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Seismic retrofitting solutions for reinforced concrete (RC) school building types in high-seismic regions are extensively reported in the state-of-the-art. Conversely, limited studies have focused on the extent of retrofitting needed for RC school buildings in low- to moderate-seismic regions. To explore this aspect, seismic retrofitting options for RC school buildings in Sri Lanka are investigated. Three retrofitting options are examined: (1) adding/altering masonry infill walls (MI walls) to reduce irregularity in buildings, (2) RC jacketing of columns and (3) a combination of adding/altering MI walls and RC jacketing. These retrofit options are applied to a common typology of Sri Lankan MI-RC school buildings, considering two and three storey height variations. A simplified numerical modelling approach that accounts for the contribution of MIs, the shear failure of RC column and torsional effects is adopted to analyse the performance of the school buildings with and without retrofit. Based on the analyses, three damage states are defined: damage limitation (DL), significant damage (SD) and near collapse (NC). Finally, a multi-criteria decision making (MCDM) method is used to determine the optimal retrofitting option for the considered school building typology, considering engineering and economic parameters. The optimal retrofit solution for the three-storey MI-RC school building is found to be jacketing of ground floor columns. Conversely, for the two-storey MI-RC school building, alteration of infill walls (MI walls) is deemed optimal. Finally, a sensitivity analysis is carried out on the MCDM method. [ABSTRACT FROM AUTHOR]

This study explored rehabilitation of severely damaged bridge pier models using reinforced concrete jacketing method adopting a plastic hinge relocation technique. The test specimens were tested under cyclic loading. The important seismic performance parameters were evaluated for the rehabilitated specimen. A 3D finite element model was simulated considering concrete-rebar interaction and jacket-pier interface shear. Calibration of the numerically simulated model was done by comparing with experimentally observed responses. Additionally, a parametric study was conducted to investigate the effect of shear connectors on performance of the rehabilitated test specimen. Rehabilitation of severely damaged scaled bridge pier model by RC jacketing following a systematically adopted step-by-step procedure. Shifting of plastic hinge location from the pier-foundation interface. Calibration of numerical model of RC jacketed pier incorporating concrete-rebar interaction and jacket-pier interface shear by comparing with experimentally observed responses. A parametric study to investigate the effect of shear connectors on the performance of RC jacketed bridge pier model. [ABSTRACT FROM AUTHOR]

The existing Reinforced Concrete (RC) School buildings are often characterized by low-rise, very long beams, and relatively large-valued uniformly distributed loads. Beams present a large amount of longitudinal reinforcement so that they exert huge shear stresses, on the joint panels into which they frame, during a seismic event. For this reason, the main cause of seismic inadequacy of these buildings is often the premature shear failure of the joints. These latter can result either (a) completely devoid of reinforcing stirrups, or (b) not suitably reinforced, or (c) under-dimensioned. In some cases, due to either the results of the feasibility study or to instructions given by the Contracting Authority, the seismic rehabilitation of such buildings must be obtained by local interventions. To rehabilitate an existing RC building with under-dimensioned joints, by means of local interventions, the availability of pourable and self-compacting High-Performance Fiber Reinforced Cementitious Composites (HP-FRCC) renders the RC jacketing the ideal solution. In cases in which an aggressive environment is concomitant with the susceptibility to earthquakes, an Engineered Cementitious Composite designed ad hoc could be an even better option. The RC jacketing of the joint allows to enlarge it, thus making it work in un-cracked regime. However, when the joint panel is enlarged, the appropriate hierarchy of resistances must be restored throughout the length of the column, to guarantee the activation of the desired global dissipative mechanism. In this article a calculation strategy to follow in these cases, in compliance with the Building Regulations, is proposed and described by referring to a case-study. Finally, a discussion is provided on some aspects, which are still susceptible to improvement, to refine the intervention technique. [ABSTRACT FROM AUTHOR]

One of the most common retrofitting processes for columns is reinforced concrete (RC) jacketing. This process enhances the ductility, stiffness, flexural capacity, and shear capacity of columns, and increases the axial load-carrying capacity of the RC column when subjected to an axial load. According to previous research, the most typical way to improve the lateral capacity of a column in composite action is column retrofitting with RC jacketing. The lateral strength of a single RC column and a single bay, single-story RC frame (retrofitted with concrete jacketing) is numerically analyzed in this work. To obtain a complete overview, an RC frame analysis is conducted even though a single column represents the column in the frame. Finally, parametric analysis is performed to assess the influence of reinforcement ratio, compressive strength of jacket concrete, and yield strength of reinforcement. The axial load carrying capacity and flexural capacity of retrofitted columns increase with the increase in RC jacket compressive strength, reinforcement percentage, and yield strength of the reinforcement. If the compressive strength of the jacket concrete stays constant, 1% reinforcement increment leads to a 10% to 20% increase in the axial capacity of the column. The parametric study reveals that retrofitted columns and frames exhibit a substantial increase in lateral capacity (10 to 12 times) compared to existing columns. [ABSTRACT FROM AUTHOR]

Revision of design standards, new regulations, and planned/unplanned increases in loadings potentially make buildings vulnerable to changes in capacity demands. This may result in some structural components of an existing structure requiring strength or stiffness improvement. Strengthening of capacity deficit members is widely recognised as the preferred and sustainable choice in terms of conserving resources and minimising the carbon footprint. Strengthening techniques such as Reinforced Concrete (RC) jacketing, steel jacketing and Fibre Reinforced Polymer (FRP) wrapping have been explored and widely used. However, when adopted solely, each scheme has its drawbacks. RC jacketing results in a considerable rise in the dead load, influencing the loading on the foundation and restricting the functional space available post-jacketing. Steel jacketing often lacks flexibility as a significant effort is required for installation. FRP improves the flexural strength response but produces low stiffness enhancement and lesser ductility. A hybrid strengthening option, i.e. a combination of the above strengthening methods, presents a viable option to address the limitations cost-effectively. This paper presents a pioneering attempt focused on improving the flexural performance of RC beams using a hybrid strengthening scheme comprising of RC jacketing and FRP. Four large-scale RC beams were prepared, and three of them were strengthened. The first was strengthened by RC jacketing, the second by a combination of RC jacketing and flexural FRP, and the last by the combination of RC jacketing, flexural FRP and FRP 'U wrap' in the shear zones. The specimens were tested using the four-point bending test. The flexural behaviour was assessed in terms of the peak moment capacity, flexural stiffness, load-displacement response, crack pattern and failure modes. The test results confirmed that hybrid strengthening leads to strength gain and stiffness gains of 105% and 90% respectively, as compared with the unstrengthened beam. The efficacy of the theoretically predicted strengths against the test strengths is assessed. • Development of hybrid strengthening scheme for capacity deficit RC beams. • Various combinations of strengthening schemes were explored. • Large-scale tests conducted to determine the efficacy of the strengthening schemes. • Feasibility of combining RC jacketing & FRP for strength/stiffness gains is confirmed. • Theoretically predicted strengths match with the test strengths with sufficient accuracy. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical finite element study on effect of different strengthening techniques on structural behavior of the reinforced concrete frames subjected to earthquake motion. In the study, diagonal cross bracing, masonry infill wall, externally CFRP laminates, and reinforced concrete jacketing for columns are considered as different strengthening techniques of one story reinforced concrete bare frame. Time history analysis for the frame subjected to EL-Centro (1940 N-S) earthquake ground motion is carried out using ETABS software. The obtained results from the time history analysis including axial and shear stresses, lateral displacement, acceleration and base shear with time are illustrated and discussed with/ or without the strengthening techniques. The study illustrates the influence and enhancement of each technique on the performance of one story bare frame subjected to EL-Centro earthquake motion. [ABSTRACT FROM AUTHOR]

In Southern European countries several existing reinforced concrete (RC) buildings were designed before the introduction of modern seismic codes and thus they may be potentially vulnerable to horizontal loads. Recent seismic events have also shown that RC buildings designed without specific seismic provisions can be subjected to meaningful damages or even collapse during moderate-to-strong earthquakes. In this framework, straightforward methodologies for a preliminary and suitable seismic assessment and retrofitting of existing RC buildings are required, along with reliable and effective seismic rehabilitation techniques. In this study, a simplified displacement based procedure using non-linear static analyses is applied to obtain a preliminary estimation of the overall inadequacy of an under-designed four-storey RC frame and to propose suitable retrofitting interventions based on different rehabilitation strategies. To this aim, accurate numerical models are developed to simulate the seismic response of the RC frame in the original and retrofitted configurations. The effectiveness of three different retrofitting solutions countering the main structural deficiencies of the RC frame is evaluated through the displacement based approach. Then, non-linear dynamic analyses are carried out to assess and compare the seismic performance of the RC frame in the original and retrofitted configurations. A combined use of different approaches may represent a valuable tool to accurately address the retrofitting interventions and to assess their effectiveness in order to reduce the seismic vulnerability of poorly designed RC buildings. [ABSTRACT FROM AUTHOR]

Among the different strengthening techniques to repair RC structures, reinforced concrete (RC) jacketing is one of the most commonly adopted, especially for columns. Its wide application is due to its easy application and relatively reduced cost with respect to other methods (e.g. FRP wrapping, Shape Memory Alloy active confinement). The target of RC jacketing is to increase axial, flexural capacity and ductility of weak existing members by means of two main effects: confinement action provided by the jacket and composite action between external jacket and inner concrete. Different theoretical studies have been carried out to calculate the strength enhancement due to confinement action and most of these are based on the adaptation of classical models for confinement evaluation in RC members. However, experimental investigations on compressive behaviour of RC jacketed sections have shown that the actual axial capacity could be substantially lower than that analytically evaluated with classical confinement models. This fact could be explained by taking into account the presence of tensile stresses developing in the jacket, due to the different dilatation of the inner core and the external jacket. This phenomenon is relevant especially in members where the concrete properties of the jacket are different with respect to those of the core, causing the premature failure of the external layer. This paper presents a simplified approach able to evaluate these effects. In particular RC jacketed circular columns are analysed as case study and the member is studied by considering the different concrete properties of core, jacket and cover. Circumferential and radial stresses are firstly calculated under the assumption of linear elastic behaviour and plane strain state. The model is extended in the non-linear range by adopting a secant constitutive law. Finally, comparisons are made with experimental data available in the literature, showing good agreement and explaining some experimental results. [ABSTRACT FROM AUTHOR]

Bridge’s piers can be damaged by the action of intense earthquakes. These elements are the most important component of the structural stability of a bridge system. After the occurrence of an earthquake or after updates in the design code, the piers may require an intervention to improve their strength and stiffness capacities. For a long time, RC jacketing has been used as a suitable technique to provide additional strength and to improve the ductile behavior of columns under seismic loads. There are several studies related to the behavior of bridges retrofitted with RC jacketing, but there is a lack of quantitative evaluations for this technique regarding the seismic vulnerability of a bridge. This work investigates RC bridges with a substructure conformed with a bent cap joined to a RC column retrofitted with RC jacketing. The effects of the retrofit system are based on fragility curves estimated through a parametric analysis that considers six retrofit cases, and several seismic scenarios to determine four limit states. The results quantify the effect of the jacket thickness and the longitudinal reinforcement ratio for the probability of reaching several limit states of behavior. In all cases, RC jacketing increases the shear capacity of the pier, and it reduces the bridge’s seismic vulnerability. It was also found that the longitudinal steel ratio is the parameter with the most influence on the piers’ seismic vulnerability, and the results allow inferring the most suitable parameters that can be used as RC jacketing. [ABSTRACT FROM AUTHOR]

Reinforced concrete (RC) jacketing is a common method for retrofitting existing columns with poor structural performance. It can be applied in two different ways: if the continuity of the jacket is ensured, the axial load of the column can be transferred to the jacket, which will be directly loaded; conversely, if no continuity is provided, the jacket will induce only confinement action. In both cases the strength and ductility evaluation is rather complex, due to the different physical phenomena included, such as confinement, core–jacket composite action, preload and buckling of longitudinal bars. Although different theoretical studies have been carried out to calculate the confinement effects, a practical approach to evaluate the flexural capacity and ductility is still missing. The calculation of these quantities is often related to the use of commercial software, taking advantage of numerical methods such as fibre method or finite element method. This paper presents a simplified approach to calculate the flexural strength and ductility of square RC jacketed sections subjected to axial load and bending moment. In particular the proposed approach is based on the calibration of the stress-block parameters including the confinement effect. Equilibrium equations are determined and buckling of longitudinal bars is modelled with a suitable stress–strain law. Moment–curvature curves are derived with simple calculations. Finally, comparisons are made with numerical analyses carried out with the code OpenSees and with experimental data available in the literature, showing good agreement. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental study on retrofitting RC beams using FRP strengthening and RC jacketing. Three-point loadings are applied to twelve strengthened beams and three reference specimens to put the two techniques into perspective. Formation of first cracks, FRP debonding, and onset of concrete crushing are compared and discussed. Load–deflection behavior, failure modes, and crack propagation patterns are studied extensively. The experiments demonstrate that the RC jacketed beams exhibit a flexural failure identical to the reference beams. However, the FRP strengthened beams fail in a relatively small deflection due to the premature FRP debonding. Furthermore, the both techniques noticeably enhance strength and energy dissipation capacity. The applied RC jacketing can also improve ductility; however, the FRP method cannot provide sufficient ductility. Therefore, the RC jacketing can be used in seismic zones as an alternative to the FRP method. In addition, conventional analytical methods are applied for evaluation of ultimate moment, and the results are compared with the experiments. Good agreement between the results verifies the analyses. [ABSTRACT FROM AUTHOR]

The main objective of this study is to analytically investigate the effectiveness of different strengthening solutions in upgrading the seismic performance of existing reinforced concrete (RC) buildings in Nepal. For this, four building models with different structural configurations and detailing were considered. Three possible rehabilitation solutions were studied, namely: (a) RC shear wall, (b) steel bracing, and (c) RC jacketing for all of the studied buildings. A numerical analysis was conducted with adaptive pushover and dynamic time history analysis. Seismic performance enhancement of the studied buildings was evaluated in terms of demand capacity ratio of the RC elements, capacity curve, inter-storey drift, energy dissipation capacity and moment curvature demand of the structures. Finally, the seismic safety assessment was performed based on standard drift limits, showing that retrofitting solutions significantly improved the seismic performance of existing buildings in Nepal. [ABSTRACT FROM AUTHOR]

• An innovative seismic retrofit methodology combining stiffness retrofit with inerters is proposed for low-rise buildings. • The approach resolves shortcomings of traditional stiffness retrofit techniques. • The approach reduces the required stiffening and foundation strengthening. • Both displacement and acceleration responses can be reduced. • The practicality of the retrofit approach is addressed. Stiffness retrofit is a very effective technique to increase the seismic capacity of low-rise buildings. However, the shift of the fundamental period might lead to potential adverse effects when considering the spectral content of the ground motions. To address this issue, this paper proposed a fundamental-period-preserving (FPP) retrofit methodology. By augmenting traditional stiffness retrofit techniques with inerters, the fundamental period can be preserved. In addition, the normalized seismic input force and higher mode effects can be reduced. Different retrofit examples were evaluated through a series of nonlinear static and nonlinear dynamic analyses. Different performance objectives, including roof drift, story drift, story acceleration, base shear, and the force generated by inerters were used to demonstrate the effectiveness and the applicability of FPP approach in structure retrofit. The results showed that the FPP approach is an attractive alternative to the seismic retrofit of low-rise buildings. [ABSTRACT FROM AUTHOR]

Reinforced concrete (RC) jacketing is nowadays one of the most common techniques adopted for seismic retrofitting of existing RC columns. It is used to increase load-carrying capacity and ductility of weak existing members by means of a simple and cheap method. The structural efficiency is related to two main effects: – the enlargement of the transverse cross section; – the confinement action provided by the external jacket to the inner core. Several theoretical and experimental studies were addressed in the past to investigate on how it is possible to calculate the strength enhancement due to these effects and to highlight the main key parameters influencing the structural behavior of jacketed columns. Most of theoretical studies analyzed members subjected to axial compression while the case of axial force and bending moment was adapted only with complex formulations based on numerical approaches, which require the use of a suitable algorithm (e.g. non-linear finite element analyses, sectional fiber models). This paper presents a simplified approach, able to calculate the strength domains for jacketed columns subjected to axial force and uniaxial bending moment. The model takes into account the effects of confinement with proper stress-block parameters, the latter adapted for confined concrete, and of the composite action of jacket and core; buckling of longitudinal bars is considered and discussed with an appropriate stress–strain law for steel in compression. Results are compared with numerical analyses carried-out with the fiber model approach implemented in a commercial software (SAP2000), showing the accuracy of proposed method. Comparisons are also made with experimental results available in the literature in order to validate the model. Finally parametric considerations are made on the basis of adopted model, useful for design/verification purposes. [ABSTRACT FROM AUTHOR]

An original reinforced concrete (RC) column and four strengthened specimens, two with RC jackets and two with wing walls, were tested in this study. The original column specimen was designed to comply with older (pre-1999) design standards so that the usual detailing deficiencies in existing school buildings in Taiwan could be simulated. Two different structural details were chosen to fabricate the full-scale specimens for each retrofitting technique. The study confirmed that either RC jacketing or the installation of wing walls with two different structural details can effectively improve the stiffness and strength of an existing column. RC jacketing shows a better improvement in energy dissipation and ductility when compared to the columns with wing walls installed. This is because the two RC jacketed columns experienced a flexural failure, while a shear failure was found in the two columns with the wing walls installed, and thus led to a drastic decrease of the maximum lateral strengths and ductility. Since many factors may affect the installation of a post-installed anchor, it is better to use standard hooks to replace post-installed anchors in some specific points when using RC jacketing or installing wing walls. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the results of cyclic load tests on masonry walls performed for the purpose of evaluation of in-plane shear behaviour and identification of shear strength, stiffness and energy dissipation. Eight walls in two series were assembled in laboratory conditions. The first series consisted of four unreinforced masonry walls constructed from solid clay bricks and lime mortar. The walls from the second series were strengthened by application of RC jackets on both sides. These were constructed of the same material and were characterized by the same geometry properties and vertical load levels as those of the walls from the first series. The main goal of the tests was to compare the behaviour of the unreinforced and strengthened walls under cyclic horizontal load. The results from the tests showed that the application of the strengthening method contributed to a significant improvement of the shear resistance of the jacketed walls. Analytical models were used to predict the shear resistance of the walls. Good agreement with the experimental results was obtained with a model based on tensile strength of masonry. [Copyright &y& Elsevier]

This article presents a novel approach for deriving Retrofit Design Spectra (RDS) that are intended for use in preliminary development and assessment of seismic upgrading scenarios of existing structures. The new spectral representation relates the characteristics of the intervention method chosen as the core of the upgrading strategy, with the ductility and strength demand of the retrofitted structure. The methodology utilized for the derivation of the RDS is based on the Capacity Spectrum Method where the capacity curve is described by relationships for global and local intervention methods that are parameterized in terms of fundamental response quantities. The proposed spectra provide direct insight into the complex interrelation between the characteristics of the intervention method and the implications of the upgrading scenario on demand. Alternative retrofit solutions are thus assessed in an efficient way. A case study is used to illustrate practical application of the new approach. [ABSTRACT FROM AUTHOR]

• The problem of beams coupled by a non-linear interface is studied. • Non-linear static monotonic analyses are performed in OpenSees. • Comparisons with the monolithic assumption highlights differences in stiffness and lateral load capacity. • Comparisons with experimental results confirm the reliability of the approach. The use of reinforced concrete (RC) jacketing is a common technique for retrofitting existing RC columns with poor structural performances. Literature studies highlighted that the relative slip occurring at the interface between the new and the old concrete can affect substantially the flexibility and the strength of the retrofitted member. For this reason, several interface treatments are usually applied, aiming to induce a monolithic behaviour to the column. However, despite the commonness of RC jacketing, a limited number of studies investigated numerical modelling strategies for including these phenomena, especially with reference to the development of numerical models for design-oriented applications. This paper presents the results of a finite element modelling (FEM) approach for evaluating the flexural behaviour of members reinforced with RC jackets. The proposed method takes advantage of fiber-section beam elements connected through a non-linear interface. Monotonic static non-linear analyses are performed and results highlights the differences with the usual assumption of monolithic behaviour. The comparisons with experimental results confirm the reliability of the approach, which allows to obtain accurate results with a substantial reduction of the computational cost with respect to brick-based FEM. [ABSTRACT FROM AUTHOR]

Reinforced concrete (RC) columns often need to be strengthened or rehabilitated to allow them to carry the loads applied to them. In previous studies, RC columns have been strengthened by jacketing, without considering the occurrence of cracking. In this study, the behavior of RC columns strengthened externally by jacketing after cracking is analyzed. The accuracy of the existing models was verified by analyzing the performance of fifteen RC columns with different cross-sections to determine the effect of new variables, such as the column size, amount of steel reinforcement, and whether the column was cracked or not, on the effectiveness of strengthening. The analysis demonstrated that this strengthening technique could effectively improve both the ductility and strength of RC column cross-sections. The results indicate that the model suggested by the ACI-318 code can predict the ultimate load capacity of RC columns without strengthening, or strengthened by RC jacketing before or after cracking, with higher accuracy and material efficiency. The RC columns without strengthening met the safety limit of the ACI-318 model. However, for strengthened columns, a reduction coefficient must be used to enable the columns to meet the safety limit, with values of 94% and 76% for columns strengthened before and after cracking, respectively. Furthermore, strengthening after cracking affects the ultimate load capacity of the column, with 15.7%, 14.1%, and 13.5% lower loads for square, rectangular, and circular columns than those strengthened before cracking, respectively. [ABSTRACT FROM AUTHOR]