Reinforced Concrete Frame Structures Research Articles

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.

Effect of three-dimensional space on progressive collapse resistance of reinforced concrete frames under various column removal scenarios

In this study, high fidelity finite element (FE) model was adopted to investigate the progressive collapse resistance of reinforced concrete (RC) frame structures under various column removal scenarios. The validity of the numerical model was verified by comparing with the existing experimental results. Afterwards, to facilitate the research on possible simplified boundary conditions served for further various column removal scenarios, the effect of multi-span continuous beams was firstly evaluated and the simplified methods were compared. Then, the effect of various column removal scenarios addressing the three-dimensional effect on the progressive collapse resistance of RC frame structures with and without slab was systematically evaluated. The analysis results showed that the continuous span beams increased the internal compressive force of the beam section at compressive arch action (CAA) stage, which effectively improved the CAA strength of the substructure. For simple modeling, the use of horizontal constraints at the end of the beam could replace the continuous span beams. The presence of transverse beams and slab provided additional load transfer paths for collapse resistance. The compressive membrane action (CMA) and tensile membrane action (TMA) of the slab significantly improved the progressive collapse resistance of the substructure in the small and large deformation stages, respectively. The location of the removed column significantly affected the contribution ratio of transverse beams and slab to the progressive collapse resistance of the substructure, which was quantitatively evaluated for practical design.

Shaking table tests on inclined collapse mechanism of RC frame structures subjected to earthquakes

Inclined collapse of structures under strong earthquakes threatens the adjacent infrastructures. The collapse mode and collapse direction should be considered with caution to curb the secondary disaster in the urban area. However, current studies on the inclined collapse mechanism are predominantly limited to the qualitative and intuitive understanding. To fill this gap, this study presents an insight into the inclined collapse mechanism of the reinforced concrete (RC) frame structures with two typical features subjected to earthquakes, i.e., the structures with slab opening on one side of the first floor and the structures with disparate axial compression ratios of the first story columns on the opposite sides. First, shaking table tests were performed on two 1/10-scale three-story RC frame structure models undergoing inclined collapse. Then the multi-scale finite element models were established, validated against the test results and used for parametric studies. In the parametric studies, two coefficients were considered to describe the typical features of the RC frame structures, i.e., the strong column-weak beam ratio η and differential rate of the axial compression ratios λ. The test and numerical results found that the inclined collapse mechanism was attributed to the different distributions and discrepant resistance deterioration of the plastic hinges among the first story columns of the RC frame structures with the two features. This causes the inconsistent damage levels among the first story columns, the dominant direction of the lateral displacement, and the inclined collapse of the structures. Finally, the collapse mode and collapse direction can be controlled by appropriately adjusting the two coefficients η and λ for the structures with the typical features concerned in this study.

On the progressive collapse performance of RC frame structures under impact column removal

Currently, progressive collapse studies are mostly conducted based on an event-independent assumption. With studies employing an event-dependent premise mainly concerning explosion or fire events, the aftermath of impact loading is seldom reported. Meanwhile, interactions between reinforced concrete (RC) members and superstructures under impact loading need further evaluation. In this paper, finite element models of RC structures subjected to impact loading and progressive collapse are established and validated utilizing LS-DYNA. A valuing methodology of erosion parameters for the continuous surface cap model (CSCM) considering element size is proposed in this process. The influence of impact column removal (ICR) on the progressive collapse performance of RC frame structures is studied at sub-assemblage and structure levels. The parametric study indicates that the ICR process can be described by an impact loading stage and a gravity load stage. It is also found that structures experiencing ICR are exposed to a higher risk of progressive collapse, with the downward force exerted by the impacted columns being a significant contributing factor. Dynamic analyses demonstrate that the acceleration of the column removal point (CRP) can be used to validate and quantify the downward force. The hybrid force-displacement boundary conditions of frame columns give rise to the development of downward force. Recommendations for resisting progressive collapse considering ICR are proposed based on the analytical results of the paper.

Comparing seismic performances of single-span RC frames with and without wing wall retrofitting by shaking table tests

It is necessary to retrofit existing single-span reinforced concrete (RC) frames which could not meet the current seismic codes due to lack of multiple seismic lines of defence. Adding wing wall is one of the retrofit method that can improve the redundancy of single-span RC frames. To study the effectiveness of single-span frames retrofitted by wing walls, shaking table tests were conducted on two 1:5 specimens to compare the seismic behaviour of single-span RC frames with and without wing wall retrofitting. First, a retrofit scheme for the frame using wing walls was selected. Two specimens—a single-span RC frame and an identical frame with a wing wall retrofit—were placed on the same shaking table board and tested simultaneously. Contrast tests were conducted for 20 earthquake cases. The main conclusions are as follows: (1) Adding wing walls to a single-span frame in bi-direction is a reasonable strengthening scheme. (2) For the single-span frame without retrofitting, cracks appeared in the early stages and rapidly developed, forming plastic hinges at the ends of the beams and columns. For the single-span frame with retrofitting, cracks first appeared in the construction joints between the wing walls and beams, and then extended to wing walls and beams. The test phenomena indicate that adding wing walls to a single-span frame can increase system redundancy, and act as the first anti-seismic line of defence. (3) From the measured data, it is shown that adding wing walls to a single-span frame can improve its seismic capacity and control the lateral deformation. The peak floor displacement response of the retrofitted frame reduced by 30%–40% compare with that of the frame without retrofitting. (4) When input PGA was 0.15g (design based earthquake), slight damage occurred to the frame retrofitted by wing wall, while damage of the frame without retrofitting was moderate. When input PGA was 0.30g (rare earthquake), moderate damage occurred to the frame retrofitted by wing wall, while damage of frame without retrofitting was severe. Therefore, adding wing walls is an effective method for retrofitting single-span RC frame structures.

Experimental investigation of retrofitting seismically damaged reinforced-concrete frames using steel jacketing

Retrofitting seismically damaged structures provides significant economic and social benefits. To investigate the rationality of using steel jacketing to retrofit seismically damaged reinforced concrete (RC) structures, we initially tested a half-scale single-span two-story frame structure under pseudo-static loads to achieve a severe damage state. Subsequently, the structure was retrofitted using steel jacketing and tested under another round of pseudo-static loads. The seismic performances of the original and retrofitted structures were compared. The test results indicated that both the original and retrofitted specimens sequentially developed plastic hinges at the beam ends and the bottom of the columns, indicating a generally similar damage evolution mode. Comparing to the original structure, the retrofitted specimen exhibited an increase of 33% and 78.5% in initial stiffness and strength, respectively. Although the ductility ratio of the retrofitted specimen was reduced by about 14%, the ultimate drift ratios of both specimens exceeded the threshold value (1/50), indicating an adequate deformation capacity. The retrofitted specimen exhibited a certain degree of pinching behavior on the hysteretic curve because of the presence of extensive inclined shear cracks at the mid span of the beams and the progressive failure of the anchoring of the strengthening steel angle at the beam ends and column base. Consequently, the energy-dissipation coefficients of the retrofitted specimen were lower than those of the original specimen. Based on the test, the finite element analysis modeling method for RC frame structures retrofitted using steel jacketing was also studied. By comparing the numerical simulation analysis results with the experimental ones, the reliability of the proposed numerical simulation method was verified. This study can serve as a foundation for the practical application of steel jacketing to retrofit seismically damaged RC structures.

An Improved Anchorage System for L-Shaped FRP Composites to Enhance the Seismic Response of Beam-Column Joints in a Low-Strength Substandard Reinforced Concrete (RC) Frame

Reinforced concrete buildings are prone to collapse during seismic events due to the brittle shear failure of non-seismic beam-column joints (BCJ). In this study, two one-third scale reinforced concrete (RC) frames incorporating various non-seismic details were tested under lateral cyclic loading. One of the RC frames was used as control, while the other was strengthened using externally bonded carbon-fiber-reinforced polymer (CFRP) sheets in a L-Shaped configuration with particular attention to anchorage to evade debonding. For the strengthening process, L-shaped CFRP sheets were bonded to the inner face of columns, extended on beams both above and below the joint up to a hinge length. To avert debonding, the L-shaped CFRP sheets were fully wrapped with CFRP sheets around the column, both near the joint and at the end of the sheet. The sheets were also wrapped around the beam, through two slots in the slab that were adjacent to the beam-column interface and at the far end of the sheet. Test results confirmed that the installation of CFRP sheets in an L-shaped configuration altered the brittle-shear failure mechanism of the beam-column joints to a ductile failure by repositioning the hinges away from the columns. Additionally, the proposed anchorage method successfully eradicated the debonding and peel-off of the CFRP sheets. Moreover, strengthening with the CFRP sheets in the L-shaped configuration enhanced the strength and ductility of the RC frame by 45% and 43%, respectively. According to the findings of this study, the application of L-shaped CFRP sheets proved effective in strengthening RC frame structures.

Seismic performance loss evaluation of reinforced concrete frame structure based on updatable damage model

AbstractAs structural seismic damage develops, the internal force transfer and damage weight relationship of component are constantly changing, and the fixed weighted coefficient is adopted by the traditional weighted method, which cannot reflect the changes in the structural damage state. The structure is a network formed by connecting a large number of components, and the network state undergoes complex changes with the development of component damage. In this paper, a novel method for component damage transfer to structural seismic performance loss is proposed using network shortest paths. First, a directed weighted complex network based on force transfer direction and component stiffness is constructed for reinforced concrete (RC) frame structures. Then, the component damage is calculated by the degradation of its stiffness and is used to update the weight distribution of the damage network. Finally, the structural performance loss is evaluated by the degradation of damage network efficiency. Results show that the proposed method with an updated component weight relationship is able to reflect the true loss of structural performance according to the finite element dynamic elastic‐plastic analysis of RC frame structures. For the structural performance loss degree (none, slight, moderate, severe, complete), the proposed method and the weighted method both exhibit high accuracy (number correctly classified/total number of samples) when structural performance loss is none, severe, or complete loss. The accuracy of the proposed method is significantly higher than that of the weighted method when structural performance loss is slight and moderate loss, with 58.1% and 84.2% improvement, respectively. For structural typical damage modes (global, local, and weak layer damage mode), the proposed method and the weighted method both exhibit high accuracy when structures exhibit global damage mode. The accuracy of the proposed method is significantly higher than that of the weighted method when structures exhibit local and weak layer damage mode. The accuracy of the proposed method is stable at 95.2%, while the accuracy of the weighted method is 76.2% and 70.2%, respectively. Overall, the weighted method exhibits high accuracy when structural performance loss is less than slight loss or greater than moderate loss, while the proposed method exhibits high accuracy throughout the entire process of structural performance loss, especially in moderate loss.

Numerical Investigations on the Seismic Behavior of a Reinforced Concrete Frame Structure

Recent seismic events that took place during the past two decades and struck densely populated areas caused significant material losses and, sometimes, casualties. Taking into account the ageing of the building stock around the world coupled with unavoidable degradations, urgent measures need to be taken to either strengthen or retrofit such structures based on data obtained from analytical, experimental or numerical investigations. The present paper presents the results of numerical simulations, using time-history analyses, on a scaled-down reinforced concrete (RC) frame structure subjected to two multidirectional seismic scenarios. The obtained results are discussed from the point of view of maximum displacements, accelerations and equivalent stresses. Considering the dynamic characteristics of the model, neither considered seismic event were able to produce large lateral displacements or accelerations. Damages were localized at the beam-column joints in case of Turkey 2023 earthquake scenario.

Time-History Analyses on the Effect of Partial Infill Walls on the Seismic Response of a Reinforced Concrete Frame Structure

Time-history analysis (THA) is based on direct step-by-step integration of the equation of motion in time domain considering as input either the recorded accelerograms of real or artificially generated earthquakes. Numerical simulations are sensitive in terms of material properties, geometry, boundary conditions and applied loading scenarios correct definition. Therefore, additional care is required in the pre-processing stage of the analysis when the numerical model is defined. Infill walls play an important role on the seismic behaviour of structures during earthquakes. The response of the structure to seismic action changes due to the presence of the infill walls. The paper presents the results obtained by means of THA on the effect of partial infill walls on the seismic response of a scaled-down RC frame structure subjected to four consecutive uniaxial seismic motions. The parameters of the research were the presence of the partial infill wall and the thickness of the wall. The obtained results are discussed from the point of view of maximum lateral displacements and response spectra expressed in terms of accelerations. The results contribute towards the setting up of the subsequent shake table tests of scaled-down RC frame models with different configurations that would validate and help calibrate the numerical model.

Seismic Response of a Tall Reinforced Concrete Frame Structure Subjected to both Horizontal and Vertical Strong Motions

The continuous development of urban areas coupled with the ageing of built-stock and the variety of structures built following different design code recommendations may result in different behaviours during seismic events. Hence, reliable information is needed in order to assess the risk of severe damage or collapse during earthquakes. Accurate numerical simulations coupled with analytical models provide essential information on the seismic performance of civil engineering structures. The results obtained from realistic numerical simulations may serve as entries in data-bases designed to train and calibrate early warning systems (EWS). Their continuous development is of paramount importance taking into account their potential to save lives and prevent economic loses in case of a severe seismic event. The paper presents the preliminary results obtained from numerical simulations on the seismic response of a tall reinforced concrete (RC) frame structure subjected to both horizontal and vertical string motions produced by an earthquake. The obtained results, in terms of response spectra in terms of accelerations, may be used for training EWS sensors to identify a seismic event and transmit warning messages at local, regional and even national level.

Seismic fragility analysis of high‐strength concrete frame structures reinforced with high‐strength steel bars

SummaryTo investigate the influence of using high‐strength steel bars in columns on the seismic resistance capacity and seismic resilience of frame structures, seismic fragility evaluation of three 8‐story reinforced concrete (RC) frame structures was conducted based on the incremental dynamic analysis (IDA) using 11 ground motion records. The main parameter is the longitudinal reinforcement configuration in the frame columns, where the first structure is reinforced with HRB 600 grade steel bars in the columns, the second structure is replaced with equal area ultra‐high‐strength (UHS) steel bars (i.e., with a yield strength of approximately 1425 MPa), and the third structure is replaced with equal strength UHS steel bars. A numerical model of the RC frame structure was developed and then validated using previous experimental results. The exceeding probabilities at various performance limit states were calculated based on two typical engineering demand parameters (EDPs) of maximum interstory drift and residual interstory drift. The results showed that using UHS longitudinal steel bars instead of HRB 600 grade steel bars in frame columns could reduce the energy dissipation capacity of the structure, inevitably leading to an increase in the maximum interstory response of the frame. However, much lower exceedance probability was observed in the UHS‐enhanced frame under the repair available limit state based on the residual interstory drift, indicating that the UHS‐enhanced RC frame had higher seismic resilience. In addition, compared to equal area substitution, equal strength substitution is a more ideal design method that can use fewer UHS steel bars to achieve comparable reparability and a smaller increase in maximum interstory drift.

A Comparative Study of Seismic Performance Evaluation of Reinforced Concrete Frame Structures Using Chinese and African Seismic Codes

The evaluation of various earthquake codes, it is one of the significant challenges in the study area of earthquake engineering. However, according to the literature review, most research works have not addressed comparing Chinese and African seismic codes. Therefore, the purpose of this study is to identify each code’s advantages by comparing assessment of the seismic efficacy of moment resistance frame reinforced concrete (MRF-RC) frames using four different codes: the Ethiopian Building Code Standard (EBCS-8), the Egyptian Code for Design and Construction of Reinforced Concrete Structures (ECP-201), the Algerian Seismic Regulations (RPA-99), and the Chinese Code for Seismic Design of Buildings (GB-50011), the first three are the major codes used in Africa. The seismic provisions of these codes are compared and evaluated using nonlinear time-history analysis (NL-THA) and nonlinear static pushover to validate the results. These analyses are performed on four MRF-RC frame models with different heights. The results include various parameters that reflect the seismic performance of the structures. The study revealed that the Chinese code is more conservative and overestimates seismic performance compared with African codes. However, the Chinese code can be applied in African projects considering the African soil classifications, and seismic weight are adjusted to meet the African design criteria.

Damage analysis of a pseudoclassic reinforced concrete frame structure under the action of the Ms 6.8 Luding earthquake in China

Reinforced concrete (RC) moment-resistant frame structures are very commonly used worldwide. However, previous earthquakes have revealed the occurrence of strong beam–weak column (SBWC) failure, causing heavy human casualties and economic losses. This study analyzed the SBWC failure mechanism in RC frame structures with the pseudoclassic construction named "Que Ti" near the top of a slope topography, based on the author's recent investigation of the Ms 6.8 Luding earthquake in China. Eight refined finite element models (FEMs) were developed to simulate and analyze the influence of slope topography, pseudoclassic construction, and some structural characteristics on structural damage during an earthquake. Furthermore, discussions on using column-to-beam flexural strength ratios (CBFSRs) to mitigate SBWC failure are presented. The results demonstrated the proposed FEMs effectively simulated the performance of the pseudoclassic structure, especially the SBWC failure mode, as validated by their consistency with the observed structural damage during the earthquake. Moreover, it revealed that structures closer to the slope top experienced amplified ground motions during earthquakes, resulting in more severe damage. In addition, the Que Ti and infill walls increased the vulnerability to column damage and maximum requirement for the column shear force. Finally, to achieve the design objective of a strong column–weak beam (SCWB), the CBFSR should be at least increased to 1.8 for this pseudoclassic RC frame. This ensures a probability of the SBWC failure mode of less than 17% (meet less than 20% of recommendations) and proportion of the beam plastic energy dissipation exceeding 80% of the total plastic energy dissipation. The findings of this study are significant for enhancing seismic design and preventing earthquake disasters for the pseudoclassic structure.

Experimental Study on 3D RC Frame Structures with VE Energy Dissipation Braces Using STM32-Matlab-OpenSees Hybrid Test Platform

This study aims to reveal the control mechanism of viscoelastic (VE) energy dissipation braces with strong mechanical nonlinear characteristics on the seismic response of reinforced concrete (RC) frame structures. Firstly, a new STM32-Matlab-OpenSees joint hybrid test platform is developed by introducing OpenSees, which is convenient for structural modeling and fast for structural calculation. The overall architecture and operation of the hybrid test platform are comprehensively analyzed. Then, a series of simulated seismic hybrid tests on three-dimensional (3D) RC frame structures with VE energy dissipation braces under different seismic waves are successfully carried out. Lastly, the hybrid test results of this structure system at different working conditions are compared and analyzed comprehensively. The research results show that the developed platform breaks the boundary limitation that OpenSees relies on the framework software OpenFresco and can only be connected to specific control systems in the hybrid test. Thus, it greatly expands the application scope of flexible configuration controller and actuator categories for the hybrid test platform, and it also provides a good reference value for the establishment and implementation of similar hybrid test platforms. Under the action of various seismic waves, VE energy dissipation braces has good universality for the shock absorption of RC frame structures. The damping control effect and energy dissipation efficiency of VE energy dissipation braces are significantly different under various seismic waves. In the design of RC frame structures with VE energy dispersion braces, it is very necessary to conduct the seismic analysis of the structures under multiple ground motions with wide frequency domain. The additional position of VE energy dissipation braces should be an important analysis indicators in the optimization design of the structures.

The impact of infill wall distribution on the mechanical behavior and failure patterns of multi-story RC frame structures: An acceleration–strain coupled testing approach

Historically, earthquakes have caused extensive damage to multi-story buildings, often leading to partial or total collapses. Nevertheless, these severely damaged structures rarely exhibit the "strong column–weak beam" failure pattern, and the performance of their vertical load-bearing components varies notably. Our study reveals that the uneven distribution of infill walls leads to divergent mechanical behaviors in columns located at different positions within the frame structure. To address this issue, an acceleration–strain coupled testing system was developed and implemented in a typical reinforced concrete (RC) frame structure of a veranda-style teaching building. This system, utilizing strong-motion seismographs for trigger acquisition, collected synchronized strain data from both columns adjacent to the veranda and those partially constrained by infill walls. Two sets of records were obtained, preliminarily confirming the internal force concentration due to the infill walls' constraints. Furthermore, the varied mechanical behaviors of components along two axes emerge as the primary factors causing significant structural damage and collapse. Based on these characteristic behaviors, quasi-static tests were conducted on frame columns, both unrestricted and partially restricted by infill walls. Results indicate that the constraining effect of the partial infill walls intensifies with deformation, markedly affecting column behavior. Notably, the load-bearing capacity, ductility, and ultimate displacement exhibited substantial differences between the two column types. The coexistence of these mechanically diverse components within the same structure leads to a sequence of failures, thereby altering the mechanisms of structural damage and collapse.

An efficient framework for structural seismic collapse capacity assessment based on an equivalent SDOF system

Assessing the structural collapse capacity efficiently and accurately is a key issue in earthquake engineering. Here, an efficient framework for structural seismic collapse capacity assessment based on an accurate equivalent single-degree-of-freedom (ESDOF) system is proposed. The corresponding ESDOF system is calibrated by matching the cyclic static pushover (SPO) curves of a more complex multi-degree-of-freedom (MDOF) system (e.g., high-fidelity finite element models) through a meta-heuristic optimization method. In this way, both the backbone curve and hysteretic characteristics (the stiffness and strength deterioration, and pinching behavior) of the complex MDOF system are considered in the corresponding ESDOF system. Then, incremental dynamic analysis (IDA) is carried out to assess the structural seismic collapse capacity using the ESDOF system instead of the corresponding MDOF system to improve the computational efficiency. The efficiency and accuracy of the proposed framework have been validated through three case studies, including a bare reinforced concrete (RC) frame, a steel frame, and an infilled wall RC frame. These results affirm that the proposed framework can accurately and efficiently assess the collapse capacity of low-rise bare and infilled RC frame structures, as well as steel frame structures.

Progressive collapse resistance of RC beam-slab substructure under two-adjacent-edge-columns removal scenario

The progressive collapse resistance of reinforced concrete (RC) frame structures has attracted much attention following a series of catastrophic accidents of collapse in the history. Of the numerous studies conducted in the past, the majority devoted to single-column removal cases. However, more than one column may initially fail when the structures are subjected to accidental actions. Meanwhile, the peripheral edge-columns of the structures are most vulnerable to threats due to their accessibility to the public. To this end, this study aims to investigate the progressive collapse resistance of the RC frame structures after removing two-adjacent-edge-columns. First, a quasi-static push-down experiment was performed on an RC beam−slab substructure under the scenario of two-adjacent-edge-columns removal. The experimental results found that the resistance of the beam−slab substructure at the beam mechanism stage was mainly provided by the flexural action (FA) of the beams and slabs, as well as the compressive arching action (CAA) of the beams and the compressive membrane action (CMA) of the slabs along the edge direction. The resistance increased again after entering the catenary mechanism stage, which mainly benefited from the development of the catenary action (CTA) of the edge-beams and the tensile membrane action (TMA) of the slabs along the edge direction. Second, a validated numerical model was employed to further investigate the collapse-resistant mechanism of the substructure. The numerical results indicated that the contribution of the CTA in the edge-beams to the progressive collapse resistance was approximately 51 % at the ultimate state, which was greater than that of the TMA of the slabs. Thereafter, parametric studies were conducted to quantify the contribution of the slabs and the influence of the number of removed columns on the progressive collapse resistance of the substructure. It was found that the ultimate resistance of the beam−slab substructure was 61 % higher than that of the substructure without slab, and the equivalent uniform load that the substructure could resist in the case of two-adjacent-edge-columns removal was about two-thirds of that in the case of one edge-column removal. Finally, the first peak resistance of the RC beam−slab substructure was predicted using yield-line method, resulting in a conservative result because the CAA of the beams and the CMA of the slabs were not considered.

Experimental studies on improving progressive collapse resistance of RC beam–column assemblies using kinked steel plates

One of the commonly applied methods to improve the progressive collapse resistance of reinforced concrete (RC) frame structures is to increase the amount of longitudinal rebars in beams. However, this method may lead to the unfavorable failure mode of “strong beam-weak column” for the RC frame structures under earthquakes. To prevent from this failure mode, a novel method is presented in this study using kinked steel plates (KPs) that were locally debonded and longitudinally embedded in the RC beams to improve the progressive collapse resistance of the RC frames. First, quasi-static loading tests were conducted on four 1/3 scale RC beam − column assemblies, including an equal-span specimen without the KPs, an equal-span specimen containing the KPs, an unequal-span specimen without the KPs, and an unequal-span specimen containing the KPs. Test results indicated that the ultimate resistance was improved by 89% and 81%, and the deformation capacities were increased by 39% and 38% for the equal-span and unequal-span beam − column assemblies containing the KPs compared to those without the KPs, respectively. Afterwards, high-fidelity finite element (FE) models were established to analyze the resistance contributions of the RC beam − column assemblies at the catenary action stage. Combining the experimental and FE analysis results, the mechanism was revealed for the RC beam − column assemblies using the KPs, i.e., the KPs could improve the ultimate rotational capacity of the beam-ends and induce the behavior of “two batches of plastic hinges”, resulting in fully mobilizing the catenary action to resist progressive collapse. Finally, an analytical model was developed to predict the ultimate resistance of the RC beam − column assemblies.

Experimental and Numerical Investigations of the Seismic Performance of Reinforced Concrete Frames Strengthened with CFRP Sheets

To study the seismic strengthening of damaged reinforced concrete (RC) frames using CFRP sheets, this study designed and tested the scaled 2-bay and 2-storey RC frame specimens. After applying a low cyclic horizontal load to simulate the initial damage to the specimen in an earthquake disaster, CFRP was used to strengthen the joints of the damaged RC frame. Pseudo-static tests of strengthened specimens and counterpart specimens were then carried out. Seismic performance, including stiffness, load capacity, ductility and energy dissipation were further analyzed. The failure mode of strengthened RC frame structures showed excellent ductility. The results demonstrated that the strengthening method involving wrapping CFRP can significantly improve the maximum horizontal bearing capacity, initial stiffness and energy dissipation capacity of the non-ductile reinforced concrete frame structure. The average displacement ductility coefficient of strengthened specimen can be enhanced to 3.41 compared with that of counterpart specimen (3.00). The pushover analysis based on the OpenSees model determined that the prototype frame with CFRP strengthening can maintain structural integrity and safety, with its maximum interstorey displacement angle below the limit of seismic specification (i.e., 1/50 in a severe earthquake). This study can contribute to the development of practical and efficient methods for restoring and improving the performance of damaged RC frames in seismic-prone regions.