand exceeds the corresponding capacity, specified for a certain performance level, for given seismic intensity measures. However, the structural properties of RC bridges change over time due to the onset of corrosion in the reinforcing steel. Therefore, seismic fragility of RC bridges changes during a bridge lifetime. This paper proposes a method to estimate the seismic fragility of corroding RC bridges. Structural capacities are defined using probabilistic models for deformation and shear capacities of RC columns. Probabilistic models are also used to estimate the corresponding demands for given seismic intensity measures. The capacity and demand models are then combined with probabilistic models for chloride-induced corrosion and time-dependent corrosion rate to model the dependency on time of the seismic fragility of RC bridges. In particular, the loss of reinforcing steel is modelled as a function of the thickness of the cover concrete for each reinforcing bar in the RC columns. The stiffness degradation in the cover concrete over time due to corrosion-induced cracking is also considered in the fragility estimates. Seismic fragility estimates are then formulated at the column, bent, and bridge levels. The fragility formulations properly incorporate the uncertainties in the capacity and demand models, and the inexactness (or model error) in modelling the material deterioration. The proposed method accounts for the variation of structural capacity and seismic demand over time due to the effects of corrosion in the reinforcing steel. As an application, seismic fragility estimates are developed for a corroding RC bridge with 11 two-column bents over a 100-year period.     

Collapse safety margin and seismic loss assessment of RC frames with equal material cost

With the premise of equal material cost, a collapse safety margin‐based collapse resistance optimization strategy for passively controlled reinforced concrete (RC) frames is proposed based on seismic fragility analysis, collapse safety margin analysis, and seismic hazard loss assessment. The efficiency of introducing buckling restrained braces or lead–rubber bearings on the performance of RC frames is studied by so‐called collapse margin ratio (CMR) suggested by FEMA P695 and the modified rigidity‐to‐gravity ratio (RGR). The proposed strategy is developed from the case study on 4 low‐rise and medium‐rise RC frames and then verified on a high‐rise RC frame. The study indicates that lead–rubber bearings can cause a significant improvement at all damage levels. The contribution of buckling restrained braces to structural stiffness and collapse resistance can be maximized when they are located in potential weak stories determined through inelastic time history analysis. CMR exhibits a better linear relation with the minimum modified RGR. Increasing the equivalent story lateral stiffness and the minimum modified RGR simultaneously can give rise to a significant improvement in seismic capacity, especially CMR. Base isolation is proved to be desirable not only for improving the collapse safety margin of RC frames significantly but also for reducing seismic hazard loss.     

Seismic fragility analysis of reinforced concrete structures considering reinforcement corrosion

钢筋锈蚀是影响钢筋混凝土结构耐久性的一个重要因素，会引起结构的自振周期延长、地震需求变化及抗震能力衰减，使得锈蚀钢筋混凝土结构的地震易损性分析不同于未锈蚀钢筋混凝土结构。以一栋按我国规范设计的RC框架结构为研究对象，分别建立了未锈蚀和锈蚀结构的非线性有限元模型并进行了模型验证。分别采用云图法和条带法计算得到了钢筋混凝土结构在未锈蚀和锈蚀两种工况下的地震易损性曲线和函数参数，对锈蚀钢筋混凝土结构地震易损性分析的特殊性及其对地震易损性分析结果的影响进行了分析。分析结果表明：不考虑钢筋锈蚀引起的结构自振周期延长会错误估计锈蚀钢筋混凝土结构的地震易损性水平。采用云图法分析锈蚀钢筋混凝土结构的地震易损性会出现锈蚀结构的极限状态失效概率低于未锈蚀结构的情况。而条带法比云图法可以更好地反映钢筋锈蚀对钢筋混凝土结构地震易损性的影响。忽略钢筋锈蚀引起的结构抗震能力衰减会低估锈蚀钢筋混凝土结构地震易损性水平，建议在锈蚀钢筋混凝土结构地震易损性分析中采用基于Pushover的极限状态定义方法。     

锈蚀钢筋混凝土结构地震易损性分析

钢筋锈蚀是影响钢筋混凝土结构耐久性的一个重要因素,会引起结构的自振周期延长、地震需求变化及抗震能力衰减,使得锈蚀钢筋混凝土结构的地震易损性分析不同于未锈蚀钢筋混凝土结构。以一栋按我国规范设计的RC框架结构为研究对象,分别建立了未锈蚀和锈蚀结构的非线性有限元模型并进行了模型验证。分别采用云图法和条带法计算得到了钢筋混凝土结构在未锈蚀和锈蚀两种工况下的地震易损性曲线和函数参数,对锈蚀钢筋混凝土结构地震易损性分析的特殊性及其对地震易损性分析结果的影响进行了分析。分析结果表明：不考虑钢筋锈蚀引起的结构自振周期延长会错误估计锈蚀钢筋混凝土结构的地震易损性水平。采用云图法分析锈蚀钢筋混凝土结构的地震易损性会出现锈蚀结构的极限状态失效概率低于未锈蚀结构的情况。而条带法比云图法可以更好地反映钢筋锈蚀对钢筋混凝土结构地震易损性的影响。忽略钢筋锈蚀引起的结构抗震能力衰减会低估锈蚀钢筋混凝土结构地震易损性水平,建议在锈蚀钢筋混凝土结构地震易损性分析中采用基于Pushover的极限状态定义方法。     

A machine-learning-based two-step method for failure mode classification of reinforced concrete columns

钢筋混凝土柱在侧向地震力作用下具有弯曲、剪切和弯剪三种失效模式。不同的失效模式下钢筋混凝土柱具有不同的地震损伤特征。因此，准确地判别钢筋混凝土柱的失效模式对于准确评估钢筋混凝土结构的抗震性能具有重要意义。利用已有的钢筋混凝土柱滞回加载试验数据，采用机器学习方法，提出了一种钢筋混凝土柱失效模式两阶段判别方法。其中，第一阶段以钢筋混凝土柱的基本设计参数为输入变量，采用机器学习中的回归算法，建立钢筋混凝土柱的受弯承载力、受剪承载力、弯曲变形和剪切变形预测模型。第二阶段以钢筋混凝土柱的受弯承载力、受剪承载力、弯曲变形和剪切变形作为输入变量，采用机器学习中的分类算法，对钢筋混凝土柱的失效模式进行自动判别，实现了准确判别钢筋混凝土柱失效模式的目的。研究结果表明：极端随机树、AdaBoost、随机森林和梯度提升算法分别对受弯承载力、受剪承载力、弯曲变形和剪切变形的预测效果最佳；极端随机树、梯度提升算法和最近邻居法分别对弯曲失效、剪切失效和弯剪失效具有最佳的分类效果；相比已有的钢筋混凝土柱失效模式分类方法，提出的两阶段分类方法具有与真实失效模式最为接近的分类结果，分类精度可以达到96%。     

基于机器学习的钢筋混凝土柱失效模式两阶段判别方法增强出版（附加材料可在文章详情页“评审附件”下载）

钢筋混凝土柱在侧向地震力作用下具有弯曲、剪切和弯剪三种失效模式。不同的失效模式下钢筋混凝土柱具有不同的地震损伤特征。因此,准确地判别钢筋混凝土柱的失效模式对于准确评估钢筋混凝土结构的抗震性能具有重要意义。利用已有的钢筋混凝土柱滞回加载试验数据,采用机器学习方法,提出了一种钢筋混凝土柱失效模式两阶段判别方法。其中,第一阶段以钢筋混凝土柱的基本设计参数为输入变量,采用机器学习中的回归算法,建立钢筋混凝土柱的受弯承载力、受剪承载力、弯曲变形和剪切变形预测模型。第二阶段以钢筋混凝土柱的受弯承载力、受剪承载力、弯曲变形和剪切变形作为输入变量,采用机器学习中的分类算法,对钢筋混凝土柱的失效模式进行自动判别,实现了准确判别钢筋混凝土柱失效模式的目的。研究结果表明：极端随机树、AdaBoost、随机森林和梯度提升算法分别对受弯承载力、受剪承载力、弯曲变形和剪切变形的预测效果最佳;极端随机树、梯度提升算法和最近邻居法分别对弯曲失效、剪切失效和弯剪失效具有最佳的分类效果;相比已有的钢筋混凝土柱失效模式分类方法,提出的两阶段分类方法具有与真实失效模式最为接近的分类结果,分类精度可以达到96%。     

Reliability-based service life assessment for deteriorating reinforced concrete buildings considering the effect of cumulative damage

Most engineering systems used in maintenance strategies must consider deterioration and seismic structural damage. To identify the effects of deterioration and earthquakes simultaneously on structural performance, this study applies an integral simulation method. Compared with that of previous studies, the feature of the proposed method is its analysis of the time-dependent structural capacity of a deteriorating reinforced concrete (RC) building and the simulation of life-cycle earthquake events within a specified service period, while considering cumulative damage induced by deterioration and earthquakes. In addition, the proposed assessment method is applied to derive the reliability-based service life of a deteriorating RC building located in a region with high seismic hazard. Briefly, for deteriorating RC buildings, the proposed reliability-based service life assessment method provides useful information related to maintenance based on both serviceability and safety.     

近场地震下混凝土框架考虑不确定性的#br# 抗震风险与恢复性评估

结构的抗震恢复性表征了结构震后维持和恢复其功能的能力,是近年来地震工程学界研究热点之一。文章以我国8度区典型中低层混凝土框架为例进行系统性的抗震恢复性评估。考虑结构设计参数以及震后修复行为的不确定性,探讨混凝土框架在远近场地震下的恢复性差异。通过增量动力分析的方法得到结构的易损性曲线,并在此基础上分析计算结构的经济损失、震后残余功能以及结构功能恢复过程。研究结果表明：相较于远场地震,近场地震对结构的破坏作用显著;近场地震显著增大了结构的易损性,同时造成更大的经济损失,更小的结构震后瞬时残余功能以及更长的功能恢复过程,降低结构的抗震恢复性;忽略结构不确定性会低估经济损失,一定程度上高估震后残余功能和结构抗震恢复性。研究结果可为考虑近场地震的混凝土框架结构的新建设计或者维护加固提供参考。     

Incremental dynamic collapse analysis of RC core‐wall tall buildings considering spatial seismic response distributions

Despite wide‐ranging studies on fragility analysis and collapse safety assessment of short to medium‐rise reinforced concrete (RC) structures, a new interest in the topic is still valuable and even necessary for tall RC buildings. This study aims at establishing fragility relationships as well as collapse probability of high‐rise RC core‐wall buildings under maximum considered earthquake ground motions. This study is carried out in a probabilistic framework on a case study of a fully 3‐dimensional numerical model developed to simulate seismic behavior of a 42‐story building having a RC core‐wall system. Proposing planar and vertical distributions of ductility and damage indices, the incremental dynamic analysis, and the multi‐direction nonlinear static (pushover) analyses were employed to reach the research goal. Median collapse‐level capacities were defined in terms of seismic responses (e.g., ductility/damage indices) as well as several intensity measures by employing statistical analyses and cumulative density functions. Available and acceptable collapse margin ratios were next estimated to quantify collapse safety at maximum considered earthquake shaking level. On an average basis, the statistics indicated 9%–10% and 5%–6% collapse probability of the building subjected to near‐ and far‐field ground motions, respectively.     

Seismic design requirements for reinforced concrete piers considering aftershock-induced seismic hazard

The main purpose of this study is to develop an estimation procedure of seismic design level setting for reinforced concrete (RC) piers considering aftershock-induced seismic hazards. This work develops an assessment method of the seismic hazards induced by aftershocks and takes an example of the Chi–Chi Earthquake in Taiwan. The number of aftershocks is assumed to follow the modified Gutenberg–Richter law with lower and upper bounds when analysing the cumulative density function of the magnitude of the aftershock within a specified post-mainshock period for the earthquake. Additionally, this work considers the spatial uncertainty in the hypocentres of aftershocks to assess the aftershock-induced seismic hazards. Fragility curves and residual factors of damaged RC piers are used in the transition probability matrix of Markov Chain model for considering the cumulative damage induced by aftershocks by incorporating uncertainty into aftershock events, as well as into structural capacity and residual factors corresponding to a specified damage state, the exceedance probabilities for various damage states can be estimated using Markov Chain model and Monte Carlo Simulation. Finally, in the case study, the proposed procedure is used to determine the important factor in the preliminary seismic design of typical RC piers for the Chi–Chi Earthquake in Taiwan.     

锈蚀钢筋混凝土柱基于变形的性能指标研究

采用ABAQUS有限元软件,对按GB 50011-2010《建筑抗震设计规范》和GB 50010-2010《混凝土结构设计规范》设计的342个锈蚀钢筋混凝土柱进行有限元分析,研究钢筋的锈蚀率、轴压比、剪跨比、配箍率和纵向钢筋配筋率等参数对钢筋混凝土柱变形性能的影响。结果表明：当钢筋的锈蚀率较小时,构件的变形性能受锈蚀率影响不大;当钢筋的锈蚀率达到10%以后,构件的破坏形态发生改变,变形能力急剧退化;随着轴压比的增大,构件的变形能力降低;轴压比较小的构件变形性能受剪跨比影响比轴压比较大的构件明显;随着配箍率和纵向钢筋配筋率的增大,构件的变形能力有所提高;提出了锈蚀钢筋混凝土柱抗震性能等级和性能界限状态的划分方法,以塑性转角作为变形性能指标,对数值计算结果进行统计分析,得到了锈蚀钢筋混凝土柱各性能界限点的塑性转角统计特征值。研究成果可为钢筋混凝土结构全寿命的性能化抗震设计和抗震性能评估提供参考。     

Numerical simulation on seismic retrofitting performance of reinforced concrete columns strengthened with fibre reinforced polymer sheets

This paper evaluates the seismic performance of reinforced concrete columns retrofitted with fibre reinforced polymer (FRP) sheets through numerical simulations of the load–deformation response using two-dimensional finite element analysis (2D-FEA). The relatively rational mesh configuration is verified through comparison of analysis results obtained from the different mesh configurations. The seismic performance of three reinforced concrete (RC) columns strengthened with FRP sheets is assessed through a series of parametric studies, and the applicability of existing crack models and constitutive relationships on crack discontinuity and concrete compressive behaviour are validated. Comparisons of analysis results with tests shows that an equivalent uniaxial strain model and a failure criterion can be used to accurately simulate nonlinear behaviour and the failure of concrete under a biaxial stress state, respectively. Moreover, it is shown that a modified confinement model can be simply adopted to evaluate confined effects from hoop steel and FRP on concrete, which generally operate in three-dimensional confinement. Lastly, the seismic retrofitting performance of RC columns wrapped with FRP sheets is verified by analysing load–deformation responses and the progression of stress–strain at inflection points and bottoms of the columns.     

Stochastic seismic collapse and reliability assessment of high‐rise reinforced concrete structures

To provide knowledge beyond the conventional engineering insights, attention in this work is focused on a comprehensive framework for the stochastic seismic collapse analysis and reliability assessment of large complex reinforced concrete (RC) structures. Three key notions are emphasized: the refined finite element modeling and analysis approach towards structural collapse, a physical random ground motion model, and an energy‐based structural collapse criterion. First, the softening of concrete material, which substantially contributes to the collapse of RC structures, is modeled by the stochastic damage constitutive model. Second, the physical random ground motion model is introduced to quantitatively describe the stochastic properties of the earthquake ground motions. And then the collapse‐resistance performance of a certain RC structure can be systematically evaluated on the basis of the probability density evolution method combining with the proposed structural collapse criterion. Numerical results regarding a prototype RC frame‐shear wall structure indicate that the randomness from ground motions dramatically affects the collapse behaviors of the structure and even leads to entirely different collapse modes. The proposed methodology is applicable in better understanding of the anti‐collapse design and collapse prediction of large complex RC buildings.