Reinforced Concrete Moment-resisting Frames Research Articles

Seismic capacity evaluation of reinforced concrete moment-resisting frames with steel damper columns using incremental critical pseudo-multi impulse analysis

Steel damper columns (SDCs) are energy-dissipating members that are suitable for reinforced concrete (RC) moment-resisting frames (MRFs) and those often used for multistory housing. In a previous study, the authors proposed an energy-based prediction procedure for the peak and cumulative response of an RC frame building with SDCs. In this procedure, the accuracy of the equivalent velocity of the maximum momentary input energy (VΔE1*)–peak equivalent displacement (D1*max) relationship is essential for improved prediction. In this article, the seismic capacity curve (VΔE1*–D1*max relationship) of RC MRFs with and without SDCs is evaluated using incremental critical pseudo-multi impulse analysis (ICPMIA). In the ICPMIA, which is based on a study by Takewaki and coauthors, the structure is subjected to various intensities of critical pseudo-multi impulsive lateral force. An ICPMIA of planer four 8- and 16-story RC MRFs with and without SDCs is performed to obtain their structural behaviors under various intensities of pulsive input. Then, the seismic capacity curve obtained from the ICMPIA results are compared with the predicted results based on the simplified equations. The main findings of this article are as follows. (i) The seismic capacity curve of RC MRFs without SDCs strongly depends on the number of impulsive lateral forces (Np). As Np increases, the seismic capacity decreases. The predicted seismic capacity curve severely underestimates that obtained from the ICMPIA in the case of large Np. This trend is notable when the level of pinching behavior in the RC members is severe. (ii) In the case of an RC MRF with SDCs, however, the influence of Np on its seismic capacity curve is small. The predicted seismic capacity curve agrees very well with that obtained from the ICMPIA. The influence of the level of pinching behavior in RC members on the seismic capacity curve is small.

Critical pseudo-double impulse analysis evaluating seismic energy input to reinforced concrete buildings with steel damper columns

Steel damper columns (SDCs) are energy-dissipating members that are suitable for reinforced concrete (RC) buildings and are often used for multistory housing. The evaluation of the peak deformation and hysteretic dissipated energy of such building structures is essential for the rational seismic design of RC buildings with SDCs. In a previous study, the authors proposed an energy-based prediction procedure for the peak and cumulative response of an RC frame building with SDCs. In this procedure, the accuracy of the equivalent velocity of the maximum momentary input energy (VΔE1*)–peak equivalent displacement (D1*max) relationship is essential for high quality predictions. In this article, the VΔE1*–D1*max relationships of RC moment-resisting frames with and without SDCs are investigated using a critical pseudo-double impulse (PDI) analysis based on a study by Takewaki and coauthors. The results show that the VΔE1*–D1*max relationship obtained from the critical PDI analysis agrees well with that calculated from the equations proposed in the previous study.

Finite element analysis of RC moment-resisting frames to assess effective slab width; a parametric study

In reinforced concrete (RC) moment-resisting frames (MRFs) with cast-in-situ slabs, the slab action as tension flange during lateral loading enhances the flexural capacity of beams in negative moments. This enhancement may lead to strong beam-weak column unfavorable failure mode in these structures. Field observations of RC MRFs during strong ground motion indicated that the effective slab width values considered by the seismic provisions of the design codes are not adequate to guarantee the achievement of the design method of weak beam-strong column. An accurate estimation of effective slab width needs to assess factors such as geometrical characteristics of connection, continuity of models, and imposed story drift level. A 3D spatial two-story RC MRF was developed using finite element analysis (FEA) to explore the effect of mentioned factors on effective slab width. For finite element (FE) modeling of RC MRF, the concrete damaged plasticity (CDP) model is applied. The CDP model parameters are defined using a mesh-independent methodology introduced in the literature. Comparing the FEA results and results obtained from three half-scale RC MRFs, tested under reversed cyclic loading conditions showed an excellent correlation. The results predicted by the FE model in terms of lateral load–displacement response, reinforcement yielding sequence, crack patterns, and failure modes had a considerable agreement with test observations. The validated FE model was used to develop the other models to conduct a parametric study in the next step. Geometric parameters studied in this paper through numerical modeling were longitudinal and transverse beam length and height, and cross-sectional column dimension. Results of the parametric study showed that the effect of the ratio of transverse beam length to height is significantly more than the ratio of longitudinal beam length to height. The trend of effective slab width versus story drift depends on connection geometry, especially the ratio of transverse beam length to transverse beam height. Finally, an equation was driven to predict the effective slab width using linear regression analysis.

Shake-table testing and numerical simulation to select the FRCM retrofit solution for flexure/shear deficient RC frames

This paper presents shake-table and numerical investigations to select a retrofit strategy for reinforced concrete (RC) frame structures using fiber-reinforced cementitious matrix (FRCM) composites. Two FRCM alternatives are investigated, namely polyparaphenylene benzobisoxazole (PBO) and carbon FRCM composites, denoted as ‘P-FRCM’ and ‘C-FRCM’, respectively. The study focuses on implementing the selected retrofit material with spike anchorage systems to delay the debonding failure and addresses different failure modes of RC framing systems, including shear or flexure failure. The first phase of this study involves pre shake-table test (STT) simulation to investigate the effectiveness of the selected retrofit technique in delaying local and global damage states and failure modes. STT campaigns are undertaken in the second phase to assess the seismic performance of framing systems retrofitted with different FRCM materials and verify their fiber-based modeling approach. The verified numerical modeling approach of the retrofit techniques is utilized in the third phase to upgrade a benchmark RC moment-resisting frame (MRF) building. A parametric study involving inelastic pushover analyses (IPAs) and multi-record incremental dynamic analyses (IDAs) is finally conducted to assess the vulnerability of the retrofitted building with different FRCM alternatives. It is concluded that the adopted two retrofit techniques significantly reduced the collapse damage state at the design and maximum considered earthquake intensities by up to 59% and 46%, respectively. Although the P-FRCM composite has 38% higher tensile strength and 76% higher ultimate tensile strain than C-FRCM, it slightly improves the overall seismic performance by up to 9% based on the adopted seismic performance index. Owing to the higher cost of P-FRCM compared with its counterpart, a performance-to-cost index confirmed that the C-FRCM composite is the preferred retrofit technique for MRF structures.

Effect of subterranean levels on the dynamic response of RC-MRF buildings

The dynamic response of reinforced concrete moment resisting frame (RC-MRF) building structures is mostly altered by the substructure system such as the type of foundations and features of subterranean levels, as well as subsurface soil conditions. The majority of low-to high-rise buildings, in urban areas, are designed with one or multi-level subterranean levels for parking and other services. However, many designers overlooked incorporating these subterranean levels and soils in the seismic resistance analysis assuming that the building is fixed at the ground surface. The present study assessed the effect of the subterranean levels on the dynamic response of RC-MRF buildings. Several two-dimensional (2D) nonlinear finite element seismic response analyses were carried out in ABAQUS software for medium-rise buildings without and with one to multi-level subterranean levels resting on a deep homogenous soft soil profile under the effect of strong seismic input motion. It was observed that the subterranean levels considerably alter the dynamic characteristics and demands of the building structures. Hence, explicitly incorporating the subterranean components and foundation soil during the seismic resistance design of building structures is important.

Unified Simplified Capacity Model for Beam-Column Joints into RC Moment Resisting Frame

Seismic vulnerability of reinforced concrete (RC) buildings is strongly influenced by beam-column joints. Horizontal and vertical structural members converge in beam-column joints, as extremely delimited areas where the internal forces in concrete core and reinforcing bars have elevated stress gradients. In order to fully understand the seismic behavior of RC buildings and the related failures of T- and X-shaped beam-column joints (external corner and internal-positioned), an analytical model of RC joint behavior is proposed in a unified and simplified way. The equilibrium equations of cracked joint portions allow the assessment of internal stresses’ evolution at increasing values of column shear forces. In this way, the strength hierarchy is evaluated in terms of capacity for the different potential failure modes. This can drive, as a useful tool for designers, the design of new efficient structures or the assessment of existing ones to occur, with subsequent interventions to move an initial undesired failure mode to a more suitable one. Nearly 500 experimental results of tests available in the literature are compared with the analytical predictions of the proposed model.

Peak and Cumulative Response of Reinforced Concrete Frames with Steel Damper Columns under Seismic Sequences

The steel damper column is an energy-dissipating member that is suitable for reinforced concrete (RC) buildings and those used for multistory housing in particular. However, the effectiveness of steel damper columns may be affected by the behavior of surrounding members, and this effect can be severe in the case of seismic sequences. This article investigates the nonlinear response of building models with an RC moment-resisting frame (MRF) with and without steel damper columns under seismic sequences. The applicability of the concept of the momentary energy input to the prediction of the peak response of RC MRFs with damper columns under seismic sequences is also investigated. The main findings of the study are summarized as follows. (1) The peak response of RC MRFs with damper columns subjected to sequential accelerations is similar to the peak response obtained considering only the mainshock, whereas the cumulative strain energy of RC MRFs accumulates more for sequential accelerations. (2) The steel damper column is effective in reducing the peak and cumulative responses of RC MRFs in the case of sequential seismic input. (3) The relation of the hysteretic dissipated energy during a half cycle of the structural response and the peak displacement of the first modal response can be properly evaluated using the simple model proposed in this study.

Assessment of Different Retrofitting Methods on Structural Performance of RC Buildings against Progressive Collapse

Progressive collapse refers to the spread of primary local damages within the structure. Following such damages due to removing one or more load-bearing columns, the failure spreads in a chain and causes structural failure. This study represents a report investigating the influence of various retrofitting methods on the progressive collapse resistance of multistorey reinforced concrete (RC) structures. To this end, eight different cases were considered. The first one included a thirteen-story RC moment-resisting frame (bare frame), while the others were frames upgraded with the application of X-brace, diagonal brace, inverted V-brace, the viscous damper in the central bay, viscous damper in two inner bays, viscous damper only in certain stories and carbon fiber reinforced polymer. Moreover, three different column removal scenarios were considered as a column failure at stories one, six, and thirteen of each case study structure. The analysis results indicated that the redistribution of loads after the column’s failure and the RC buildings’ collapse resistance was increased depending mainly on the type of approach used for upgrading the bare frame.

Robust Design of Reinforced Concrete Moment-Resisting Frames

Reinforced concrete moment resisting frame (RCMRF) is one of the most popular structural systems. Conventionally, buildings with RCMRF systems are designed to satisfy the relative displacement, resistance, and flexibility requirements defined by the design codes. Structural design codes have given different ranges of design parameters that the designers and engineers must consider in the design process of structures and the values selected for these parameters affect the seismic behaviour of the structures. However, performance assessment of the RCMRF under the earthquake loading to limit the probable levels of damage has a complicated and difficult procedure that is time-consuming for designing of ordinary buildings. In this study, to prevent this time-consuming process, tighter ranges for design parameters have been attempted to improve the seismic performance of the RCMRFs. In this regard, databases of RCMFs were created for different ranges of design parameters. The Particle Swarm Optimization (PSO) algorithm is used to create these databases and RCMRFs are optimally designed according to ACI 318-14 code. Then, nonlinear time history analysis according to ASCE/SEI 7-16 code was performed on the RCMRFs in each one of the databases and the statistical analysis of local and global results acquired from the nonlinear time history analysis is carried out. Finally, tighter ranges of design parameters have been determined to achieve more robust structures without involvement in time-consuming processes.

An approximate method for determining the behavior factorof RCMRFs with vertical irregularity

Recently, designing irregular structures has been interested among civil engineers. The existing seismic codes like Iranian one do not present the separated response modification factor for regular and irregular structures. In this study, a procedure is presented to estimate the response modification factor of irregular Reinforced Concrete Moment Resisting Frames (RCMRFs). To do so, firstly, several irregular RCMRFs with various types of irregularity are designed based on the behavior factor of Iranian seismic code. Then, the inter-story drifts of these frames subjected to the proportional seismic ground motions are achieved and evaluated by the acceptance criteria. Secondly, the behavior factors of the aforementioned frames resulting from pushover analysis are acquired and the frames are redesigned based on these behavior factors. After that, the inter-story drifts of redesigned frames are again obtained and it is shown that these frames satisfy the admissible restrictions. In addition, two new relations are presented to acquire the behavior factor and the main period of irregular RCMRFs. Six new irregular RCMRFs are designed using the behavior factors achieved via the proposed relations. The behavior factors and the fundamental periods of these frames, which are computed from pushover and modal analyses respectively, are compared with those achieved via the presented relations. This comparison validates the accuracy of the suggested relations.

Proposal of Damage Index Ratio for Low- to Mid-Rise Reinforced Concrete Moment-Resisting Frame with Setback Subjected to Uniaxial Seismic Loading

A vertical irregularity setback in reinforced concrete (RC) building affects its performance and response especially subjected to earthquake ground motions. It is necessary to understand how the seismic damage is established due to setbacks and avoid damage concentration on the irregularity section. The objective of this study is to propose a formula to estimate the damage distribution along the height of the setback building from a geometric measure of the degree of irregularity. First, previous experimental tests for two types of setback buildings, a towered and a stepped setback frames, were analyzed to verify the accuracy of the frame analysis. The results of the frame analysis considerably matched the experimental test results. Furthermore, to study the relationship between the degree of setback and the distribution of damage, a parametric study was conducted using 35 reinforced concrete setback frames, consisting of models with stepped setback type and towered setback type with different degrees of setback. The inelastic dynamic analyses of all the frames under three earthquake ground motions were conducted. The irregularity indices proposed in literature were adopted to express the degree of setback and the structural damage was expressed by the Park–Ang damage index. Using nonlinear regression analysis, formulas to estimate damage index ratio between two main structure parts (tower and base) from setback indices were proposed. Finally, the proposed formula was applied to the experimental test results to confirm its validity.

Performance evaluation of RC-MRFs with UHPSFRC and SMA rebars subjected to mainshock-aftershock sequences

Although structures are generally subjected to mainshock-aftershock sequences, the current structural design process merely considers the main seismic event. This study aims to compare the performance of Reinforced Concrete Moment Resisting Frames (RC-MRFs) with Shape Memory Alloy (SMA) and Ultra-High-Performance Steel Fiber Reinforced Concrete (UHPSFRC) under the influences of mainshock and aftershock sequence. To achieve this aim, 3, 6, and 8-story five-span frames were simulated considering three different approaches: (i) RC frames; (ii) RC frames with SMA rebars in the plastic hinge length of the beams; (iii) RC frames with UHPSFRC and SMA in the plastic hinge length of the beams.Each frame was analyzed under two different scenarios, with and without considering seismic sequences to assess the post-earthquake performance of damaged RC frames. Time history analyses were conducted using seven real accelerograms. Maximum transient inter-story and roof drifts, as well as residual inter-story drifts, were determined and compared in undamaged and damaged structures to assess the effect of seismic sequence. As the performance evaluation of the damaged structures appears to be essential to make repair policy decisions and rescue operations, damage states were defined according to FEMA-356 based on maximum transient and residual drifts. Numerical results illustrate the salient effect of using SMA and UHPSFRC synergistically in reducing transient and residual drifts and enhancing the functionality of structures in both mainshock and aftershock.

On the accuracy of enhanced pushover procedures for seismic performance evaluation of code-conforming RC moment-resisting frame buildings subjected to pulse-like and non-pulse-like excitations

In this paper, the applicability and validity of enhanced pushover procedures in estimating the seismic demands of mid- and high-rise reinforced concrete (RC) building frames under pulse-like near-fault ground motions having forward-directivity and fling-step effects as well as non-pulse-like near-fault and far-fault ground motions are assessed through the comparison with the nonlinear response history analysis (NL-RHA). The enhanced pushover procedures including the modal pushover analysis (MPA), force-based adaptive pushover (FAP), displacement-based adaptive pushover (DAP), extended N2 method (EN2), single-run multi-mode pushover (SMP), and non-adaptive displacement-based pushover (NADP) procedures, as well as a conventional pushover method based on the first mode of the structure, are performed. To this end, three plan-symmetric buildings including the 9-, 12-, and 18-story RC frames designed in accordance with the provisions of ASCE 710 and ACI 318-14 are examined. The engineering demand parameters considered are the interstory drift ratios and story shears, which are representatives of the deformation and force demands, respectively. Additionally, the seismic performance of the RC buildings is assessed using the conventional and the enhanced multi-modal pushover procedures in accordance with the systematic evaluation scheme of the ASCE 41-17 standard. The comparative evaluation demonstrates that the accuracy of the NSPs varies with the type of the ground motions. Some methods result in satisfactory performance for pulse-like ground motions, some for non-pulse-like ones, and some for both of them. All in all, the MPA, EN2, and SMP procedures provide a relatively better prediction of the seismic demands of RC building frames.

Nonlinear Simulation of Reinforced Concrete Moment Resisting Frames under Earthquakes

Nonlinear Simulation of Reinforced Concrete Moment Resisting Frames under Earthquakes

Seismic Performance Evaluation of Code-compliant RC Moment-resisting Frame Buildings Subjected to Near-fault Pulse-like and Non-pulse-like Ground Motions

ABSTRACT The close proximity of some populated cities all over the world to the active faults shows the necessity to investigate the consequences of the prominent characteristics of near-fault pulse-like ground motions (i.e., the forward directivity and fling-step) on the seismic performance of the code-compliant multi-story buildings. This paper attempts to quantify the effects of pulse-like near-fault ground motions on the seismic responses of reinforced concrete (RC) moment-resisting frame (MRF) buildings and to compare the results with those induced by non-pulse-like ground motions. For this purpose, three archetype buildings with 3-, 9-, and 18-story heights were designed in accordance with the provisions of ASCE 7–10 and were subjected to 48 ground motions in four sets including the forward-directivity, fling-step, non-pulse near-fault, and far-fault records. The results demonstrate that the contribution of higher modes to the seismic responses is more predominant in the non-pulse-like ground motions than their pulse-like counterparts. Moreover, the maximum seismic demands imposed by the pulse-like ground motions are higher than those for the non-pulse-like motions. A detailed assessment of the seismic performance of the RC buildings based on Tier 3 systematic evaluation of the ASCE 41–17 standard is another objective of this study. Findings from this evaluation reveal that the 3-story frame fails to meet the acceptance criteria under the pulse-like ground motions, whereas both the 9- and 18-story frames satisfy the performance criteria of ASCE 41–17 for both the pulse-like and non-pulse-like ground motions. This indicates that the performance of the buildings designed in accordance with ASCE 7–10 does not necessarily satisfy the performance expected in ASCE 41–17.

Strength, Deformation and Fragility assessment of Reinforced Concrete Moment Resisting frame designed by Force Based Design and the Performance Based Plastic Design method for Seismic loads

A 20 storied Reinforced Concrete (RC) Moment Resisting Frame (MRF) has been analysed and designed by Force Based Design (FBD) method and the Performance Based Plastic Design (PBPD) method. The PBPD frames were designed for all the three performance levels namely Immediate Occupancy, Life safety and Collapse Prevention of the basic safety objective. The guidelines of the Indian Standard codes along with established principles of design have been incorporated in the study. Nonlinear static pushover analysis was the tool used to evaluate the seismic performance of the frames in terms of strength and deformation. The fragility assessment has been carried out in terms of Spectral displacement obtained from the pushover analysis results for different specified damage states. Results indicate that PBPD method excels the FBD method, both in terms of seismic performance and fragility.

Peak Velocity of Viscous Dampers for Direct Displacement Based Design of RC Moment Resisting Frames

ABSTRACT Previous researchers have extended the direct displacement-based design (DDBD) procedure, to design reinforced concrete moment-resisting frame (RC-MRF) buildings with linear viscous dampers (LVDs). In those studies, the viscous damper forces are found to be larger than the design viscous damper forces because the actual relative structural velocity is different from the pseudo-velocity and also due to higher-mode effects. In the present study, a damper velocity correction factor is introduced along the height of the RC-MRF with LVDs, to calculate the corrected velocity of the LVD from the design velocity of the LVD, when the DDBD procedure is used. The damper velocity correction factor is proposed using the results of nonlinear response history analysis (NLRHA) for a set of RC-MRFs with LVDs, which are designed using the DDBD procedure. The DDBD procedure is then used to design RC-MRFs with nonlinear viscous dampers (NLVDs) making use of the proposed damper velocity correction factor, by calculating the NLVD characteristics using the equal energy (EE) dissipation approach or the equal power (EP) consumption approach. The accuracy of the proposed damper velocity correction factor is evaluated by carrying out NLRHA of two sets of RC-MRFs with NLVDs, one set where the damper velocity correction factor is not used, and the other set where it is used. When the damper velocity correction factor is used; the design LVD forces either agree with NLRHA results or are slightly more conservative, the design NLVD forces are close to the NLRHA results, and the maximum value of peak inter-story drift ratio (IDR) along the height of the RC-MRF from NLRHA is close to the design IDR limit. There is no significant difference in the peak story shear forces whether the damper velocity correction factor is used or not.

Effects of ground-motion sequences on fragility and vulnerability of case-study reinforced concrete frames

This study investigates the effects of ground-motion sequences on fragility and vulnerability of reinforced concrete (RC) moment-resisting frames (MRFs). Two four-storey, four-bay RC MRFs are selected as case studies. These structures, which share the same geometry, are representative of distinct vulnerability classes in the Mediterranean region and are characterized by different material properties, cross-section dimensions, and detailing. The first case study is a ductile MRF designed according to Eurocode 8 (i.e., a special-code frame), while the second is a non-ductile MRF designed to sustain only gravity loads (i.e., a pre-code frame). The influence of masonry infills on their seismic performance is also investigated. Advanced numerical models are developed to perform cloud-based sequential nonlinear time history analyses using ground-motion sequences assembled by randomly pairing two real records via Latin hypercube sampling. Different structure-specific damage states are considered to derive fragility curves for the undamaged structures, when subjected to a single ground-motion record, and state-dependent fragility curves by considering the additional damage induced by a second ground-motion record within the sequence. Damage-to-loss models are then used to derive mean vulnerability relationships. Results of the analysis show the importance of considering the effect of damage accumulation in the pre-code frames. Moreover, the presence of infills shows an overall positive contribution to the seismic performance of both frame types. Vector-valued vulnerability relationships accounting for the damaging effect of two ground-motion records are finally presented in the form of mean vulnerability surfaces.

Seismic performance of reinforced concrete interior beam-column joints with novel reinforcement detail

Horizontal stirrups are normally required in reinforced concrete (RC) beam-column joints (BCJs) for resisting shear forces in seismic design. For RC moment-resisting frames subjected to a high lateral load, a large number of stirrups are needed in joint cores. This may cause reinforcement congestion, leading to construction difficulty and insufficient concrete compaction, which can result in poor seismic performance. In this study, a novel reinforcement detail in the form of unbonded diagonal bars mechanically anchored at beam ends is proposed for RC interior BCJs. The detail alleviates the reinforcement congestion through partially replacing horizontal stirrups, and improves the seismic performance of BCJs by plastic hinge relocation and input shear force reduction mechanisms. Four 2/3-scale RC interior BCJ specimens were prepared and tested under quasi-static cyclic load, including one specimen designed with the current code and three specimens adopting the proposed reinforcement detail. Test results show that the proposed reinforcement detail is able to relocate the plastic hinges away from beam-joint interfaces as well as improve the loading capacity, energy dissipation capacity, stiffness, and bonding condition of beam reinforcements within the cores of BCJs. The combined use of stirrups and the proposed reinforcement detail significantly enhances the cracking resistance and reduces joint distortion, while additional amount of stirrups results in a marginal improvement. Moreover, an analytical model considering plastic hinge relocation and input joint shear force reduction is proposed for BCJs with the novel reinforcement detail. The model can adequately predict the failure mode and loading capacity of BCJ specimens.

Experimental Study on Seismic Retrofitting of Reinforced Concrete Frames Using Welded Concrete-Filled Steel Tubes

Buildings constructed with non-seismic details are at risk of damage and collapse when an earthquake occurs due to a lack of strength, stiffness, and ductility. For reinforced concrete (RC) moment-resisting frames, seismic retrofitting methods that can enhance strength or ductility should be applied. However, such strategies have many disadvantages related to constructability, serviceability, securing integrity, and cost. In this paper, a welded concrete-filled steel tube (WCFST) system was examined in order to resolve the problems of the existing seismic retrofitting methods for RC moment-resisting frames. To evaluate the seismic performance of the proposed system, two specimens, one with non-seismic details and another reinforced with a WCFST seismic system, were manufactured for the cyclic loading tests. As a result of the experiments, the specimen retrofitted with the WCFST system showed maximum load, effective stiffness, and energy dissipation capacity values approximately 3, 2, and 2.5 times greater, respectively, than those obtained from the existing reinforced concrete frame specimen. The experimental results indicate that the proposed WCFST system is expected to be effective at improving the seismic performance by enhancing both the strength of the existing reinforced concrete frame structures and the dissipation of the seismic energy.