Life-cycle evaluation of deteriorated structural performance of neutralised reinforced concrete bridges

For existing reinforced concrete (RC) bridges, the structural performance is highly dependent on the changing properties of concrete and reinforcing steel due to neutralisation-induced corrosion. As neutralisation progresses, the corrosion could become serious enough to deteriorate not only the serviceability, but also the maintainability, of the structural performance. To study the influence of neutralisation on the existing RC bridges, the inspected data and test results collected from 21 bridges in Taiwan were examined to obtain the essential parameters through regression analyses. The regressive parameters related to service time can be employed in evaluating the variation of material and sectional properties in both reinforcements and concrete, and, accordingly, the change of structural performance from time to time could be obtained quantitatively via structural analysis. As a consequence, the performance degradation curve of an existing RC bridge can be predicted and, if necessary, the appropriate timing for repair or retrofit could be suggested. The results obtained could facilitate the minimisation of life-cycle cost for the neutralised RC bridges and enhance the functionality of a bridge management system (BMS).     

扇形铅黏弹性阻尼器加固RC框架的抗震性能试验研究

针对钢筋混凝土框架结构梁柱节点震害严重的抗震问题,提出采用扇形铅黏弹性阻尼器加固框架结构的 方法,结合扇形阻尼器型式提出外包箱形钢板和外包U形钢板两种不同的加固连接方式。为研究扇形铅黏弹性阻 尼器加固框架的抗震性能,设计并制作了3榀框架试件,分别为空框架和两榀不同连接的扇形铅黏弹性阻尼器加 固框架,通过低周反复加载试验分析了其滞回性能、承载能力、刚度退化、耗能能力等参数。试验结果表明：采 用扇形铅黏弹性阻尼器加固的框架滞回曲线饱满、耗能能力强、加固效果良好;扇形铅黏弹性阻尼器为框架提供 了一定的抗侧刚度,提高了框架的水平承载能力,延缓了框架塑性铰的产生,使框架具有良好的耗能能力;外包 U形钢板和外包箱形钢板用于扇形铅黏弹性阻尼器与主体结构的连接都是有效的,这两种不同的连接方式对加固 框架的整体抗震性能影响不大。     

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.     

磁流变阻尼智能加固体系的试验研究

提出利用磁流变阻尼器(MRD)对有损伤结构进行智能加固。根据MRD的特点,结合我国抗震设计规范,给出MRD智能加固的三个等级性能目标,对其进行量化,并确定MRD智能加固的设计要点。利用MRD对一有损伤钢筋混凝土框-剪偏心结构进行抗震加固,并进行振动台试验。结果表明,采用MRD智能加固后的结构抗震性能大大提高,优于用被动控制方法加固的结构。     

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.     

氯离子侵蚀效应对RC桥墩抗震性能的影响

为研究混凝土桥梁结构在服役期内由于环境氯离子侵蚀效应引起钢筋、混凝土锈蚀退化等导致结构抗震性能退化的规律,以某多跨钢筋混凝土连续梁桥为例,采用OpenSees软件建立非线性动力分析模型,根据已有研究成果并基于概率方法研究了墩柱截面主筋和箍筋锈蚀的开始时间和锈蚀率大小,进而建立了钢筋的直径及屈服强度退化模型;针对考虑纵筋锈蚀、考虑箍筋锈蚀、同时考虑纵筋和箍筋锈蚀3种情况,分别分析了材料退化对桥墩抗震性能的影响。结果表明:同等条件下箍筋锈蚀比纵筋锈蚀更早;随着时间的推移,氯离子侵蚀效应会导致桥墩抗震能力下降,结构的抗震需求明显增加;与以往只考虑纵筋锈蚀的情况相比,同时考虑箍筋和纵筋锈蚀时桥墩抗震性能退化更严重。     

Reliability-based capacity design for reinforced concrete bridge structures

In the seismic design of reinforced concrete (RC) bridge structures, there should be no brittle failures, such as shear failures, in the components, and a plastic hinge should be formed at the bottom of the bridge pier. These are important concepts in capacity design to guarantee the safety of bridges subjected to severe earthquakes. These concepts can maximise post-event operability and minimise the cost of repairing bridges after a severe earthquake. In this article, a reliability-based methodology to carry out capacity design with partial factors is proposed and applied to the seismic design of RC bridge structures. This ensures that (i) all of the components undergo the desired ductile failure mode, (ii) the damage due to an earthquake is induced only at the bottom of the bridge pier and (iii) the probability of failure is at most equal to a specified value.     

消能支撑加固底部框架砌体房屋的弹塑性地震反应分析

本文采用弹塑性时程分析方法,对底部框架多层砌体房屋在不同烈度罕遇地震作用下进行了抗震性能评估;研究了在底部框架内增设消能外包混凝土无粘结钢支撑对房屋抗震性能的影响;对增设消能支撑加固方案和增设抗震墙加固方案进行了对比分析。分析结果表明,在底层框架砌体房屋的底层增设消能支撑可显著降低底层框架的最大层间位移,控制结构的塑性损伤,提高结构的抗震性能。增设消能支撑加固方案与增设抗震墙加固方案相比,结构加固更合理、更易实施。     

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.     

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%。     

Collapse safety margin-oriented seismic retrofit strategy for corroded reinforced concrete frames using fibre reinforced plastics

Considering residual service life, this paper presents a collapse safety margin-oriented seismic retrofit strategy for corroded reinforced concrete (RC) frames using fibre reinforced plastics (FRP). With the assumed uniform corrosion model, corrosion-induced initial damage combined with subsequent earthquake-induced damage is identified by the multi-mode-based global damage model developed previously. The collapse-level earthquake intensity determined by incremental dynamic analyses (IDA) with the damage model and the maximum considered earthquake (MCE)-level intensity considering residual service life are combined to generate the time-variant collapse safety margin assessment of corroded RC structures. Based on this assessment, the collapse safety margin-oriented FRP seismic retrofit strategy is proposed and demonstrated on a 4-storey frame. The damage model originally developed for earthquake scenarios has also exhibited its rationality for characterising corrosion-induced initial damage and its influence on coupled damage development with subsequent earthquake excitation. Seismic retrofitting with FRP composites should consider the effects of the corrosion development stage, target collapse safety margin and residual service life. FRP retrofits can cause decreases in the MCE-level collapse probability and increases in the collapse resistance of corroded RC structures. Retrofitting carried out at different times achieves different efficiencies and different time-variant collapse safety margins within the residual service life.     

预制自保温钢筋混凝土墙板加固震损框架试验研究

为了达到震后快速加固修复和建筑节能改造等综合加固改造目的,提出采用三明治式预制自保温钢筋混凝土墙板（PISW）对震损钢筋混凝土框架进行快速加固的方法。开展了4榀采用预制自保温钢筋混凝土墙板加固震损框架模型的低周往复加载试验,研究了预制自保温钢筋混凝土墙板与损伤框架之间的连接构造以及墙板配筋率对加固效果的影响,分析了试件的破坏特征、滞回性能、骨架曲线、耗能能力等。结果表明：采用预制自保温钢筋混凝土墙板加固方法可大幅度提高震损框架的承载能力;预制自保温钢筋混凝土墙板与损伤框架之间的界面强度是影响加固试件承载能力的主要因素;由于墙板破坏不充分,墙板配筋率的变化并未对承载力产生明显影响。基于试验研究结果提出了预制自保温钢筋混凝土墙板加固震损框架受剪承载力计算方法。     

Modelling and analysis of retrofitted and un-retrofitted masonry-infilled RC frames under in-plane lateral loading

In the context of various developing countries where many old structures require retrofitting or strengthening work to mitigate earthquake hazards, a cost-effective method is the retrofitting of damaged masonry-infilled reinforced concrete (RC) frames using ferrocement overlays, and the strengthening of existing infilled RC frames with ferrocement. However, no reliable mathematical or computational tool is accessible in the open literature to estimate the effect of such a retrofitting technique quantitatively. The present study is a numerical investigation of the retrofitting effect of masonry-infilled RC frames using ferrocement. A finite element technique has been used effectively to develop a computational model for analysing bare RC frames, together with un-retrofitted and retrofitted masonry-infilled RC frames. The proposed model accounts for the material nonlinearities of both concrete and masonry, and the yielding of reinforcing steel. It is shown that the proposed model can be used effectively in predicting the load carrying capacity of existing RC frames, as well as the required degree of strengthening when ferrocement overlays are applied as a retrofitting scheme. A parametric study was performed using the proposed model on bare and infilled frames to quantify the effects of different parameters. This enabled the development of a simplified equation for predicting the ultimate load carrying capacity of masonry-infilled RC frames, which proved to be reasonably accurate and which was validated by both experimental and numerical results.     

Seismic performance parameters of RC beams retrofitted by CARDIFRC

A new high performance fibre-reinforced cementitious composite material (designated CARDIFRC^®), to be used for retrofitting concrete members, has been developed at Cardiff University. The material is compatible with concrete and possesses favourable strength and ductility properties desirable for seismic retrofit. It overcomes some of the problems associated with the current techniques based on externally bonded steel plates and fibre-reinforced polymer (FRP) laminates caused mainly by the mismatch of their tensile strength and stiffness with that of the concrete member being retrofitted.

This paper reports on the results of three-point and four-point flexure tests conducted on a number of non-ductile and ductile small scale RC beams retrofitted with externally bonded CARDIFRC^® strips. A number of different strip configurations and two strip thicknesses are investigated and their effect on the retrofitting process examined. Some strength and seismic performance parameters including toughness and ductility are evaluated for each retrofitted beam and compared with those of the corresponding control beams. It is shown that precast CARDIFRC^® strips, adhesively bonded to the surface of the RC beam can greatly increase the strength and ductility of the retrofitted beam; hence, they can be used successfully to enhance the seismic performance of both damaged and undamaged RC beams.     

塑性铰区埋入高阻尼隔震橡胶层厚度的数值模拟分析

钢筋混凝土(RC)桥墩一旦出现塑性铰后,混凝土的破坏主要集中在塑性铰区域,而且塑性铰区混凝土的损伤及破坏程度在较大程度上影响RC桥墩的抗震性能。本文希望通过在RC桥墩塑性铰区域埋入橡胶层来提高其变形能力、增强延性和耗能能力,改善传统RC桥墩的抗震性能。基于橡胶、混凝土和钢筋的本构模型及承载力—变形理论,采用条带法,本文提出了不同轴压比、不同层间位移角所需要橡胶层厚度的计算方法,并编制了计算程序;分析得到了不同轴压比、不同层间位移角的RC柱中所需埋入的橡胶层的最小厚度,为确定RC柱塑性铰区所埋入橡胶层厚度提供参考。     

Seismic fragility increment functions for deteriorating reinforced concrete bridges

Fragility increment functions are developed to estimate the seismic fragility of reinforced concrete (RC) bridges subject to deterioration due to the onset and progression of corrosion of the reinforcement. For each mode of failure considered, the fragility at time t of a deteriorating bridge is obtained by multiplying the initial fragility of the undeteriorated bridge by a corresponding increment function expressed in terms of the environmental conditions, the original material properties, time, a measure of the seismic demand, and a set of unknown model parameters. The developed increment functions account for the effects on the fragility estimates of the loss of the reinforcement and of the increasing uncertainty over time. As an application, the developed increment functions are used to estimate the seismic fragility of an example RC bridge. The proposed fragility increment functions are useful to estimate the fragility of deteriorating bridges without any extra reliability analysis once the fragility of the undeteriorated bridge is known. In particular, the proposed fragility increment functions can be used to assess the time-variant fragility of bridges for applications such as reliability-based design, life-cycle cost analysis, and risk analysis.     

U形预应力钢板箍加固T形截面钢筋混凝土梁受剪性能试验研究

为改进T形截面钢筋混凝土(RC)梁的受剪加固工艺,提出了一种采用U形预应力钢板箍对T形截面RC梁进行受剪加固的技术。采用简支梁跨中集中力加载的方法,完成了1个足尺未加固RC梁和5个足尺加固RC梁的单调静载试验,研究U形钢板箍间距和预应力水平对加固RC梁受剪性能的影响。通过试验得到了试件的损伤发展过程、破坏形态、荷载-变形曲线以及钢板箍、钢筋和混凝土的应变曲线。研究表明：采用U形预应力钢板箍可有效提高既有T形截面RC梁的斜截面受剪承载力,试件受剪承载力随钢板箍间距的减小而提高,最大提高幅度可达46.1%。当钢板箍间距小于400mm时,加密钢板箍对试件受剪承载力的提高作用逐渐减弱。钢板箍预应力水平在0.35以上时,进一步提高预应力水平对试件受剪承载力无明显影响。提出了U形预应力钢板箍加固T形截面RC梁受剪承载力的计算式,计算结果与试验结果吻合良好,可为既有T形截面RC梁的加固设计提供参考。     

混凝土横孔空心砌块填充墙-RC框架抗震性能试验

通过对3榀单层单跨RC（钢筋混凝土）框架足尺模型进行低周反复荷载作用下的抗震性能试验,着重探讨混凝土横孔空心砌块填充墙对RC框架抗震性能的影响。对试件的试验破坏特征、滞回曲线、骨架曲线、刚度退化、耗能性能等抗震性能指标进行对比分析,并对混凝土横孔空心砌块填充墙 RC框架的抗震性能进行评估。结果表明:混凝土横孔空心砌块填充墙-RC框架属于强框架、弱填充墙类型,最终破坏形态与空框架破坏形态接近;混凝土横孔空心砌块填充墙对RC框架具有刚度效应,较大程度提高了框架的水平承载力和抗侧刚度;混凝土横孔空心砌块填充墙与框架结构共同参与滞回耗能,混凝土横孔空心砌块填充墙-RC框架表现出良好的抗震性能。     

SEISMIC RETROFITTING FOR BRIDGES IN OSAKA CITY

In the wake of the Hanshin-Awaji Earthquake, the Osaka Municipal Government has planned and executed seismic retrofitting for bridges in Osaka.

This report first discusses the present condition of bridges in Osaka, and changes in bridge design standards. Next, it explains the basic concept underlying bridge seismic retrofitting, derived from the experience of the Hanshin-Awaji Earthquake.

In planning retrofitting, evaluation criteria on the importance and seismic capacity of all bridges in Osaka were established. Based on these criteria, the bridges were classified into five ranks each. Evaluation results were indicated in the form of a matrix.

Using the matrix, retrofitting methods and priority were determined. According to plan, about half of the bridges in Osaka will be retrofitted.     

碳纤维增强复合材料修复震后玻璃纤维增强复合材料管钢筋混凝土桥墩柱抗震性能试验

为了研究碳纤维增强复合材料（CFRP）修复震后玻璃纤维增强复合材料（GFRP）管混凝土桥墩柱的抗震性能,对4根CFRP修复震后破坏GFRP管混凝土桥墩柱进行拟静力试验,对比修复后钢筋混凝土桥墩柱与原型桥墩柱的滞回曲线、骨架曲线、延性和耗能能力。试验结果表明,修复后的墩柱承载力和延性指标均能得到较好的恢复甚至提高。