Effect of URM infills on seismic vulnerability of Indian code designed RC frame buildings

Unreinforced Masonry(URM) is the most common partitioning material in framed buildings in India and many other countries.Although it is well-known that under lateral loading the behavior and modes of failure of the frame buildings change significantly due to infill-frame interaction,the general design practice is to treat infills as nonstructural elements and their stiffness,strength and interaction with the frame is often ignored,primarily because of difficulties in simulation and lack of modeling guidelines in design codes.The Indian Standard,like many other national codes,does not provide explicit insight into the anticipated performance and associated vulnerability of infilled frames.This paper presents an analytical study on the seismic performance and fragility analysis of Indian code-designed RC frame buildings with and without URM infills.Infills are modeled as diagonal struts as per ASCE 41 guidelines and various modes of failure are considered.HAZUS methodology along with nonlinear static analysis is used to compare the seismic vulnerability of bare and infilled frames.The comparative study suggests that URM infills result in a significant increase in the seismic vulnerability of RC frames and their effect needs to be properly incorporated in design codes.     

罕遇地震作用下的框架-摇摆刚架结构体系动力可靠度研究

针对钢筋混凝土框架结构的受力特点,采用增设摇摆刚架的抗震设计方法,以提高罕遇地震下建筑结构的安全性。建立了框架一摇摆刚架结构体系的计算模型,结合状态空间法与虚拟激励法,求解结构的平稳随机响应,并根据计算所得随机响应对框架一摇摆刚架体系的动力可靠度进行分析。以西部地区某已建成的6层框架结构为算例,探讨了罕遇地震作用下不同刚度比的摇摆刚架对新结构体系动力可靠度的影响。结果表明,通过增设不同刚度比的摇摆刚架,可以有效协调结构体系的变形模式,充分发挥结构的耗能能力,降低整体结构的条件失效概率。     

板柱结构中柱节点抗震性能的试验研究

通过5个板柱结构中柱节点的水平低周反复荷载试验,研究了中柱节点的延性、滞回特性、耗能能力和破坏形态。实验结果表明,水平荷载作用下中柱节点主要有弯曲破坏和冲切破坏2种破坏形态,板中钢筋的配筋率是影响节点破坏形态的主要因素之一,板柱结构中柱节点的抗震性能要比框架结构的梁柱节点差。     

Seismic resilience assessment of corroded reinforced concrete structures designed to the Chinese codes

The natural landscape in China exposes many existing RC buildings to aggressive environments.Such exposure can lead to deterioration in structural performance with regard to resisting events such as earthquakes.Corrosion of embedded reinforcement is one of the most common mechanisms by which such structural degradation occurs.There has been increasing attention in recent years toward seismic resilience in communities and their constituent construction;however,to date,studies have neglected the effect of natural aging.This study aims to examine the effect of reinforcement corrosion on the seismic resilience of RC frames that are designed according to Chinese seismic design codes.A total of twenty RC frames are used to represent design and construction that is typical of coastal China,with consideration given to various seismic fortification levels and elevation arrangements.Seismic fragility relationships are developed for case frames under varying levels of reinforcement corrosion,i.e.,corrosion rates are increased from 5%to 15%.Subsequently,the seismic resilience levels of uncorroded and corroded RC frames are compared using a normalized loss factor.It was found that the loss of resilience of the corroded frames is greater than that of their uncorroded counterparts.At the Rare Earthquake hazard level,the corrosioninduced increase in loss of resilience can be more than 200%,showing the significant effect of reinforcement corrosion on structural resilience under the influence of earthquakes.     

基于能量方法设计的RC框架结构易损性分析

基于“强柱弱梁”的屈服机制,依据能量平衡方法设计了某6层RC框架结构,采用震级-震中距条带地震动记录选取方法,选取12条随机地震动,利用Perform-3D有限元分析软件对结构进行增量动力（IDA）分析,得到了结构的地震易损性曲线、破坏状态概率曲线以及结构破坏概率矩阵。分析结果表明:该方法设计的结构能够形成预设的“强柱弱梁”屈服机制,可以保证结构中梁充分参与耗能,同时结构具有较强的抗倒塌能力,可以满足“小震不坏,中震可修,大震不倒”的性能要求。     

Interior wide beam-column connections in existing RC frames subjected to lateral earthquake loading

The seismic performance of two RC interior wide beam-column connections representative of existing frames designed and detailed according to past construction practices in the moderate-seismicity Mediterranean area was investigated experimentally. The specimens were subjected to axial loads, moderate levels of gravity loading and cyclic displacements up to failure. The specimens exhibited a “strong column-weak beam” type of flexural yielding mechanism. The wide beams did not reach the expected capacities corresponding to the formation of a full-width plastic hinge. The wide-beam longitudinal bars exhibited significant slippage, and the transverse beams underwent severe torsion cracking and even failure; this caused severe pinching in the load versus displacement hysteretic loops and exacerbated the intrinsic flexibility of this type of connection. The average drift ratios at first yielding of the wide beam longitudinal reinforcement and at failure were 2.7 and 4.5%, respectively. The displacement ductility ratio was about 2.8. The ultimate energy dissipation capacity of each specimen—obtained by dividing the total plastic strain energy by the product of the yield load and yield displacement—was approximately 9, which is about one fourth of the value recommended for providing adequate seismic performance. Finally, a simple approach is suggested for prediction of the bending capacity of existing connections.     

Pushover analysis of reinforced concrete frames considering shear failure at beam-column joints

Since most current seismic capacity evaluations of reinforced concrete(RC) frame structures are implemented by either static pushover analysis(PA) or dynamic time history analysis,with diverse settings of the plastic hinges(PHs) on such main structural components as columns,beams and walls,the complex behavior of shear failure at beam-column joints(BCJs) during major earthquakes is commonly neglected.This study proposes new nonlinear PA procedures that consider shear failure at BCJs and seek to assess the actual damage to RC structures.Based on the specifications of FEMA-356,a simplified joint model composed of two nonlinear cross struts placed diagonally over the location of the plastic hinge is established,allowing a sophisticated PA to be performed.To verify the validity of this method,the analytical results for the capacity curves and the failure mechanism derived from three different full-size RC frames are compared with the experimental measurements.By considering shear failure at BCJs,the proposed nonlinear analytical procedures can be used to estimate the structural behavior of RC frames,including seismic capacity and the progressive failure sequence of joints,in a precise and effective manner.     

不同支撑形式高强钢组合偏心支撑抗震性能研究

耗能梁段作为偏心支撑结构的耗能元件,在大震作用下通过弹塑性变形吸收地震能量,保护主体结构处于弹性受力状态。现行规范基于强度的设计理论,为了保证耗能梁段进入塑性或破坏,梁柱构件需要进行放大内力设计,导致截面过大,而且基于强度的设计方法很难保证结构的整体破坏状态。目前,抗震设计越来越重视基于性能的设计思想,该方法能够评估结构的弹塑性反应。对于高强钢组合偏心支撑,其中耗能梁段和支撑采用Q345钢,框架梁柱采用Q460或者Q690高强度钢材,高强钢不仅带来良好的经济效益,而且能够推广高强钢在抗震设防区的应用。利用基于性能设计方法设计了4种不同形式的高强钢组合偏心支撑钢框架,包括K形、Y形、V形和D形,考虑4层、8层、12层和16层的影响。通过Pushover分析和非线性时程分析评估该结构的抗震性能,研究结果表明:4种形式的高强钢组合偏心支撑钢框架具有类似的抗震性能,在罕遇地震作用下,几乎所有耗能梁段均参与耗能,而且层间侧移与耗能梁段转角沿高度分布较为均匀。其中:D形偏心支撑具有最大的抗侧刚度,但延性较差,而Y形偏心支撑的抗侧刚度最弱,但延性最佳。     

Seismic Performance of RC Frames Designed to Eurocode 8 or to the Greek Codes 2000

For the first time after the finalisation of the European Norm for seismic design of buildings (Eurocode 8 – EC8),the performance of RC buildings designed with this code is evaluated through systematic nonlinear analyses. Regular 4-, 8- or 12-storey RC frames are designed for a PGA of 0.2 or 0.4 g and to one of the three alternative ductility classes in EC8. As the Eurocodes are meant to replace soon existing national codes, design and performance is also compared to that of similar frames designed with the 2000 Greek national codes. The performance of alternative designs under the life-safety (475 years) and the damage limitation (95 years) earthquakes is evaluated through nonlinear seismic response analyses. The large difference in material quantities and detailing of the alternative designs does not translate into large differences in performance. Design for either Ductility Class High (H) or Medium (M) of EC8 is much more cost-effective than design for Ductility Class Low (L), even in moderate seismicity. It is also much more cost-effective than design to the 2000 Greek national codes.     

Low‐seismic damage strategies for infilled RC frames: shake‐table tests

Unreinforced masonry (URM) infill panels are widely used as partitions in RC frames and typically considered as non‐structural elements in the design process. However, observations from recent major earthquakes have shown that under seismic excitation, the structural interaction between columns and infill walls can significantly alter the structural behaviour, thus causing catastrophic consequences. The purpose of this research was to propose and test an innovative low seismic damage detailing method, which isolates the infill panel from bounding columns with finite width vertical gaps during the infill panel construction phase and deploys steel wire connections in mortar layers anchored to columns. Taking into account the similitude requirements, a total of six one‐third scale, single‐storey single‐bay RC frames with different infill configurations and flexible connection details were carefully designed and tested on a shake‐table. Three real earthquake records were selected and scaled to ascending intensity levels and used as input signals. A series of thorough investigations including dynamic characteristics, hysteretic behaviour, failure mechanisms, out‐of‐plane vulnerabilities and the effect of different gap filling materials and load transfer mechanisms were rigorously studied. The experimental results indicate that the undesirable interaction between infill panels and bounding frame is significantly reduced using the proposed low seismic damage detailing concept. Direct shear failure of columns at an early stage is prevented, and structural redundancy at high levels of excitation can be provided. In general, the structural stability and integrity, and displacement ductility of infilled RC frames can remarkably be improved. Copyright © 2017 John Wiley & Sons, Ltd.     

Experimental behavior of transfer story connections for high‐rise SRC structures under seismic loading

Results from an investigation aimed at assessing seismic behavior of transfer story connections for high‐rise building consisting of steel‐reinforced concrete (SRC) frame and reinforced concrete (RC) core tube are presented. Two types of transfer story connections were experimentally evaluated for adequate strength, ductility and energy dissipation. For each type of connection, two large‐scale subassembly tests were carried out under monotonic and cyclic lateral displacement, respectively. Detailed observations and behavior responses were obtained to contrast the differences between monotonic and cyclic performance of the connections. Test results showed that the SRC column failed before connection collapse and that loading types have little effect on the strength but greatly affect the failure modes and the ductility of the connections. All specimens exhibited good properties for earthquake resistance since they all kept a stable inelastic behavior up to the interstory drift demand suggested by the AISC Seismic Provisions. Based on test observations, support stiffeners with appropriate width‐to‐thickness ratio and mechanical connectors connecting bars with the steel plate are recommended for design purposes in order to achieve more ductile and reliable seismic behavior of transfer story connections. Copyright © 2010 John Wiley & Sons, Ltd.     

碳纤维布增强混凝土平面框架节点抗剪性能试验研究与机理分析

混凝土平面框架节点核心区不存在直交梁的影响,可以应用碳纤维布直接进行加固,以增强地震作用下节点核心区的抗剪承载力。根据足尺混凝土框架节点低周反复加载试验结果,对碳纤维布的增强机理进行了分析和讨论,并在此基础上提出了实用的加固计算公式。     

Study on the effect of the infill walls on the seismic performance of a reinforced concrete frame

Motivated by the seismic damage observed to reinforced concrete (RC) frame structures during the Wenchuan earthquake, the effect of infill walls on the seismic performance of a RC frame is studied in this paper. Infill walls, especially those made of masonry, offer some amount of stiffness and strength. Therefore, the effect of infill walls should be considered during the design of RC frames. In this study, an analysis of the recorded ground motion in the Wenchuan earthquake is performed. Then, a numerical model is developed to simulate the infill walls. Finally, nonlinear dynamic analysis is carried out on a RC frame with and without infill walls, respectively, by using CANNY software. Through a comparative analysis, the following conclusions can be drawn. The failure mode of the frame with infill walls is in accordance with the seismic damage failure pattern, which is strong beam and weak column mode. This indicates that the infill walls change the failure pattern of the frame, and it is necessary to consider them in the seismic design of the RC frame. The numerical model presented in this paper can effectively simulate the effect of infill walls on the RC frame.     

A novel structural detailing for the improvement of seismic and progressive collapse performances of RC frames

Earthquake-induced building collapse and progressive collapse due to accidental local failure of vertical components are the two most common failure modes of reinforced concrete (RC) frame structures. Conventional design methods usually focus on the design requirements of a specific hazard but neglect the interactions between different designs. For example, the progressive collapse design of an RC frame often yields increased reinforcement and flexural strength of the beams. As a result, the seismic design principle of “strong-column-weak-beam” may be violated, which may lead to unfavorable failure modes and weaken the seismic performance. To avoid these adverse effects of the progressive collapse design on the seismic resistance of RC frames, a novel structural detailing is proposed in this study. The proposed detailing technique intends to concurrently improve the seismic and progressive collapse performances of an RC frame by changing the layout of the newly added longitudinal reinforcement against progressive collapse without introducing any additional reinforcement. A six-story RC frame is used as the prototype building for this investigation. Both cyclic and progressive collapse tests are conducted to validate the performance of the proposed structural detailing. Based on the experimental results, detailed finite element (FE) models of the RC frame with different reinforcement layouts are established. The seismic and progressive collapse resistances of different models are compared based on the incremental dynamic analysis (IDA) and nonlinear dynamic alternate path (AP) methods, respectively. The results indicate that the proposed structural detailing can effectively resolve the conflict between the seismic and progressive collapse designs.     

不同设防标准RC框架结构基于易损性分析的抗震性能评估

借助非线性动力时程分析,对严格按照规范Ⅶ、Ⅷ、Ⅸ度设计的5个三跨6层钢筋混凝土框架结构开展易损性分析,建立了基于峰值加速度的易损性曲线。从易损性的角度对不同设防标准RC框架结构的抗震性能做了定量评价,并探讨了设防标准对RC框架结构易损性的影响。分析表明,对应于设防小震、中震及大震水平的峰值加速度,结构“小震不坏”、“中震可修”和“大震不倒”的失效概率均在18%以内,可认为结构满足三水准的性态控制目标。随着结构设防标准的提高,其易损性随之降低,相同峰值加速度对应的各个破坏状态的超越概率均有所降低。此外,将框架结构的设防烈度提高1度,其“大震不倒”的失效概率会明显减小。而将框架结构的设防烈度降低1度,其“大震不倒”的失效概率会显著增加,最高可达4倍。     

Seismic vulnerability of reinforced concrete buildings with discontinuity in columns

Irregular reinforced concrete (RC) buildings constitute a significant portion of the existing housing stock. A common type of irregularity is in the form of discontinuity in the vertical framing elements, which can exacerbate their seismic vulnerability. The design guidelines available in seismic design codes essentially cater to only regular buildings, and the safety of such buildings, even when the other guidelines of the codes are followed, is doubtful. This article evaluates the vulnerability of RC frame buildings with discontinuity in columns designed for modern seismic codes, in the form of seismic collapse capacity, collapse resistance against maximum earthquake demand level, and failure mechanism. The adequacy and limitations of the provisions of the seismic design codes are evaluated for such buildings. Analysis results show that the sequential analysis of buildings considering the construction staged effects, considerably affects the design and hence the collapse failure mechanism of even low- and mid-rise buildings. The results also underline the importance of strong column–weak beam design in the seismic performance of the floating column buildings. The vertical component of ground motion is also observed to be relatively more crucial in floating column buildings.     

Field investigation on severely damaged aseismic buildings in 2014 Ludian earthquake

The 2014 magnitude 6.5 Ludian earthquake caused a death toll of 617, many landslides and tens of thousands of collapsed buildings. A field investigation to evaluate the damage to buildings was carried out immediately after the occurrence of the earthquake. Severely damaged aseismic buildings, which were basically observed in the downtown of Longtoushan Town, were carefully examined one by one with the aim to improve design codes. This paper summarizes the damage observed to the investigated aseismic buildings in both the structural and local levels. A common failure mode was observed that most of the aseismic buildings, such as RC frame structures and confined masonry structures, were similarly destroyed by severe damage or complete collapse of the first story. The related strong ground motion, which was recorded at the nearby station, had a short duration of less than 20 s but a very large PGA up to 1.0 g. The RC frames based on the new design codes still failed to achieve the design target for "strong column, weak beam". Typical local failure details, which were related to the interaction between RC columns and infill walls and between constructional columns and masonry walls, are summarized with preliminary analyses.     

Optimal seismic design of reinforced concrete structures under timehistory earthquake loads using an intelligent hybrid algorithm

A reliable seismic-resistant design of structures is achieved in accordance with the seismic design codes by designing structures under seven or more pairs of earthquake records. Based on the recommendations of seismic design codes, the average time-history responses(ATHR) of structure is required. This paper focuses on the optimal seismic design of reinforced concrete(RC) structures against ten earthquake records using a hybrid of particle swarm optimization algorithm and an intelligent regression model(IRM). In order to reduce the computational time of optimization procedure due to the computational efforts of time-history analyses, IRM is proposed to accurately predict ATHR of structures. The proposed IRM consists of the combination of the subtractive algorithm(SA), K-means clustering approach and wavelet weighted least squares support vector machine(WWLS-SVM). To predict ATHR of structures, first, the input-output samples of structures are classified by SA and K-means clustering approach. Then, WWLS-SVM is trained with few samples and high accuracy for each cluster. 9- and 18-storey RC frames are designed optimally to illustrate the effectiveness and practicality of the proposed IRM. The numerical results demonstrate the efficiency and computational advantages of IRM for optimal design of structures subjected to time-history earthquake loads.     

An alternative approach for the seismic rehabilitation of existing RC buildings using seismic isolation

Usually, buildings with seismic isolation are designed to comply with an operational building performance level after strong earthquakes. This approach, however, may limit the use of seismic isolation for the seismic rehabilitation of existing buildings with low lateral strength or substandard details, because it often requires invasive strengthening measures in the superstructure or the use of expensive custom‐made devices. In this paper, an alternative approach for the seismic rehabilitation of existing buildings with seismic isolation, based on the acceptance of limited plastic deformations in the superstructure under strong earthquakes, is proposed and then applied to a real case study, represented by a four‐storey RC frame building. Nonlinear response‐time histories analyses of an accurate model of the case‐study building are carried out to evaluate the seismic performances of the structure, comparing different rehabilitation strategies with and without seismic isolation. Initial costs of the intervention and possible (future) repair costs are then estimated. Based on the results of this study, values of the behavior factor (i.e. response modification factor) higher than those adopted in the current codes for base‐isolated buildings are tentatively proposed. Copyright © 2015 John Wiley & Sons, Ltd.     

Seismic performance and global ductility of RC frames rehabilitated with retrofitted joints by CFRP laminates

This paper presents a new FRP retrofi tting scheme to strengthen local beam-column joints in reinforced concrete(RC) frames.The new retrofi tting scheme was proposed following a preliminary study of four different existing retrofi tting schemes.A numerical simulation was conducted to evaluate the effectiveness of FRP-strengthened reinforced concrete frames by bridging behavior of local joints to the whole structure.Local confi nement effects due to varying retrofi tting schemes in the joints were simulated in the frame model.The seismic behavior factor was used to evaluate the seismic performance of the strengthened RC frames.The results demonstrated that the new proposed retrofi tting scheme was robust and promising,and fi nite element analysis appropriately captured the strength and global ductility of the frame due to upgrading of the local joints.