Reinforced Concrete Frame Research Articles

Confinement effects within the seismic design of reinforced concrete frames: reliability assessment and comparison

The current Italian code “NTC2018” and Eurocode “EC8” provide specific design requirements in terms of flexural overstrength factors for reinforced concrete (RC) buildings in seismic areas. This study proposes to improve the design methodology by including explicitly the confinement effects within the seismic design of a new regular RC frame due to their influence on the capacity design principles (at structural and member level). Specifically, the seismic performance of a RC frame designed according to codes is compared, in reliability terms, with the one of the same frame designed including the concrete confinement effects. Moreover, a comparison between three constitutive models for confined concrete is performed. Selecting L’Aquila (Italy) as reference site, the two design methodologies combined with the three confinement models are numerically implemented. Scaling 30 non-frequent natural ground motions to increasing seismic intensity measures, according to the site seismic hazard, non-linear incremental dynamic analyses (IDAs) have been developed. Assuming the interstory drift indices as engineering demand parameters, the peak responses have been fitted through log-normal distributions to define IDA curves. Subsequently, appropriate limit state thresholds have been adopted to define the seismic fragility curves. Finally, through the convolution integral and Poisson model, the seismic reliability curves have been derived showing the importance of the proposed design methodology. In fact, for the frame under investigation, the seismic performance strongly improves in terms of ductility respecting all the reliability objective levels if the confinement effects are explicitly considered in the seismic design. Furthermore, the different confinement models provide similar results when the proposed seismic design is adopted, implying a reduction of the model uncertainty.

Assessment of RC Frame Structure under Different Seismic Condition

Abstract: Structures in seismic zones are especially prone to collapse during significant ground motion occurrences. This susceptibility is sometimes linked to flaws in seismic design principles, notably in the selection of suitable structural systems. Equivalent static analysis, which is often employed in seismic design, may not accurately account for the structure's strength and stiffness. As a result, ground floor columns may become highly reinforced, thereby lowering ductility, and causing poor performance. This thesis offers a parametric study of an RC frame structure developed for various seismic zones. The finite element software is used to model the structure to investigate performance levels. Pushover analysis was performed to determine the structure's seismic capacity and behavior under various seismic conditions. Parametric research employing RC Frames with demand variation was done to investigate the structure's behavior in different seismicity zones across the country

Seismic performance of HECC/RC beam–column external joint with no stirrup and reduced anchorage length in core region

In this paper, steel–polyethylene hybrid fiber-reinforced strain-hardening cementitious composites (HECC) are applied in beam–column external joint core region to form a novel HECC/reinforced concrete (RC) composite beam–column joint with reduced anchorage length of beam longitudinal rebar and eliminated transverse rebar, and thus to alleviate rebar congestion and simplify the construction process efficiently especially for precast RC frame structures. Five external beam–column joint specimens are constructed and tested under cyclic loading to illustrate the influences of the anchorage length of beam longitudinal rebar and longitudinal rebar ratio on the seismic performance of beam–column joint. The effects of design parameters on seismic performance, including hysteresis behavior, degradation of strength, energy dissipation capacity, and cracking patterns are discussed in detail. Experimental results indicate that the replacement of normal concrete with HECC in beam–column joint core region could apparently reduce the amount of stirrups in the joint core area while maintaining reliable seismic behavior. Remarkably, specimen with no stirrups in the core area exhibits nearly equivalent seismic behavior to that of the control RC specimen. Furthermore, the application of HECC in joint core area allows for a substantial reduction of the required anchorage length for the beam longitudinal rebar to 9d, which further simplifies the construction process considerably.

Numerical Simulation on Aftershock Fragility of Low-Ductility RC Frames under Different Mainshock-Induced Damage Conditions

Aftershocks typically occur multiple times following major earthquakes, potentially inflicting enhanced damage. It is crucial to quantify the impact of aftershocks on the seismic performance of low-ductility reinforced concrete (RC) frames with different mainshock-induced damage states. For this purpose, this study investigated the aftershock fragility of low-ductility RC frames with different damage states induced by mainshocks via the case study of a six-story RC frame without a seismic design. The models of the low-ductility RC frames with different damage states were established via OpenSees software 2.5.0. Incremental dynamic analysis (IDA) was carried out to establish damage states induced by a mainshock. Then sixty-five real aftershocks were inputted to analyze the structures with different main-induced damage states. Aftershock fragility curves of low-ductility RC frame structures with different damage states were obtained. The results show that the exceedance probability of the low-ductility RC frame with mainshock-induced damage is higher than that of the intact low-ductility structure, corresponding to each limit state. The severity of the mainshock-induced damage directly amplifies the demand for a low-ductility RC frame in the subsequent aftershocks. As the mainshock-induced damage increases, the exceedance probability of the low-ductility RC frame undergoing more severe damage under aftershocks significantly increases.

Determination of dynamic collapse limit states using the energy-based method for multi-story RC frames subjected to column removal scenarios

The Alternative Load Path method is widely adopted to perform progressive collapse analyses for reinforced concrete (RC) frame structures subjected to column removal scenarios. Considering that the progressive collapses are usually dynamic phenomena, nonlinear dynamic analyses can yield the most accurate results. However, the associated computational cost is high. Alternatively, the energy-based method (EBM) can be employed to efficiently calculate the maximum dynamic responses. Unfortunately, collapse or dynamic limit states (DLS) cannot be directly determined when the EBM is adopted for the progressive collapse analyses, because it is essentially a quasi-static approach. To tackle this issue, in this paper an EBM-Intersection Point-based (EBM-IP-based) method is proposed to effectively determine the DLS. The method is considering that the dynamic instability will occur once the dynamic capacity curve exceeds the corresponding static capacity curve. Hence, it is possible to determine the DLS by making a correction on the intersection point of the aforementioned two curves without performing nonlinear dynamic time-history analyses. Compared with the other energy components, the kinetic energy at the moment of the peak displacement response is relatively small and the effectiveness of the EBM is verified. Through both static pushdown analyses and incremental dynamic analyses for 48 column removal scenarios of six different multi-story RC frames, the effectiveness of the proposed EBM-IP-based method is verified and the associated coefficients are calibrated. Statistical information and (joint) probabilistic density functions (PDF) of the coefficients are identified. Further, stochastic analyses are carried out in order to quantify the model uncertainties when adopting the EBM-IP-based method. On the basis of all the calculation results, a Gumbel distribution is adopted to capture the PDF of the model uncertainty in relation to displacements, while a lognormal distribution is adopted to fit the PDF of the model uncertainty in terms of resistances. Moreover, a multi-variate Gaussian distribution model with two components is employed to fit the joint PDF.

Seismic responses and loss evaluation of RC frame with slotted infill walls

Infill walls are a primary source of structural loss under service level and design basis earthquakes, so developing high-performance infill wall is an effective strategy to reduce seismic loss. While several high-performance infill walls have been proposed, most studies only reveal the damage characteristics of infill walls at the component level. However, the interaction between infill walls and structures can alter the seismic response of structures and consequently affect the loss of infill walls under earthquakes. Therefore, it is necessary to investigate the seismic responses and loss of high-performance infill wall at the structural level. In this paper, the working mechanism and damage characteristics of the slotted infill wall (SIW) are firstly introduced and verified through cyclic loading test. Then, an in-plane hysteresis model of SIW is developed, and the calculation method of key parameters is proposed. Finally, a numerical analysis model of a 10-story frame with slotted infill wall (SIW-F) is developed to study the seismic response and seismic losses. The results indicate that SIW-F’s displacement response is larger than that of the frame with ordinary infill wall (OIW-F) on most floors. However, the floor acceleration response is less. The seismic loss of SIW-F is less than that of OIW-F, particularly under design basis earthquakes. The repair cost ratio of infill wall decreases from 19% to 8%, the total repair cost of the SIW-F decreases by 41%, and the total repair time also reduces by 25%.

Cyclic performance of RC frame joints with drilled openings

The construction industry commonly employs reinforced concrete (RC) beams with utility pipe openings to facilitate the passage of pipes through structures. These openings can be made through pre-construction or post-construction processes. Pre-construction openings in RC members are included in the design, and in most cases, designers include reinforcements around the opening to deal with the stress concentration and failure mechanism caused by the presence of the opening. However, post-construction or drilled openings are added after the structure is built, potentially disrupting established load paths, and developing stress concentrations that are not anticipated by the designer. In this study, three-dimensional nonlinear numerical finite element models (FEM) models were developed and validated according to four sets of experimental data. These models formed the basis for a parametric study exploring different design parameters. Parameters such as the width and depth of the specimen and the size of the opening were investigated. The results suggest that the presence of the opening leads to a brittle behavior and a sudden drop in the strength and stiffness of the specimen after the ultimate load. Increasing the depth of the beam results in more brittle behavior in specimens with openings and decreased the drift ratio where the specimen failed by up to 75%. However, increasing the width of the specimen increases the ductility of these specimens. Finally, opening size was found to lead the specimen to behave more brittle in all cases, and decrease the maximum drift ratio from 5% to 1.5%, and decrease the load capacity in 1.5% drift ratio by up to 61%. This study will help engineers and designers to understand the effect of beam-column joints with post-construction openings under cyclic loading, whether these openings are intentional or by human error.

Collapse mechanisms and fragility curves based on Lumped Damage Mechanics for RC frames subjected to earthquakes

Reinforced concrete structures under earthquakes present nonlinear behavior with cracking evolution and plastic strains that can lead to failure due to large displacements with development of collapse mechanisms. This manuscript presents a methodology to apply Lumped Damage Mechanics (LDM) in a Performance-Based Earthquake Engineering (PBEE) framework, to evaluate the seismic vulnerability of reinforced concrete frames. The lumped damage model represents the evolution of damage and plasticity lumped into local inelastic hinges at the nodes of the elements. This paper proposes a novel procedure to identify and characterize global collapse mechanisms from the internal variables of damage using system reliability theory. In this way, the internal variables of damage are used as engineering demand parameters (EDPs) to calculate the fragility curves of the structure, which are also compared with usual interstory drift fragilities. To evaluate the seismic vulnerability, incremental dynamic analyses are conducted. The main results demonstrate efficiency of LDM in a PBEE framework, and that the internal variables of damage are objective indicators of collapse for RC frames. Results show how to identify the global failure mechanisms that are more likely to appear for each frame. For regular frames, high correlation is observed between the local hinges forming each global collapse mechanism. This allows collapse mechanisms to be readily characterized by the strongest link in a parallel system.

Advanced tree-based machine learning methods for predicting the seismic response of regular and irregular RC frames

In this study, predicting the maximum drift ratio (MDR) for two types of structures: 8-story regular (R8) and vertically irregular (IR8) reinforced concrete (RC) frames are focused. To accomplish this, advanced tree-based machine learning (ML) methods, namely Random Forest (RF), eXtreme Gradient Boosting (XGBoost), and Stochastic Gradient Boosting (SGB) are utilized. 2300 ground motion (GM) records, includes twenty-one input parameters and their nonlinear time history analyses results are considered as a dataset. The performance of the RF, XGBoost, and SGB models was examined considering the impact of the data pre-processing issue to test whether the use of data pre-processing approaches in model development could improve the prediction performance of the advanced ML models. Various data pre-processing techniques, including outlier detection using boxplot, missing data imputation using RF, data transformation using log transformation, and feature selection using Hill Climbing are employed as part of ML pipeline. The models' performance is evaluated using six different well-established statistical evaluation metrics. Subsequently, the results are systematically compared to identify the most effective prediction model. Our findings highlight the substantial impact of data pre-processing on the performance of ML models when predicting seismic responses, specifically the MDR of RC frames. Notably, RF consistently outperformed other ML models for both R8 and IR8 structures in the case of the simple ML pipeline strategy and data pre-processing approach. The RF model achieved R2 scores of R2 = 0.87 and R2 = 0.91 on the test data before implementing data pre-processing for R8 and IR8, respectively. After data pre-processing, R2 scores showed an improvement of 12% and 6.22% in the cases of R8 and IR8. In contrast, the performance scores of SGB consistently fell behind those of RF and XGBoost models.

Vierendeel action in progressive collapse of three-storey RC structures under corner column removal scenario

Previous studies have found that the Vierendeel action (VA) is one of the main mechanisms against progressive collapse of multi-storey reinforcement concrete (RC) frame structures in the event of corner column failure. However, the knowledge remains inadequate in terms of the behavior, mechanism, and quantitative contribution of the VA to the collapse resistance. To fill this gap, quasi-static tests were performed on a 1/3 scale single-storey RC substructure and a three-storey RC substructure with identical geometry, material, reinforcements and boundary conditions under corner column removal scenario. Test results found that, compared with the single-storey substructure experiencing no VA, the three-storey substructure developed additional plastic hinges at the beam ends near the corner column and at the lower end of the corner column as a result of the VA. The ultimate resistance of the three-storey substructure was 10.6% more than the triple ultimate resistance of the single-storey substructure. Then, numerical analysis was conducted to cognize the formation sequence of the plastic hinges and the effect of slabs. Furthermore, two coefficients were defined to reveal the mechanism of the VA in the three-storey substructure, i.e., the displacement-related coefficient ed and the coefficient associated with ultimate resistance eu. Finally, a simplified analytical model was proposed to predict the ultimate resistance of the three-storey structures with consideration of the VA.

Sensitivity Analysis of Modal Parameters of an RC Joint Subject to Progressive Damage under Cyclic Loads

This paper presents the results of an experimental study that focused on the gradual modification of the modal parameters of reinforced concrete beam–column frames subjected to progressive damage under cyclic loading. As is commonly found in structures of the 1970s, the specimen was characterized by the absence of specific shear reinforcement in the nodal panel. The frame modal parameters were investigated using the ambient vibrations test (AVT) as a modal identification technique. In particular, quasi-static cyclic tests with increasing amplitudes were performed on the reinforced concrete frame specimen and the modal parameters were assessed at various stages of frame degradation. By establishing a correlation between the changes in the modal parameters and the mechanical indicators of the structural damage in the frame, this study aimed to determine whether the ambient vibration tests could offer meaningful insights for evaluating the structural health of this type of structural component. As a result of the damage that occurred in the tested RC frame, the residual experimental value of the first natural frequency of the specimen was found to reduce at 52.7% of the original reference value (undamaged stage). Similarly, the residual value of the frame stiffness was found to be as low as 43.82% of the initial one. Both these results confirmed that changes when monitoring the modal frequencies may provide quantitative indexes to describe the structural health of RC frames. In combination with static tests for a direct measure of the structural stiffness variations, the AVT technique was shown to have interesting potential in detecting the type, level, and distribution of the progressive damage in civil structures. In particular, exponential and polynomial regression curves were defined to describe the decay of the first natural frequency as the structural damage increased in various parts of the frame, and it was shown that the variation in the first natural frequency was determined more by the damage on the beam than by the damage on the joint.

Maximum Drift Demands of Earthquake Damaged Reinforced Concrete Columns Based on Residual Flexure Cracks

Assessment of existing reinforced concrete (RC) structures after an earthquake is a challenging task that must somehow relate qualitative and quantitative observations in the plastic hinge regions and the associated residual deformation capacity of damaged structures. Having an estimate available for the remaining drift capacity will result in more economical and informed decisions regarding demolition or strengthening options. This study aims to develop a practical methodology to estimate the maximum drift demand of an RC column based on the residual crack width. For this purpose, fiber-based frame elements are used to model the RC column considering appropriately concrete behavior in compression and tension stiffening effects. Afterwards, the accuracy and reliability of the proposed methodology are demonstrated by validating the computational approach with two cyclic experimental results from literature and new test data for a one-bay one-story RC frame conducted within the course of this study. A comprehensive parametric study is performed for RC columns with different axial loads, longitudinal and transverse reinforcement ratios, and ground motions to exhibit the stochastic behavior. The study identifies the axial load ratio as the predominant parameter. Key findings include strong correlations between maximum drift ratios and total residual crack widths, as well as maximum compressive strains, with regression analysis yielding equations for accurate drift ratio estimation. Simple predictive models are proposed to estimate the maximum deformation demands based on observed residual crack widths. Residual cracking exceeding 5 mm poses significant risk for the columns with axial load ratios above 0.4, with 90% probability of exceedance 2% drift ratio.

Structure-Structure Interaction in the Reinforced Concrete Frames-Wall

Abstract Reinforced concrete frame-wall (RC frame-Wall) structures are generally solved in a global manner by modeling the entire structure by assembling the RC frame and wall stiffness matrices into a global matrix. The bar-wall element interface is modeled at the mean line of the beam-column element, according to RPA 99, which limits the interaction to the axial and transverse components of the forces (mechanical and kinematic) thus neglecting the effects of section rotations. In order to take into account, the rotational components of the mechanical behavior of the beams (section rotation and bending moment), the interaction between the RC frame and the wall must take place at the real interface of the two substructures, i.e. at the extreme fiber of the bar elements (beams and columns). The transmission of force from the wall to the RC frame is done by transforming the tangent stresses of the wall into moment in the beam.

Risk-based life cycle cost–benefit analysis for progressive collapse design of RC frame structures

Progressive structural collapse is a typical low-probability high-consequence event. Progressive collapse design can effectively improve the robustness of structures against accidental local damage and reduce the probability of collapse. However, progressive collapse design also increases the construction cost of buildings. Hence, it is necessary to conduct cost-benefit analyses for the progressive collapse design of building structures. This study first designs a reinforced concrete (RC) frame based on a progressive collapse design program that simultaneously optimizes material consumption and structural resistance. Then, the structural failure probability is quantified using an alternative path (AP) method-based fragility analysis that considers the structural uncertainties. Based on this, the life cycle costs and benefits of the structure designed with different design load factors are investigated. A four-story RC frame is used as an example. By considering the time value of money and threat probability, the break-even points and optimized design load factors for the progressive collapse design of the RC frame are analyzed. These research outcomes can help promote progressive collapse design in engineering practice and provide references for proposing scientific and reasonable design load factors.

In-plane behaviour of masonry infill walls with openings strengthened using textile reinforced mortar

A numerical study was carried out to simulate the in-plane behaviour of a 1:2.5 scaled one-storey one-bay masonry-infilled reinforced concrete (RC) frame retrofitted with textile-reinforced mortar (TRM). The study used ABAQUS finite element analysis (FEA) software and a three-dimensional (3-D) finite element method. Using a ductile design and details, the TRM-reinforced structure encountered in-plane displacement-control cyclic loads during the test. This study assesses how accurately a representative numerical model can replicate the results of an experimental test. Parametric research was conducted following the numerical model's calibration. To develop a suitable numerical model, suitable constitutive models based on the concrete damage plasticity (CDP) approach were used to characterize the nonlinear response of TRM, masonry infill, and concrete. The experimental findings, which aggrandize the response simulation, served as the basis for calibrating the models. Using a finite element framework and the simplified micro model technique, the masonry structure is simulated at the infill component level. The numerical model demonstrated accurately the ability to simulate the in-plane behaviour of the modified RC frame with infill walls, including its initial stiffness, deterioration, deformation-resisting ability, and failure patterns. Furthermore, a parametric study was conducted to determine the significance of the full-bond scenario between the RC frame infilled with masonry and the TRM considering: (1) the location of the strengthening material on the specimen and (2) the number of layers of the strengthening material. All retrofitted wall specimens showed a considerable improvement in in-plane performance; some features of the wall's behaviour were found to be strongly influenced by the parameters under examination. It can be observed that the specimen with two layers of TRM on both sides of the infill wall exhibit the maximum lateral load carrying capacity and maximum deflection resisting characteristics while the other retrofitted specimens controlled the deflection to some extent and significantly enhancing the load carrying capacities of the masonry system.

Out-of-Plane Performance of Lightweight AAC Infills within RC Frames Using Noncontact Full-Field Strain Measurement Technique

Out-of-Plane Performance of Lightweight AAC Infills within RC Frames Using Noncontact Full-Field Strain Measurement Technique

Influence of frequency characteristics of seismic ground motion on the fragility of an RC frame structure

Ground-motion frequency characteristics have a significant impact on structural damage. A quantitative description of the impact of ground-motion spectral features on structures is an urgent topic that requires solutions. In this study, the ratio of peak displacement to peak acceleration (R d2a ) is used as a representative index of the ground-motion spectral characteristics. The seismic records collected from the Kik-net strong-motion array in Japan are grouped according to their R d2a values. By adopting the incremental dynamic analysis method, the seismic fragility curves of a six-story RC frame structure loaded by inputting seismic records with different R d2a values are calculated using the OpenSees platform. The influence of R d2a values on the structural fragility is analyzed. Ground-motion records of liquefied sites are collected, and the peak ratios of seismic records at the liquefied sites are analyzed to reveal the differences in the fragility of the six-story RC structures at liquefied and non-liquefied sites. The results of this study are expected to serve as a reference for the evaluation of structural fragility and to understand the effect of ground-motion frequency characteristics on the fragility of RC frame structures.

Numerical Analysis of RC Frame Reinforced by Steel-Plate Concrete Module System

Numerical Analysis of RC Frame Reinforced by Steel-Plate Concrete Module System

Enhancing Structural Evaluation: Machine Learning Approaches for Inadequate Reinforced Concrete Frames

Enhancing Structural Evaluation: Machine Learning Approaches for Inadequate Reinforced Concrete Frames

Study on the mechanical properties of a viscoelastic self-centering brace with rotational displacement amplification and its application in RC frames

This paper introduces a novel viscoelastic self-centering brace with a rotational displacement amplification function (VSB-RA), developed to address the issue of limited energy dissipation in traditional self-centering braces. The basic structure and working mechanism of the VSB-RA are detailed. Low-cycle reciprocating loading tests were performed on two types of VSB-RA specimens with different initial amplification angles, and a comprehensive analysis was conducted on their bearing capacity, energy dissipation performance, self-centering capability, and frequency correlation. A refined finite element model was developed in ABAQUS software to simulate the complete test loading process. Nonlinear dynamic time history analysis is then performed on an 8-story reinforced concrete (RC) frame structure with the VSB-RAs using different initial amplification angles. The results demonstrate that the hysteresis curves of the VSB-RA exhibit distinct flag-shaped characteristics and excellent self-centering performance, and its energy dissipation capacity increases exponentially with the decrease of the initial amplification angle. The numerical simulation results of the hysteresis curves align well with experimental findings. The inter-story drift, residual inter-story drift, floor acceleration, and base shear force are effectively controlled in the VSB-RA frame structures compared to the uncontrolled structure. Under rare earthquake actions, the base shear forces of the structure are reduced by 27.2 %, 27.9 %, and 30.7 % at initial amplification angles of 45°, 37.5°, and 30°, respectively.