Existing Reinforced Concrete Buildings Research Articles

Evaluation on Effective Dimensions of Base Isolators Applied at Existing RC Building due to Earthquakes

This study investigates the structural performance of High Damper Rubber Bearings (HDRB) as base isolators in reinforced concrete (RC) buildings subjected to seismic loads. Given the potential for damage or collapse in RC buildings during earthquakes, the utilization of base isolators offers a promising solution to mitigate seismic risks. Finite element analysis was employed to assess the displacement efficiency of different HDRB configurations, focusing on the SMP 5 Muhammadiyah building in Surabaya as a case study. Results indicate that HDRB type HH090X6R exhibits superior performance, achieving displacement efficiencies of 90.56% in the x-direction and 95.87% in the y-direction. These findings underscore the importance of selecting optimal base isolator configurations to enhance the seismic resilience of RC structures. The study's implications extend to seismic retrofitting and structural design practices, offering valuable insights for improving the seismic performance of existing buildings and bolstering community resilience to earthquakes.

Protection of Reinforced Concrete Columns from Pounding-Induced Effects in Adjacent Buildings

The pounding damage is one of the most common seismic damages observed after past earthquakes in adjacent reinforced concrete (RC) buildings having insufficient gap in-between. Particularly in highly populated regions, there are numerous adjacent RC buildings, in which pounding can cause severe damage or total collapse. Many previous reconnaissance reports following earthquakes have emphasized severe column damage due to pounding with the slab of adjacent building. In this paper, a retrofitting method is presented to reduce the seismic damage caused by pounding on existing RC structural elements in cases where collision is unavoidable. Pounding effects are investigated by using 4- and 7-story RC buildings with unequal floor elevations. As a retrofitting solution for pounding, steel columns and neoprene rubber pads are placed at the mid-height of the story so that the pounding takes place between the neighboring slab and added steel columns instead of existing RC columns. The mid-column pounding of existing RC buildings with and without retrofitting are numerically investigated. In the numerical model, buildings are connected by link elements to reproduce the pounding in-between. Nonlinear response history analyses are performed using 11 different real ground-motion records. Pounding forces, peak floor accelerations, inter-story drift ratios, story shear force distributions, and plastic hinge distributions are obtained and compared for each of the pounding conditions. The analysis results revealed that the proposed retrofitting method protects the existing RC columns from brittle type of shear failure and reduces the destructive effects of pounding.

Residual design life-based evaluation of structural retrofitting on high-rise reinforced concrete buildings

The deteriorating structural performance of aging high-rise reinforced concrete (RC) buildings has brought increasing attention to the necessity of retrofitting in developed regions. In this context, the present paper introduces a novel approach by reversing the conventional service life-based structural design procedure. The proposed approach establishes a residual design life evaluation framework, enabling a comprehensive assessment of the performances of existing RC buildings both before and after retrofitting. This assessment framework takes into account various load scenarios, including wind and earthquake forces, etc., and yields an intuitive index based on design codes, termed "residual design life", which serves as valuable information for decision-makers. To exemplify the framework's application, a case study involving a high-rise RC building is presented. Initially, the influence of the adopted design codes on the residual life of the aging building is investigated. Subsequently, three commonly employed structural-level retrofit methods, namely steel bracing, RC wall addition, and exoskeleton, are examined. The findings reveal that the residual design life of the existing building decreases as the adopted design code transitions from an outdated version to the current one. However, the implementation of structural-level retrofit interventions proves highly effective in enhancing the building's residual life, particularly when failure is primarily governed by column capacities and storey drift. Nonetheless, it should be noted that certain structural-level interventions may increase the load on adjacent beams, potentially leading to a negative impact on the building's residual life when the condition of the beams becomes the predominant factor. Furthermore, the obtained residual life of the retrofitted building could be used to assist in conducting life-cycle analysis and further investigations and contribute to the understanding regarding structural performance and safety in the built environment.

Field Observations and Damage Evaluation in Reinforced Concrete Buildings After the February 6th, 2023, Kahramanmaraş–Türkiye Earthquakes

On February 6, 2023, two major earthquakes (Mw 7.7 and Mw 7.6) occurred in the province of Kahramanmaraş in southern Türkiye at 9-h intervals, causing a lot of loss of life and destruction in the region where approximately 16% of the country’s population lives. This study provides a comprehensive summary of common structural deficiencies that occurred in reinforced concrete (RC) buildings in earthquake-prone cities and proposes actionable measures systematically for future disasters. The evaluated data are based on the field observations of the authors. The main reasons for damages in RC buildings in the observation field can be classified into two categories: (1) the errors during the design stage (strong beam–weak column, short column, soft story, pounding effects, the existence of short and unconfined lap splices, insufficient transverse reinforcement, column–beam connection failures), and (2) the errors during the construction stage (poor concrete quality, unribbed reinforcing steel bar and corrosion, incorrect placement of reinforcement bars, incorrect end hook angle and length, weak gable walls, workmanship defects). Furthermore, a comparative nonlinear finite element analysis of the seismic impacts caused by the first earthquake on an existing RC building fully collapsed in the earthquake was conducted as a case study considering Türkiye Building Earthquake Code 2018 [TBEC 2018] “Türkiye Bina Deprem Yönetmeliği,” Afet ve Acil Durum Yönetimi Başkanlığı, Ankara (in Turkish)]. The validity and accuracy of the reasons for damages, which were reached by the field observations, were also proven by numerical analysis results. The importance of complying with the regulations, conducting the performance evaluations and controls of the buildings according to the renewed earthquake regulations and, if necessary, repairing–strengthening works were emphasized. The loss of life and property after the earthquakes that we experienced on February 6, 2023, which were called the “Disaster of the Century”, showed that Türkiye’s current building stock, especially those built before a certain date, is insufficient against earthquakes.

Reliability Estimation of the Compressive Concrete Strength Based on Non-Destructive Tests

The uncertainty in the concrete compressive strength is one of the most challenging issues in safety checking of existing reinforced concrete (RC) buildings. The concrete compressive strength used in the assessment can highly influence the vulnerability results and thus the retrofit strategies. The need to use less expensive and less invasive in situ measurements such as the non-destructive tests should be balanced with a careful check of their structural reliability. The compressive concrete strength is characterized herein based on a large database of both in situ destructive and non-destructive results measured on the same structural members. The data are obtained from existing RC buildings mainly located in the Campania region, Southern Italy. Probabilistic linear and multilinear regression models are developed for calculating the compressive concrete strength based on non-destructive tests. Furthermore, the implementation of the concrete strength based on ultrasonic test results are investigated together with the relative measurement error through a fully probabilistic workflow. Accordingly, the relative weights of non-destructive data for calculating concrete compressive strength are estimated and compared with those recommended by the Italian national code. The results demonstrate that the effective weights of the non-destructive data are very close to the code-based recommendation.

Structural Health Monitoring of Fiber-Reinforced Concrete Prisms with Polyolefin Macro-Fibers Using a Piezoelectric Materials Network under Various Load-Induced Stress

This experimental study investigates the influence of synthetic macro-fibers added in fiber-reinforced concrete (FRC) prismatic specimens on their flexural response and overall cracking performance. Application of a novel structural health monitoring (SHM) system that implements the electromechanical impedance (EMI) technique and the use of piezoelectric lead zirconate titanate (PZT) transducers installed in the FRC prisms are also included. The applied PZT-enabled EMI-based monitoring system was developed to diagnose damage and the overall performance in reinforced concrete (RC) structural members subjected to cyclic repeated loading, simulating seismic excitations in existing RC buildings. The paper also aims to determine the sensitivity of the real-time, wireless, and portable monitoring technique corresponding to the location, the distance, the direction of polarization of the PZT transducers and the location and magnitude of damage due to flexural cracking. Further, the influence of the effect of stresses corresponding at various loading levels and the observed changes in the ΕΜΙ frequency response of the PZT transducers are also examined. Test results indicated that cracking detection is achieved using this SHM system by prompt damage level assessment due to the FRC’s flexural load at early seismic loading stages in existing RC buildings.

Shear strength formula for interior beam-column joints with plain bars in existing buildings

Beam-column joint behavior in Reinforced Concrete (RC) structures is a key issue for earthquake engineering, because, under seismic loads, these elements are subjected to high stresses, which could lead to unexpected collapses, due to the development of brittle failure mechanisms. In Italy, many existing RC buildings constructed before the mid-1970's present structural deficiencies, as they were designed in the absence of the prescriptions of modern seismic codes. In particular, the use of plain reinforcing bars, inappropriate anchorage solutions and the lack of joint horizontal hoops are rather widespread.This paper presents an in-depth overview on seismic behavior of interior beam-column joints reinforced with plain bars, under cyclic loads, based on experimental results available in the literature. The inadequacies of certain reinforcement arrangements and details of tested joints are highlighted. A critical discussion of the damages and failure mechanisms developed in the joints, in relation to the reinforcement details and to the effects of the main influencing parameters, is provided. These parameters are the mechanical properties of materials, the geometry of the joint and the converging elements, and the column axial load. A formula to predict shear strength of joints with plain bars is proposed.Due to the lack of similar papers in the literature, the present analysis constitutes the basis for a comprehensive understanding of behavior of interior joints with plain bars under seismic actions. Moreover, due to the absence in the main current Codes of design formulas for the shear strength assessment of existing beam-column joints with plain bars, a design formula is also proposed. This formula is a valuable tool to design the retrofit intervention of existing buildings reinforced with plain bars, constituting a not negligible part of the built heritage.

Effect of buoyancy loads on the tsunami fragility of existing reinforced concrete frames including consideration of blow-out slabs

Currently available performance-based methodologies for assessing the fragility of structures subjected to tsunami neglect the effects of tsunami-induced vertical loads due to internal buoyancy. This paper adopts a generalized methodology for the performance assessment of structures that integrates the effects of buoyancy loads on interior slabs during a tsunami inundation. The methodology is applied in the fragility assessment of three case-study frames (low, mid and high-rise), representative of existing masonry-infilled reinforced concrete (RC) buildings typical of Mediterranean region. The paper shows the effect of modelling buoyancy loads on damage evolution and fragility curves associated with different structural damage mechanisms for existing RC frames with breakaway infill walls including consideration of blow-out slabs. The outcomes attest that buoyancy loads affect the damage assessment of buildings during a tsunami, especially in the case of mid and high-rise structures with blow-out slabs. The rate of occurrence of slabs uplift failure increases with the number of stories of the building, indicating the need to account for such damage mechanism when assessing the performance of structures. It is also found that buoyancy loads slightly affect the fragility curves associated to other structural damage mechanisms for existing RC buildings commonly monitored for fragility assessment.

Evaluation of a strengthening approach for existing RC buildings in terms of resilience and cost efficiency

The aim of the present study is to assess the performance of a reinforced concrete (RC) building been retrofitted with a new precast insulated steel and concrete composite sandwich wall. The applied retrofitting method, as well as the selection of the performance level for design have been studied with reference to their impact on the retrofit cost and the estimated cost due to possible earthquake losses. As a case study an existing 8-story RC building has been selected that is strengthened with inverted-V steel bracings as well as with the application of the proposed insulated composite walls, respectively. By applying inelastic dynamic analyses with artificial accelerograms scaled to represent earthquake scenarios with specific probabilities of recurrence, the seismic response of the strengthened structure is defined for each scenario. The results of the aforementioned analyses are being correlated with the earthquake losses based on an established methodology. Eventually, the performance of each retrofit method is determined and then compared with the respective performance of the existing building. The most efficient method is defined by considering both the construction cost and the earthquake losses cost of each earthquake scenario.

Machine learning for the prediction of evaluation of existing reinforced concrete structures performance against earthquakes

Earthquakes are challenging disasters that pose a huge threat to the urbanized world. In particular, the majority of the existing reinforced concrete (RC) building stock in developing countries such as Turkey is under huge seismic risk. These structures are at risk of partial or complete collapse under the effects of strong ground motions, due to some deficiencies in the structures. Therefore, seismic evaluation of existing buildings with a predominantly RC structural system is vital to reduce the potential seismic risk. In this study, machine learning (ML) techniques have been used for the prediction of the existing RC buildings’ performance against earthquake. The k-fold cross-validation has been employed to check the accuracy of the ML techniques. Random Forest (RF) provided the highest performance among the other ML techniques used. Sensitivity analysis have also been performed to determine the most significant factors in the prediction of the performance of the buildings. The results show that the building age, concrete compression strength, maximum column stirrup distance, steel yield strength, and the existence of corrosion have a high impact on the assessment of building performance.

Influence of Masonry Infills on Seismic Performance of an Existing RC Building Retrofitted by Means of FPS Devices

The safety assessment of existing structures in areas with a relevant seismic hazard is one of the major topics for engineers since many existing reinforced concrete structures have been realized disregarding seismic design with minimal details with respect to present practice. In this context, seismic assessment is a primary issue in order to identify the best retrofitting solution with the aim of enhancing the efficiency of existing buildings. In recent years, with the aim to enhance the seismic behavior of reinforced concrete (RC) structures (with particular care to existing ones), the system of seismic isolation adopting friction pendulum (FPS) devices proved to be among the most diffuse and effective solutions. The purpose of this paper is to explore the effectiveness of the refurbishment using FPS with single concavity devices on the performance of one irregular existing RC building placed in a highly seismic area of central Italy. First, the geometric and material characteristics of the building have been determined within the approach based on the “knowledge levels”. Second, a suitable numerical model based on a fiber-modeling approach has been established using SAP2000, including relevant mechanical non-linearities. Then, a set of 21 natural seismic inputs, inclusive of 3 accelerations over vertical and horizontal directions, was adopted with the aim of performing non-linear (NL) dynamic simulations. The NL dynamic simulations have been performed considering the structural system, both inclusive and not inclusive of the FPS isolator devices. The influence of the actual distribution of infill masonry panels on the overall behavior of the structure has also been evaluated in both of the cases mentioned above. Finally, the outcomes deriving from the NL dynamic simulations were helpful in assessing the advantages of the intervention of retrofitting to improve the seismic performance of the building, highlighting the influence of masonry infills.

Energetic and Structural Retrofit of Existing RC Buildings through Precast Concrete Panels: Proposal of a New Technology and Explorative Performance Simulation

In this paper, we present a proposal for a retrofit system for existing reinforced-concrete (RC) buildings consisting in the use of precast concrete panels, designed for improving both the structural and energetic performance. In particular, the proposed system is conceived to improve both the energetic efficiency, by ensuring high-performance thermal insulation, and the capacity of the structural elements under gravity and seismic loads. Firstly, a detailed description of the proposed technology is provided, which has been tested and assessed on a real-scale prototype. Then, the efficiency of the technique was explored by means of numerical simulations for both the energetic and structural performance. Although no experimental tests are available, the results of our numerical simulations and analyses on a real case-study building reveal interesting insights, highlighting the main pros and cons of the proposal and providing a possible retrofit solution for buildings that do not meet the current European code requirements.

Pin-supported walls as seismic retrofit for existing RC frames: performance improvement through link removal

Pin-supported (PS) walls represent a possible solution for the seismic retrofit of existing reinforced concrete (RC) buildings, enabling linearization of the frame deformation along its height and consequently inhibiting soft storey collapse mechanisms. The effectiveness of this rehabilitation technology is strongly related to the characteristics of the existing frames, especially to the vertical distribution of the frame storey lateral stiffness. Since a larger 1st storey lateral stiffness may lead to a detrimental structural response of the retrofitted system, an alternative solution obtained from removing the connecting link between the 1st floor of the existing building and the pin-supported wall is investigated in this paper. An analytical method is proposed to derive the distribution of the internal actions in a dual 2D RC frame-PS wall system without the link at the 1st floor level, considering both a linear and nonlinear behaviour of the frame. It emerges that some parameters describing the structure type can provide straightforward information on the suitability of this solution in the seismic retrofit of existing RC buildings. As a result, a simplified procedure has been derived to preliminarily define the retrofit system configuration and evaluate the maximum demand in the PS wall. Such a procedure is finally applied with reference to a frame representing a RC building and finite element model analyses are carried out for validation purposes.

Fragility functions for fiber-reinforced polymers strengthened reinforced concrete beam-column joints

Post-earthquake field inspections have outlined the poor seismic performance of non-conforming reinforced concrete (RC) beam-column joints (BCJs) of existing RC buildings. Experimental and analytical studies have demonstrated that the application of externally bonded fiber-reinforced polymers (FRPs) is an efficient and cost-effective strengthening technique to improve the seismic performance of deficient BCJs. However, seismic losses for FRP-strengthened BCJs at different damage states (DSs) are currently not quantified. A potential seismic damage assessment tool is critical to obtain reliable estimations of the effects of FRP strengthening on reducing the expected damage and losses in existing buildings. Therefore, experimental-based fragility functions for FRP-strengthened corner and exterior BCJs at different limit states are proposed in this study. Experimental tests on 70 corner and 28 exterior FRP-strengthened BCJs are first collected. Then, DSs with increasing severity (i.e., light, moderate, and heavy) are defined according to widely recognized studies. These DSs are quantified on the basis of inter-story drift ratios (IDRs). After retrofitting, IDR-based fragility functions are generated for strengthened BCJs classified in terms of failure mode and achieved ductility level. The proposed functions are also compared with fragility functions available for non-conforming and conforming joints obtained from the literature. Finally, an application of the proposed fragility functions within a performance-based earthquake engineering (PBEE) framework for quantifying expected losses (ELs) and the benefits of FRP strengthening of BCJs is showed.

Experimental Analysis of the Effectiveness of Pre-Stressed Steel Strips for the Strengthening of Beam-Column Joints in Existing RC Buildings

Beam-column joints in existing Reinforced Concrete (RC) buildings are usually characterized by the absence of transverse reinforcement. The observation of post-earthquake damage, as well as several numerical and experimental studies, have soundly demonstrated the possible impact of the response of unreinforced joints on the damage – up to collapse – of existing RC buildings under seismic action. In this study, a possible strengthening approach for these elements, based on CAM® technology, is investigated through experimental tests. The results of these tests are reported, along with the results of a previous experimental campaign carried out on the same topic. Full-scale external beam-column joint specimens are tested, with and without strengthening, and with different characteristics in terms of amount of beam longitudinal reinforcement and concrete compressive strength. Different failure modes are observed for the as-built specimens (joint failure without (J) or after (BJ) beam flexural yielding). The effectiveness of the adopted strengthening approach is demonstrated, observing an increase in joint shear strength and subassemblage ductility and – unless a joint failure in compression occurs – a possible modification of the failure mode into a ductile, flexure-controlled response of the subassemblage, as for code-compliant beam-column joints.

Simplified framework for economic convenience of base isolation as seismic retrofit solution for existing RC buildings

Modern seismic retrofit strategies aim at increasing the building safety under high intensity earthquakes and minimizing the possible damage and, in turn the expected economic losses (EAL) under frequent and low intensity earthquakes. Classic techniques based on tradition or innovative material such as steel jacketing or FRP wrapping aims at increasing the strength/ductility of reinforced concrete (RC) members, but they do not significantly act in reducing the engineering demand parameters EDPs such as acceleration and drift. On the other side innovative techniques such as base isolation are capable of combining the increase of seismic safety with the significant reduction of acceleration and drift demand on the superstructure, thus, resulting in a significant reduction of the EALs. Although the number of applications is significantly increasing in Italy, its wide use is discouraged by the high cost of installation. Considering the annual savings related to the reduction of the EALs, the payback time (PBT) of the economic investment in a retrofitting by using base isolation can be comparable to that of other techniques. However simple tools to easily assess the PBT knowing major building characteristics are currently not available.This paper proposes a novel framework to estimate the EALs and the PBT of RC buildings retrofitted by means of FRP or Base Isolation based on the main building characteristics (i.e. in-plane geometry, number of floors, seismic). It is implemented in a MATLAB code to perform NLTHs on simplified structural models and calculate the EALs by using the full PBEE framework and considering the uncertainties in the hazard, damage and loss analysis. The framework is validated against the results of a case study building analyzed by means of a refined 3D FEM model and calculating the EAL by means of the FEMA P-58 approach. A parametric study is carried out varying the concrete compressive strength and the reinforcement details to check the sensitivity of the framework to these variables. The preliminary results are presented and discussed.

Seismic fragility curves for the Italian RC residential buildings based on non-linear dynamic analyses

In the present paper, Fragility Curves (FCs) of Reinforced Concrete (RC) building types with moment-resisting frame structure representative of the existing Italian building stock have been derived through an analytical approach. The proposed methodology is based on Non-Linear Dynamic Analyses encompassing all the steps required to bring about reliable as well realistic fragility results. First, prototype building types have been selected by considering the main attributes affecting the seismic vulnerability of existing RC buildings, that is: age of construction (i.e. ‘50 s, ‘70 s and ‘90 s), number of storeys (i.e. 2, 4 and 6 storeys), arrangement in elevation of infills (i.e. Bare-, Infilled-, Pilotis-frame) and design level (i.e. seismic or gravity loads). A simulated design has been used for detailing the building types at hand, whose non-linear dynamic response has been computed by using a large set of signals. The signals have been purposely selected in order to approach the elastic design spectra provided in the Italian seismic code for different return periods, being able to take into account also record-to-record variability and soil-amplification effects. A specific relationship between the considered engineering demand parameter (i.e. inter-storey drift ratio) and all damage levels proposed in the EMS-98 scale have been defined on the basis of empirical data and expert judgement. A set of FCs in terms of peak ground acceleration are finally derived and compared to point out the role of the considered vulnerability attributes.

Retrofitting of Shear Compression Failure-Critic Short Columns with a New Technique

One of the reasons that cause the collapse of buildings is deficient short columns, which need to be retrofitted to prevent the collapse of the building in a potential earthquake. External reinforced concrete (RC), steel plates, and fiber-reinforced polymer (FRP) jacketing are standard retrofitting methods to retrofit columns to increase their shear capacity. However, in compression shear failure, the effectiveness of steel and FRP jacketing is quite limited due to the premature buckling of the FRP and steel material. On the other hand, RC jacketing is not practical because it requires more labor and covers more architectural places. Thus, the main motivation of this study is to present the effectiveness of a new method to retrofit short columns, including those with dominated shear compression failure. For this purpose, HSPRCC (high-performance steel plate-reinforced cementitious composite) was adapted to retrofit such short columns. This method is a combination of high-performance concrete and perforated steel plates. Short-column specimens representing existing RC buildings were retrofitted using the HSPRCC and tested. Perforated steel plates anchored to the specimen by steel bolts and repair mortar are used as a matrix. The retrofitted specimens were found to exhibit much better performance both in terms of shear strength and deformation capacity. It was also observed that the retrofitting method is effective in contributing to increasing the compression shear capacity.

Retrofit of Existing Reinforced Concrete (RC) Buildings: Steel vs. RC Exoskeletons

The existing building stock is largely made up of reinforced concrete (RC) buildings, constructed between the post-World War II period and 1981, and mostly consists of buildings constructed very quickly to meet the great housing demand of this period, and buildings that do not adhere to anti-seismic and energy regulations. Today, after more than fifty years, these buildings have reached the end of their useful life cycle and their maintenance is not sustainable, presenting a series of structural, energy and architectural problems and criticalities. The myriad of possible retrofit interventions currently available for these RC structures drastically reduces when the main requirement for interventions is to avoid operational interruptions to buildings. In this case, an additive structure, operating exclusively from the outside as an exoskeleton, is typically used for achieving appropriate retrofit. In this paper, two solutions are proposed and addressed for the retrofit of an existing RC building in Italy, one through the application of a steel exoskeleton and the other through the application of an RC exoskeleton system. A set of push-over (PO) analyses is carried out to define the performance point of both the original and combined systems. The comparative results of these solutions are then discussed.

Seismic strengthening of existing RC buildings with external cross-laminated timber (CLT) walls hosting an integrated energetic and architectural renovation

In this study a timber-based integrated solution is presented to solve at once common issues affecting typical reinforced concrete (RC) existing buildings, such as seismic and energy performances, providing an eco-friendly alternative to steel external bracing systems. Cross-laminated timber (CLT) walls are provided perpendicularly to the external façades as strengthening elements while interposed CLT slabs are foreseen at each floor level to host new architectural units together with a new envelope. While the connections to the foundations and to the existing RC frames are provided respectively with steel brackets and axial-connectors distributed along the height of the building, a post-tensioned connection, between CLT panels (PT-CLT connection), is implemented in the system to guarantee resistance to horizontal actions acting parallel to existing façades with consequent structural independence and architectural freedom. On this regards a first look at the findings of an experimental campaign carried on the Technical University of Munich are presented. A numerical model is developed with finite element software characterizing each type of connector for linear and non-linear analyses. Modal analyses with response spectrum are performed to verify structural elements and connectors, while pushover analyses with target displacement checks are performed to assess the obtained seismic improvement. Finally, the preassembled architectural components that allow to renovate the envelope and the provided assembly procedure are revealed.