Reinforced Concrete Moment Frames Research Articles

Seismic Retrofit and Strengthening of Reinforced Concrete Moment Frames with Strongback-Braced System

Seismic Retrofit and Strengthening of Reinforced Concrete Moment Frames with Strongback-Braced System

Optimal Seismic Design Method Based on Genetic Algorithms to Induce a Beam-Hinge Mechanism in Reinforced Concrete Moment Frames

Optimal Seismic Design Method Based on Genetic Algorithms to Induce a Beam-Hinge Mechanism in Reinforced Concrete Moment Frames

Experimental investigation of concrete frames reinforced with GFRP bars under reversed cyclic loading

Fiber reinforced polymers (FRP) have been widely investigated and successfully used as internal reinforcement in new reinforced concrete (RC) members and structures such as slabs, beams, bridges, multi-storey car parks and others as means to overcome the corrosion problem of steel. The linear-elastic behaviour, brittle failure and low modulus of elasticity are some of the significant mechanical characteristics of FRP rebars which raise concerns on their capability to allow structural members to dissipate energy in seismic zones and thus limit their use in earthquake resistant RC structures (such as moment resisting frames). The behaviour of concrete frames reinforced with FRP bars is still in its early stages of research and development with only a few researchers exploring such structures under in-place cyclic loading (seismic loading). As a result, design codes do not provide guidance for RC moment frames internally reinforced with FRP. This study attempts to partially fill the gap by investigating the behaviour of glass fiber reinforced polymers (GFRP) RC frames under in-plane loading. Six 1/3 scaled RC frames were constructed and tested under reversed cyclic loading to simulate seismic loading conditions. Three were reinforced with GFRP bent bars and three (their counterparts) with conventional steel. The testing was conducted in displacement-controlled mode with the loading history according to ACI 374.1–05. Test parameters included longitudinal reinforcement ratio and arrangement in the columns and presence of links in the joints. The experimental results are presented as hysteretic curves, lateral load / drift graphs, stiffness degradation and cumulative energy dissipated with the overall indication of the feasibility of GFRP RC frames in seismic regions. All GFRP specimens successfully reached 2.75% loading drifts. The energy dissipation of GFRP specimens was lower at initial drifts compared to the steel reinforced samples, but it become greater at final drift. GFRP frames had approximately the same load bearing capacity as their counterparts and even greater in one case. Ultimate loads for G frames were reached at higher level of displacement than for S frames.

Identification of Damage in Planar Multistory Reinforced Concrete Frames Developing a Beam-Sway Plastic Mechanism Using the “M and P” Technique

The effectiveness of a recently proposed methodology for the identification of damage in planar, multistory, reinforced concrete (RC) moment frames, which develop a plastic yield mechanism on their beams, is showcased here via the examining of a group of such existing multistory frames with three or more unequal spans. According to the methodology, the diagram of the instantaneous eigenfrequencies of the frame in the nonlinear regime is drawn as a function of the inelastic seismic roof displacement by the performance of a sequence of pushover and instantaneous modal analyses with gradually increasing target displacement. Using this key diagram, the locations of severe seismic damage in an existing moment frame can be evaluated if the instantaneous fundamental eigenfrequency of the damaged frame, at an analysis step within the nonlinear area, is known in advance by “the monitoring and the identification of frequencies” using a local network of uniaxial accelerometers. This is a hybrid technique because both procedures, the instrumental monitoring of the structure and the pushover analysis on the frame (M and P technique), are combined. A ductile, five-story, planar RC moment frame with three unequal spans is evaluated in this paper by the M and P technique. The results show that the seismic roof displacement, the lateral stiffness matrix, and, finally, the damage image of this existing frame, are fully compatible with the eigenfrequencies identified by the monitoring and are calculated with high accuracy.

Comparative analysis of the sustainability and seismic performance of a social interest house using RC moment frames and bahareque as structural systems

This study compares the seismic performance and environmental impact of a social housing structure designed with reinforced concrete with a structure using Guadua angustifolia “Kunth” cane. The aim is to contrast the implementation of an ecological material such as the Guadua cane, which is an accessible alternative due to its cost and construction time, versus the traditional reinforced concrete (RC) construction method. Both applied to social housing structures. The seismic performance of both methods is analyzed through nonlinear static analysis (pushover) with the objective of establishing the performance; structural and nonstructural damage, performance point, maximum displacements, and structural elements that induce structural failure; and acting forces, against a design earthquake (established by the NEC DS 2015 Standard), with a return period of 475 years. The environmental impact is evaluated through a life cycle assessment of the structure (LCA). Thus, the embodied carbon obtained from each structural element (foundations, beams, columns, floors, and roof support elements) was determined, considering material manufacturing, transportation, and construction. The results obtained demonstrated a higher seismic performance, with 70% less environmental impact on the Guadua cane structure.

Seismic resilience of typical code-conforming RC moment frame buildings in Canada

This article examines the baseline resilience of reinforced concrete (RC) moment frame buildings conforming to the seismic design standards of Canada. Metrics for robustness, rapidity, and resilience are evaluated to capture the system’s reliability, speed of recovery, and socioeconomic impacts. Buildings of different heights are evaluated using nonlinear time-history analyses. Six damage states are defined as disjoint branches of an event tree depending on the building’s path to recovery. For a scenario earthquake of magnitude 7.3 magnitude at a distance of 30 km from Vancouver, the housing occupancy recovery trajectory is developed. Monte Carlo simulations are used to propagate uncertainty from seismic hazards to the building response to the lead time required for recovery. Buildings are found to maintain 50%–65% of their pre-event housing occupancy in the immediate aftermath. The housing occupancy is restored to 90% within 2–4 months, with a shorter recovery period for low-rise buildings, whereas the system resilience level requires 6 months to 1 year for restoration to 90%. Empirical data from the Loma Prieta (1989) and Northridge (1994) earthquakes are used to compare analytically predicted repair times. The findings from this article will facilitate the shift to resilience-based design in Canada.

Effect of Joint Flexibility on Seismic Performance of a Reinforced Concrete Ductile Moment-Resisting Frame

Currently, the seismic collapse risk of modern code-conforming reinforced concrete (RC) frame structures is often evaluated without considering the behavior of the beam-column (BC) joints since joints designed to meet modern concrete design codes are assumed to be sufficiently rigid. BC joints in RC ductile moment frames may undergo a significant amount of inclined cracking during strong earthquakes. Consequently, the stiffness, strength, and dynamic characteristics of the RC moment frames may vary. Hence, the rigid joint assumption may be unsuitable for assessing the seismic performances of RC ductile moment frames because the local strength and ductility demands of the constituent members can be misinterpreted by structural analyses based on the assumption of a rigid joint. In this study, nonlinear static and dynamic analyses are conducted to quantify the seismic response variations of low- to midrise RC-frame structures under different joint modeling assumptions. In total, six 2D continuum finite element models of four- and eight-story frames with rigid and flexible joints were constructed by simulating the actual structures as realistically as possible. The results indicate that the interior joints in the considered frames suffer light-to-moderate damage, and the effect of joint cracking is found to be significant in a four-story structure whose column-to-beam flexural strength ratio and column-to-beam area ratio are below 1.5. Such joint behavior demonstrates that current code provisions cannot ensure joint behavior compatible with the rigid joint assumption that is used in the practical structural analysis of RC-frame buildings.

Machine learning-based seismic damage assessment of non-ductile RC beam-column joints using visual damage indices of surface crack patterns

After a significant earthquake, the updated status of the structural elements is usually determined based on a qualitative visual inspection. Although visual inspection provides a prompt assessment of the damaged elements, the output of this subjective method is influenced by the experience and decision of a trained inspector, which may vary from case to case. In this study, an innovative machine learning-based procedure is developed to automate damage state identification of non-ductile reinforced concrete moment frames (RCMFs) utilizing visual indices of crack patterns of the concrete surface. An extensive database including 264 surface crack patterns is constructed corresponding to 61 non-ductile beam-column joint specimens tested under the quasi-static cyclic loads at different drift ratios. Two visual damage indices, including the cumulative length of cracking and the areal density of crushing, are extracted using image processing filtering on the collected images. Supplementary information of the specimen, such as aspect ratio and concrete compressive force, is also considered as input variables. Various machine learning-based predictive models are developed for estimating the maximum drift ratio of a damaged non-ductile beam-column joint based on the available information of the specimen. Two scenarios are considered based on the accessibility of the compressive strength of the concrete as the input data, and a predictive model is obtained for each scenario. The peak experienced drift ratio predicted by the proposed methodology is finally used as an engineering demand parameter attributed to the corresponding fragility curves to determine the damage state of non-ductile RCMFs.

Seismic Retrofit of Reinforced Concrete Moment Frame Using a BRB with U-Plate Connections

Seismic Retrofit of Reinforced Concrete Moment Frame Using a BRB with U-Plate Connections

Influence of ground motion frequency content on the nonlinear response and seismic performance of RC moment frame structures

Several parameters may affect the response of structures to seismic excitation. These include features of the structures, properties of imposed ground motion and geological specifications of building location. This research has been conducted to investigate the effects of the frequency content of earthquakes as an important property of ground motion on the seismic performance and nonlinear response of reinforced concrete moment frame (RCMF) structures. To this end, forty-two earthquake records have been selected from the stations on site classes A and B based on ASCE7-10. These input excitations have been classified into three groups based on PGA to PGV ratios namely: Low, Intermediate, and High frequency motions. Three RCMFs with 2, 6, and 10 stories have been designed according to ACI318-14 and ASCE7-10. Finally, utilizing OpenSEES platform, based on the FE method a numerical model has been built to perform incremental dynamic analysis (IDA). After interpreting the results of the IDA, fragility curves have been obtained as a function of PGA for pre-defined damage states. The results showed that the associated PGA with the median of the fragility curve is in ascending order Low, Intermediate and High frequency motions. Accordingly, it is concluded that in the same PGA level, Low frequency motions are more probable to reach a damage threshold than Intermediate and High frequency motions. Similarly, Intermediate frequency motions is more capable reaching the damage threshold in comparison to High frequency motions.

Evaluation of Intermediate Reinforced Concrete Moment Frame Subjected to Truck Collision

In this study, the progressive collapse of reinforced concrete structures due to vehicle collision to the columns of the ground floor was modeled and examined. For this purpose, a four-story reinforced concrete building with the intermediate moment frame system was designed using ETABS software followed by the simulation of impact loading by SAP2000 software. Performing non-linear time history dynamic analysis, the critical forces required to the column failure were determined via trial and error by considering different live load contribution. Then, the corresponding critical velocities for 4, 8, and 12 ton vehicles were determined. Finally, the progressive collapse of the building was examined by the sudden removal of the column. The results showed that by increasing the percentage of live load contribution, the force and critical velocity for the instability and damage of the column will decrease. Furthermore, comparing the perimeter and corner columns showed that the corner columns are the most critical columns for occurrence of the progressive collapse. In addition, during the assessment of the progressive collapse, it was found that the number of damaged springs in the corner column removal scenario is less than that of the perimeter column removal scenario.

An eco-friendly reliability-based design optimization of intermediate reinforced concrete moment frames

Failing to account for the stochastic nature of structural parameters such as material properties, external loads and geometric dimensions, can cause uncertainty about the safety of the design of an engineering structure; thus, assessing the reliability of a design seems necessary. The present study considered uncertainty related to the statistical nature of the parameters affecting the design of intermediate reinforced concrete (RC) moment frames in the process of achieving an optimal design. The reduction of CO2 emissions, which comprises a large percentage of greenhouse gas emissions, also must be considered from an environmental perspective in the design and construction of RC structures. Hence, an objective function was formulated that also minimizes CO2 emissions. In the objective function, the effective design parameters with probabilistic distributions have been considered when calculating the reliability index using the Monte Carlo simulation method. The limit state functions also are taken into account according to the design requirements in ACI 318-19. Ultimately, the performance of the formulated objective function in the reliability-based design optimization of four-story and nine-story intermediate RC moment frames is presented using four metaheuristic algorithms.

Seismic fragility analysis of buckling-restrained brace-strengthened reinforced concrete frames using a performance-based plastic design method

An easily-operated performance-based plastic design (PBPD) method for buckling-restrained braced (BRB) reinforced concrete (RC) moment frames is adopted in this research to strengthen existing moment frames by BRBs. The appropriate range of the story base shear ratio p (i.e., the ratio of the base shear resisted by the BRBs to that of the total system) is suggested through a seismic fragility analysis. With an earthquake ground motion set of thirty far-field records, the seismic fragility analysis and risk assessment of an 8-story RC moment frame structure and its strengthened structures with BRBs are reported in this article, in which the fragility curves for different limit states and the corresponding annual exceedance probability are calculated. The applicability of 16 ground motion intensity measures (IMs) in the fragility analysis of the structure before and after strengthening under three damage limit states is further reviewed. As indicated by the fragility analyses, a higher seismic resistance capacity can be obtained for the strengthened structure when the ratio p varies between 0.5 and 0.8. The seismic performance of the structures strengthened using the PBPD method can increase steadily till the advent of the preset target ultimate drift limit, after which the improvement efficiency decreases. The IM Sa,gm(Ti) that considers both effects of softened period and higher mode has a better performance in each limit state of the structure both before and after strengthening.

Lateral load performance of a reinforced concrete frame with pultruded GFRP box braces

Reinforced concrete buildings, which constitute most of the building stock in Turkey, should be examined in terms of earthquake resistance. Many studies have been carried out to increase the seismic resistance of reinforced concrete (RC) structures against earthquakes. In this study, a single-story, single-span RC frame stiffened with chevron steel braces was chosen as a reference frame and laterally loaded to failure. In the first step, the experimentally obtained static lateral load-deflection curve was verified by non-linear finite element (FE) analysis. Then, the uniaxial tensile properties of the selected glass fiber reinforced polymer (GFRP) materials were verified using FE analysis with the results found in the literature. In the second step, three different GFRP box braces with different axial stiffnesses were investigated and the results were compared with those of the steel chevron braces. Finally, the lateral load performance and expected buckling failure of the GFRP box braces in an RC moment frame have been presented and discussed in this study. Considering the lightweight of the GFRP sections, the lateral load capacity of the RC frame with GFRP braces was improved as much as the steel braces, and the maximum gain was about 47% more when the equal axial stiffness of steel brace was provided to GFRP brace. Ductility and story drift of the considered braced moment frames are presented.

Experimental investigation of strengthening of masonry-infilled RC frames using prefabricated engineered cementitious composite panels

Cement-based composite materials such as engineered cementitious composites (ECCs) are appropriate for retrofitting buildings because of their high tensile strength and strain. This research evaluated the lateral behavior of strengthened masonry-infilled reinforced concrete (RC) frames reinforced with prefabricated engineered cementitious composite (PECC) panels. The goals of the proposed technique were to increase the energy absorption capacity and the lateral capacity of infilled RC frames and protect the stability of the masonry infills after cracking and brittle failure in the out-of-plane direction, which can contribute to the seismic vulnerability of a structure. The mechanical characteristics of the ECC materials, such as the tensile strength and strain and multiple cracks, were first determined. Then, nine diagonal compression tests were done on masonry panels strengthened with PECC panels. In these tests, two types of connection were compared and the best one was selected. Next, an existing one-story RC moment frame with one bay was scaled down for testing under lateral cyclic loading. The results indicated that the strength, deformation, stiffness, and energy absorption of the RC frames increased considerably. Additionally, the effects of the precast panel thickness, performance of the connections, and type of damage sustained were compared between the different types of tests. Overall, the PECC panels connected to the infill wall with the proposed connections improved the lateral behavior of the system in the in-plane direction and markedly mitigated the risk of collapse by confining the masonry infill wall.

Improving the seismic performance of reinforced concrete frames using an innovative metallic-shear damper

In reinforced concrete (RC) frames, the applied seismic energy is dissipated through the ductile behavior of RC moment resisting frames. The main elements of the RC frames carry the gravity loads in addition to the seismic loads. Since the RC frames are sensitive to gravity loads, the ductility of the frames is reduced by increasing the gravity loads. Moreover, because of the low stiffness of the RC moment frames, huge member sizes of structural elements are required to control lateral drifts under lateral loading. In addition, the existing RC moment frame buildings with non-ductile characteristics pose a considerable hazard during earthquakes. To solve the mentioned problems, an innovative metallic damper with a shear link (metallic-passive energy damper) was developed in this paper. The proposed damper not only enjoys an easy fabrication and a good seismic performance but also can be easily replaced after a severe earthquake. Since the damper does not carry the gravity loads, replacing the damper does not affect the severability of the building during repairing. The main goal of this study is to confine the plastic deformation in the proposed damper. The numerical results indicated that the proposed damper improved the behavior of the RC frame in elastic and inelastic zones. It enhanced the shear capacity, shear stiffness, energy absorption, and ductility of the RC moment resisting frame. The results indicated that the proposed damper enhances the shear stiffness and the ultimate shear capacity from a minimum of 11% to a maximum of 24% and from a minimum of 11% to a maximum of 48%, respectively. Also, the proposed damper improved the yielding strength from a minimum of 53% to a maximum of 61%. Moreover, the dynamic analysis indicated that the damper improved the behavior of the system in the case of maximum lateral displacement and base shear. Based on the time-history dynamic analysis, dampers in the 5- and 10-story frames are more effective compared to the 15-story frame. This result confirms the suitable performance of the proposed damper. Herein the required equations and the recommendations for the design of the proposed metallic-shear damper have been presented.

Experimental dynamic characterization of friction brace dampers and application to the seismic retrofit of RC buildings

Previous earthquakes have caused extensive damage to reinforced concrete (RC) structures with insufficient lateral force resistance or energy dissipation capacity. There is a need to retrofit vulnerable existing RC buildings, particularly those not originally designed for seismic effects or designed to an outdated seismic specification. This study investigates the use of friction dampers as displacement-dependent energy dissipation devices to retrofit RC moment frame structures. First, an experimental program was conducted to characterize the dynamic behavior of friction brace dampers with several different materials, finding that a sintered metal compound provided a stable friction coefficient of 0.4. A strength-based equivalent linearization design procedure was then developed based on the required friction slip force and considering the cracked state of the existing RC structure. A four-story RC school building was then designed using the proposed retrofit design method, validated using nonlinear response history analysis and compared to a previous retrofit scheme that employed buckling-restrained braces (BRBs) and a stiffness-based equivalent linearization design method. The analysis results suggest that the proposed retrofit design method and friction brace dampers are effective in reducing the maximum story drift.

Investigation of the behavior factor of Reinforced Concrete Moment Frame with concrete Voided slab constructed by Spherical and Cubic Balls

In order to prevent damage caused by earthquakes in different types of structures including concrete structures, various methods are used by structural engineers considering terms of design and working conditions. Simple slabs, bubble hollow slabs (Spherical) and Cubic hollow slabs may be used to meet the structural requirements and also to increase performance level of the structures against earthquakes. This research aimed to evaluate the performance of these structures in different conditions and evaluate their impact on the structural behaviour coefficient. For this purpose, ABAQUS finite element software has been utilized to model a concrete moment frame with one span in one floor equipped with simple concrete slab, Spherical and Cubic hollow slabs in different conditions. Then the required analysis has been performed to evaluate the behaviour of these different type slabs. Verification of the model is implemented According to the study of Leimam et al. Then, in different models, the coefficient of structural behaviour is calculated and studied. The results show that using Cubic and Spherical slab could increase the behaviour coefficient of concrete moment frame in comparison with simple slabs. In the case of concrete moment frame with Cubic and Spherical slab, behaviour coefficient increases 32.79% and 46.3% respectively.

Load redistribution pattern in a RC moment frame due to excavation-induced 3D ground surface settlement profiles

Following abnormal events, identification of load redistribution pattern in the load-bearing elements is an important issue in the field of structural engineering. As a possible event, one can imply to initiation of excavation-induced large differential settlements under the columns of common reinforced concrete (RC) moment frames. A large portion of existing literature have studied the effect of single-column settlement scenarios or tunnel-induced ground movements on 2D frames, however. Using two series of real ground surface settlement profiles (called medium and large) representing deep excavation adjacent to the construction site, 8 analysis cases were defined to gradually impose differential settlements under the columns in this paper. Apart from different 3D settlement profiles, the structure was assumed to be located on the corner or center of excavation edge, and two directions were also considered for the building plan. Based on the findings of the numerical models, the axial compressive force significantly decreased in a series of highly settled columns. Also the columns which were relatively far from the location of maximum settlement made key contribution to redistribution of loads. Moreover, a large number of columns experienced tensile axial forces in some of the cases with larger surface settlement imposed. The change in the direction of beam flexural moments in conjunction with some columns (not limited to the column having the largest settlement) shed light on possibility of damage in an extensive area of the building plan.

Probabilistic damage evaluation of base-isolated reinforced concrete structures under near-fault pulse-like bidirectional seismic excitations

In this article, seismic fragility of a base-isolated special Reinforced Concrete (RC) moment frame is investigated under near-fault pulse-type ground motion records. The vulnerability of the displacement-dominated performance of isolator devices to a likely vibration resonance caused by such records is the focus of the study. For this purpose, Lead-Rubber Bearings (LRBs) and Triple Friction Pendulum Bearings (TFPBs) are utilized to represent various isolator behaviorisms in different period ranges. The structures are modelled in three dimensions to account for the axial-bending-bending interaction in the isolator devices. The models are then subjected to IDAs under three sets of near-fault pulse-type ground motion records with different pulse periods. Four different damage states are considered and the IDA results are used for deriving fragilities that express probabilities of exceeding these states at various intensity levels. The study revealed that despite the amplifying effects of the employed records, using isolators is still feasible. That is, even for pulse periods near to the effective period of isolators, resonance has not led to higher failure probabilities in the isolated buildings compared to their fixed-base counterparts. Furthermore, at longer pulse periods, the gradual yielding behavior of TFPBs made them more effective than the LRBs. Results also disclosed that at lower intensities, damage of base-isolated structures is mainly caused due to excessive floor acceleration while at higher intensities the maximum inter-story drift plays the dominant role. This suggest these two response parameters to be alternatively used as the Engineering Demand Parameter (EDP) when defining damage states at the medium and high intensity levels.