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.     

Characteristics of CFRP retrofitted hollow brick infill walls of reinforced concrete frames

The mechanical characteristics of infill walls retrofitted with carbon fiber reinforced polymer (CFRP) sheets are really important for the realistic prediction of seismic performance of the vulnerable reinforced concrete (RC) frames retrofitted through CFRP strengthened infill walls. In this study, 36 hollow brick wall specimens were tested either under uniaxial compression or diagonal tension before and after retrofitting externally with CFRP sheets. The test parameters are the dimensions of the walls, the orientation of holes of bricks, the type of mortar, the amount of CFRP sheets and the details of strengthening application. At the end of the tests, a significant contribution of CFRP sheets on the mechanical characteristics of hollow brick walls was observed in terms of several important structural design parameters such as Young and shears moduli, axial and shears strengths as well as the deformation capacity. Finally, the strength and deformability characteristics of the walls and frames retrofitted with CFRP sheets were predicted analytically. The predictions were in good agreement with the experimental data.     

Effect of different size of joint enlargement on seismic behavior of gravity load designed RC beam‐column connections

Effect of different size of planer joint enlargement as a noninvasive and practical strategy for seismic retrofit of gravity load designed external reinforced concrete beam‐column connections was experimentally investigated. The joint region was enlarged using steel angles that are mounted using prestressed cross‐ties. Reverse cyclic load tests on five half‐scale control and retrofitted external RC beam‐column connections were conducted. Three different size of planer joint enlargement being 180, 140, and 90 mm were considered for retrofitted specimens. The performance of the retrofitted specimens is compared with that of the control gravity load designed beam‐column connections, in terms of load–displacement hysteresis curve, energy dissipation and ductility capacities, and global strength and stiffness degradation behavior. The experimental results showed that increasing the size of planer joint enlargement significantly enhances the seismic capacity of the retrofitted connections, in terms of strength, stiffness, energy dissipation, and ductility capacity and also planer joint enlargement can relocate beam plastic hinges to outside the joint panel.     

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。     

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。     

多层砌体填充墙框架结构抗震性能试验研究

为了研究砌体填充墙沿框架层不连续布置对框架结构抗震性能的影响,进行了3榀两层单跨砌体填充墙框架结构模型、1榀单层单跨砌体填充墙框架结构模型、1榀两层单跨框架结构模型和1榀单层单跨框架结构模型的对比试验,分析了各试件的破坏特征、滞回曲线、骨架曲线、位移延性、刚度退化、承载力退化和耗能性能等抗震性能指标。结果表明：无论是单层单跨还是两层单跨的砌体填充墙框架结构,其水平峰值荷载和初始刚度比相应的纯框架结构均有较大幅度的提高,且其刚度退化程度比相应纯框架结构要缓慢;砌体填充墙的存在提高了框架结构的抗侧刚度和水平峰值荷载,使框架结构的变形由剪切型逐渐转变为弯剪型;砌体填充墙参与了结构的滞回耗能,填充墙框架的位移延性和累积耗能能力明显优于框架;砌体填充墙沿框架层不连续布置会引起框架结构层间侧移刚度和层间受剪承载力发生突变,影响框架结构的破坏形态,但由于砌体填充墙参与了框架结构的滞回耗能,故其仍具有较好的抗震性能。     

不同连接方式砌体填充墙钢筋混凝土框架抗震性能试验研究

填充墙的构造形式对框架结构的抗震性能影响较大。试验设计了7榀足尺的单层单跨钢筋混凝土框架,其中6榀为带加气混凝土砌块填充墙框架,1榀为空框架,对其进行低周反复荷载作用下的抗震性能试验。填充墙与框架的连接方式采用柔性连接和刚性连接两种。柔性连接填充墙的变化参数主要包括构造柱的构造形式和数量、框架柱中拉结筋的设置与否和填充墙上设置竖向缝的数量。分析了不同填充墙构造形式框架的破坏特征、荷载 位移滞回曲线、骨架曲线、位移延性系数、刚度退化、耗能能力。结果表明：柔性连接填充墙框架的抗震性能介于刚性连接填充墙框架和空框架之间;对于柔性连接填充墙框架,填充墙的构造措施对框架结构的抗震性能有一定的影响,但影响不大。     

Behavior and design of reinforced concrete frames retrofitted with steel bracing against progressive collapse

Steel bracing is able to improve progressive collapse resistance of reinforced concrete (RC) frames, but the bracing design is typically based on seismic retrofitting or lateral stability. There is no approach for design of steel bracing against progressive collapse. To this end, a retrofitting approach with steel braces is proposed based on analysis of macro finite element (FE) models with fiber beam elements. The FE models were initially validated through the experimental results of a braced frame and then used to investigate the effects of pertinent parameters on the progressive collapse resistance of planar frames. The results suggest the braces should be placed at the top story. Thereafter, macro FE models are built to investigate the dynamic responses of the three‐dimensional prototype RC frames under different column removal scenarios (CRS) and show the necessity of retrofitting. Accordingly, the design approach of steel bracing is proposed with incremental dynamic analysis (IDA) and assuming independent contribution of braces and frames to resistance. Finally, the fragility analysis of the frames under a corner‐penultimate‐exterior CRS is conducted through IDA and Monte Carlo simulation, and the results confirm the validity of the proposed design approach for retrofitting RC frames.     

不同连接方式下新型砌体填充墙框架结构的抗震性能

试验设计了4榀足尺框架结构,其中1榀为空框架,3榀为带新型横孔空心砌块砌体填充墙,带填充墙框架试件分为刚性连接试件和柔性连接试件2种。对各试件在恒定竖向力和水平低周反复荷载作用下的抗震性能进行试验,研究了框架在不同连接形式下的破坏特征、滞回特性、骨架曲线、位移延性、刚度退化、强度退化、耗能能力。结果表明:柔性连接试件抗震性能介于空框架和刚性连接试件之间,框架梁和填充墙采用拉结筋连接试件的抗震性能相对于未设置拉结筋试件有所改善,但提高幅度有限。     

预应力钢带加固钢筋混凝土柱抗震性能试验研究

提出一种预应力钢带加固钢筋混凝土柱的新技术,并通过预应力钢带加固高轴压比钢筋混凝土矩形截面柱的拟静力试验,对该预应力钢带加固方法进行初步研究。在试验中主要考察轴压比和预应力钢带间距两个主要参数对加固后钢筋混凝土矩形截面柱抗震性能的影响规律。结合试验研究结果,对加固试件和未加固的对比试件的滞回曲线、骨架曲线、位移延性、刚度退化和耗能性能等抗震性能指标进行了比较研究。研究结果表明,预应力钢带加固技术可以明显改善钢筋混凝土柱抗震性能,具有良好应用前景。     

对部分填充钢筋混凝土框架的试验研究

通过在合适的地方填充钢筋混凝土是一种很常用的加固方法。有时考虑到建筑上的需要,在加固中会将窗户或者门的开口留出来。但是目前我们对在水平循环荷载作用下的部分填充钢筋混凝土结构的行为还不十分了解。本研究旨在探究水平循环荷载下,通过部分填充而加固的柔性钢筋混凝土结构的性能。试验对象为9个三分之一比例的单层单开间模型,对其施加反复荷载。填充墙的纵横比(lw/hw,长度/高度)和安放的位置是试验中需要考虑的两个参数。试验结果指出,与无填充墙的结构相比,部分填充的RC结构显示出了明显更高的最终强度和更高的初始刚度。当填充墙的纵横比增大,结构的抗侧强度和刚度也有显著增加。此外,从试验中还可以看到,结构和填充墙之间的连接同样会影响填充结构的性能,那些同时连接到柱子和梁的部分填充墙模型显示出了最优的性能。     

既有混凝土框架结构减震加固有限元分析——用带防屈曲支撑的内嵌钢框架加固

历次地震表明既有混凝土框架结构在地震中表现为承载力不足、难以满足规范全部抗震设防目标。在已完成两榀用内嵌钢框架+防屈曲支撑、一榀纯框架试验模型的基础上进行有限元非线性,以便分析结构内力重分配规律,研究防屈曲支撑与原框架结构的工作性能。得出以下结论:1)有限元模拟与试验结果吻合较好,验证了有限元模型合理性;2)内嵌钢框架与原有框架结构协同工作良好,承担部分竖向力,说明该加固方法不增加原结构竖向负担;3)加固后的整体框架内力重分配规律为向着结构发挥延性的方面发展,对加固方法及需要注意的问题提出了建议,能为今后采用此类方法加固工程起到指导作用。     

考虑全填充墙作用的钢筋混凝土框架抗连续倒塌性能分析

为探究钢筋混凝土(RC)全填充墙框架在边中柱失效情况下的抗连续倒塌性能及其承载力计算方法,基于已有试验和有限元程序OpenSees建立宏观有限元数值模型展开研究。数值模型中的梁柱采用基于力的纤维梁单元模拟,填充墙则转化为等效斜撑并用桁架单元进行表征。填充墙宏观模型涉及等效斜撑的宽度、数量和相应材料属性的确定,为此对比了不同等效斜撑模型的适用性,确定了一种连续倒塌工况下全填充墙的宏观模拟方案。进而利用验证的数值模型,揭示全填充墙框架防倒塌的荷载传递机制,并研究了层数和填充墙砌体抗压强度对抗倒塌性能的影响。结果表明：全填充墙框架荷载主要通过墙体对角传递且全填充墙会与周围框架形成一种桁架机制;随填充墙砌体抗压强度降低,结构抗力峰值呈下降趋势。最后,以填充墙和周边框架竖向承载刚度比为基本参数,建立了通过求得填充墙和框架刚度以及纯框架理论弯曲承载力,便可快速评估规则RC填充墙框架防倒塌能力的回归模型。     

基于易损性的RC空间框架填充墙结构连续倒塌能力分析

为研究填充墙对RC空间框架抗竖向连续倒塌能力的影响,对一栋6层双跨RC框架结构采用OpenSees有限元软件进行了建模,利用集中塑性的beamWithHinges单元模拟填充墙的性能。考虑到结构的不确定性,在有限元模型的基础上采用改进的拉丁超立方体抽样法获得了性能不同的100个纯框架结构和100个框架填充墙结构,并采用PDA方法和竖向IDA方法对取得的样本进行了易损性分析和对比。分析结果表明：填充墙提高了框架结构的抗倒塌能力,发生连续倒塌概率相同的情况下,框架填充墙结构的承载能力要比纯框架结构的高10%~20%。此外,两种分析方法对纯框架结构的抗连续倒塌能力分析差别较小,而对于带填充墙的框架结构,PDA方法计算结果则偏于保守。     

Seismic retrofitting of nonductile beam-column sub-assemblage using FRP wrapping and steel plate jacketing

The paper discusses the aspects of repair and retrofitting technique adopted for a damaged reinforced concrete beam-column joint specimen under cyclic loading. A specimen designed based on Indian Standard specifications with consideration of seismic load but without adopting ductile detailing (NonDuctile) was investigated under reverse cyclic loading. Then, the damaged nonductile specimen was repaired with epoxy mortar and grouted using low viscous polymer, and retrofitted using fiber reinforced plastic (FRP) wrapping in beam and column components and steel plate jacketing in joint region. The experimental results showed that the retrofitted specimen not only regained its original strength and stiffness but also has overcome the deficiencies of nonductile detailing. The present study shows that a proper repair and adequate retrofitting technique can be used for strengthening and improvement of damaged regions in reinforced concrete structures.     

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。     

Investigation on the behavior of brick-infilled steel frames with openings, experimental and analytical approaches

This article deals with an experimental program to investigate the in-plane seismic behavior of steel frames with clay brick masonry infills having openings. Six large-scale, single-story, single-bay frame specimens were tested under in-plane cyclic loading applied at roof level. The infill panel specimens included masonry infills having central openings of various dimensions. The experimental results indicate that infill panels with and without openings can improve the seismic performance of steel frames and the amount of cumulative dissipated energy of the infill panels with openings, at ultimate state are almost identical. Furthermore, contrary to the literature, the results indicate that infilled frames with openings are not always more ductile than the ones with solid infill. It seems that the ductility of such frames depends on the failure mode of infill piers. This experimental investigation shows that infilled frames with openings experienced pier diagonal tension or toe crushing failure and have smaller ductility factors than those frames with solid infill. Furthermore, a simple analytical method is proposed to estimate the maximum shear capacity of masonry infilled steel frames with window and door openings.     

Experimental study for seismic retrofit of non‐seismic designed reinforced concrete structures

An evaluation of seismic performance was conducted for a reinforced concrete (RC) structure designed without seismic considerations. The seismic performances of many existing structures are often inadequate based on the current seismic design codes, and it is sure that this is the general tendency in most countries. Six specimens were designed and constructed to simulate RC frames built in the 1980s, before the introduction of earthquake‐resistance design provisions in South Korea. The specimens were composed of one control specimen without any retrofit and five specimens retrofitted using wire‐bracing, brick infill wall, steel bracing (X‐ and K‐type) and corner gusset plate. All retrofitted specimens show significant increase in strength and ductility. The behaviours of each specimen are compared in the view of maximum strength, maximum moment, stiffness and energy dissipation capacity. Overall, efficiency analysis was also demonstrated in this study. Copyright © 2010 John Wiley & Sons, Ltd.     

Analytical investigation of the seismic performance of RC frames rehabilitated using different rehabilitation techniques

The objective of this study is to investigate analytically the effectiveness of different rehabilitation patterns in upgrading the seismic performance of existing non-ductile reinforced concrete (RC) frame structures. The study investigates the performance of two RC frames (with different heights representing low- and high-rise buildings) with or without masonry infill when rehabilitated and subjected to three types of ground motion records. The ground motion records represent earthquakes with low, medium and high frequency contents. Three models were considered for the RC frames; bare frame, masonry-infilled frame with soft infill, and masonry-infilled frame with stiff infill. Four rehabilitation patterns were studied, namely: (1) introducing a RC shear wall, (2) using steel bracing, (3) using diagonal FRP strips (FRP bracings) in the case of masonry-infilled frames, and (4) wrapping or partially wrapping the frame members (columns and beams) using FRP composites. Incremental Dynamic Analysis was conducted for the studied cases. The seismic performance enhancement of the studied frames is evaluated in terms of the maximum applied peak ground acceleration resisted by the frames, maximum inter-storey drift ratio, maximum storey shear-to-weight ratio and energy dissipation capacity.     

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.