Collapse Capacity Research Articles

Seismic Collapse Risk Assessment and Fragility Analysis of Reinforced Masonry Core Walls with Boundary Elements Using the FEMA P695 Methodology

In the past, the primary objective of structural design codes was centred around ensuring human life safety, with a strong focus on preventing structural collapse. This approach predominantly relied on force- or strength-based criteria as the basis for design. Nevertheless, a notable shift has occurred recently, transitioning the design philosophy from 'strength' towards a 'performance'-based approach. This shift signifies a growing recognition that strength and performance are not identical. However, to adopt the performance-based seismic design approach, it is essential to develop precise models for estimating damage and potential losses associated with various seismic force-resisting systems (SFRSs). Fragility functions are widely interpreted among the most prevalent damage and loss models. They establish a crucial link between specific demand parameters and the likelihood of exceeding various damage states. Although reinforced masonry (RM) structures have recently gained popularity, the seismic design of mid- to high-rise RM structures is still challenging because it requires a reliable SFRS capable of providing the needed ductility and capacity. Therefore, the main objective of this study is to evaluate the key seismic performance parameters (i.e., system overstrength and ductility) and to perform a collapse capacity risk assessment of reinforced masonry core walls with boundary elements (RMCW+BEs) as the main SFRS in RM structures. The current study utilizes the applied element method (AEM) implemented in the Extreme Loading for Structures software (ELS) to model three RM structures with various heights (10-, 15-, and 20-story buildings). Nonlinear pseudo- static pushover analysis was performed to quantify the ductility and overstrength of the proposed RM system following the FEMA P695 guidelines. In addition, an incremental dynamic analysis (IDA) was carried out to assess the seismic collapse risk of RMCW+BEs by generating system-level-based fragility curves. The results showed that the proposed RM system provides the needed ductility, overstrength and deformation capacity for a ductile SFRS for typical mid- and high-rise RM buildings. Furthermore, the developed collapse fragility curves verified excellent seismic performance, negligible collapse probability values and high reserved collapse capacity at the maximum considered earthquake (MCE) design level. The findings of this study significantly contribute toward adopting RMCW+BEs as an effective SFRS for typical RM buildings in the next generations of North American masonry design standards.

Seismic Collapse Risk Assessment and Fragility Analysis of Reinforced Masonry Core Walls with Boundary Elements Using the FEMA P695 Methodology

In the past, the primary objective of structural design codes was centred around ensuring human life safety, with a strong focus on preventing structural collapse. This approach predominantly relied on force- or strength-based criteria as the basis for design. Nevertheless, a notable shift has occurred recently, transitioning the design philosophy from 'strength' towards a 'performance'-based approach. This shift signifies a growing recognition that strength and performance are not identical. However, to adopt the performance-based seismic design approach, it is essential to develop precise models for estimating damage and potential losses associated with various seismic force-resisting systems (SFRSs). Fragility functions are widely interpreted among the most prevalent damage and loss models. They establish a crucial link between specific demand parameters and the likelihood of exceeding various damage states. Although reinforced masonry (RM) structures have recently gained popularity, the seismic design of mid- to high-rise RM structures is still challenging because it requires a reliable SFRS capable of providing the needed ductility and capacity. Therefore, the main objective of this study is to evaluate the key seismic performance parameters (i.e., system overstrength and ductility) and to perform a collapse capacity risk assessment of reinforced masonry core walls with boundary elements (RMCW+BEs) as the main SFRS in RM structures. The current study utilizes the applied element method (AEM) implemented in the Extreme Loading for Structures software (ELS) to model three RM structures with various heights (10-, 15-, and 20-story buildings). Nonlinear pseudo- static pushover analysis was performed to quantify the ductility and overstrength of the proposed RM system following the FEMA P695 guidelines. In addition, an incremental dynamic analysis (IDA) was carried out to assess the seismic collapse risk of RMCW+BEs by generating system-level-based fragility curves. The results showed that the proposed RM system provides the needed ductility, overstrength and deformation capacity for a ductile SFRS for typical mid- and high-rise RM buildings. Furthermore, the developed collapse fragility curves verified excellent seismic performance, negligible collapse probability values and high reserved collapse capacity at the maximum considered earthquake (MCE) design level. The findings of this study significantly contribute toward adopting RMCW+BEs as an effective SFRS for typical RM buildings in the next generations of North American masonry design standards.

Effect of prevalent irregular configuration of infills on the seismic collapse of Indian RC buildings

Rapid urbanization and land scarcity obligated the urban population for multi-use of Reinforced Concrete (RC) buildings to meet both residential and commercial purposes together. Un-Reinforced Masonry (URM) infills are often placed irregularly in plan and or elevation of the buildings in some floor(s), particularly in the ground floor to suit the need of occupational requirements accompanied by the inherent irregularities due to openings for doors and windows to meet functional requirements for residence. Seismic performance of infilled frame highly depends upon degree of infill-frame interaction, and it is of utmost importance to classify infilled frame buildings in various Model Building Types (MBTs) based on the type of irregular placement of infills in addition to the parameters affecting seismic performance. Based on field pilot surveys carried out in Indian cities, URM infilled buildings have been classified into 14 different MBTs. Seismic performance of the prevalent categories of MBTs have been evaluated through Incremental Dynamic Analysis (IDA). It has been observed that irregular placement of infills increases the seismic collapse risk significantly, and reduces collapse capacity to approximately 50% as compared to ideal uniformly infilled building. Global collapse occurred due to failure of structural members in the vicinity of irregularity created due to infills.

Minimum magnitude boundaries in probabilistic seismic hazard analysis: an insight from structural engineering

In order to systematically advance our understanding of the minimum magnitude limit (Mmin) in the probabilistic seismic hazard analysis (PSHA) calculations, a novel and useful approach utilising a broad range of Single-Degree-of-Freedom oscillators and hazard conditions is being developed and tested. We have determined the most reasonable Mmin value for a variety of structures by examining the impact of Mmin on the mean annual frequency (MAF) of various limit states (LSs) (including the collapse capacity). The originality of the suggested methodology in the current work, known as the MAF saturation strategy, is the recommended Mmin, which is the cut-off value at which lesser magnitude events do add to the hazard but do not significantly change the MAF. The current work is the first to offer the MAF saturation strategy methodology, which searches for the cut-off magnitude at which the MAF value essentially remains constant even when smaller values of this cut-off are utilised as Mmin for hazard assessments. Therefore, given a series of carefully chosen ground motions in each oscillator instance, an incremental dynamic analysis is carried out (by applying the Hunt and Fill algorithm), and the appropriate LS (including the collapse capacity defined as global instability) points are calculated. Thus, the relationship between the distribution of LSs and the Engineering Demand Parameter and intensity measure is found. A simple point source hazard curve is convoluted with this distribution, yielding the structure-specific MAF. In order to find the cut-off lower magnitude (Mmin), this convolution is repeated for several Mmin values. This cut-off is defined as the point at which, when lower values are utilised as Mmin in the PSHA computation, the MAF’s values do not change considerably (with a five per cent threshold). The acquired data were thoroughly discussed in relation to various structural features and seismic input factors. The primary findings showed that each of the structures under consideration requires a Mmin value in the range of 4–4.3. Put otherwise, the suggestions seen in technical literature, which range from 4.5 to 5, are not cautious, at least not when it comes to probabilistic structural limit state frequency. The derived Mmin value is mostly controlled by the natural period of the structure and is largely unaffected by other structural characteristics like ductility, damping ratio and overstrength factor.

Efficient neural network-aided seismic life-cycle cost optimization of steel moment frames

In this paper, a novel and efficient neural network-based methodology is proposed to achieve seismic total cost optimization of steel moment-resisting frames in a timely manner. The computational burden of an optimization process based on performing nonlinear time-history analysis is prohibitively high. To address this crucial issue, a new and efficient neural network model is proposed in this paper to accurately predict the nonlinear time history response of steel frames during the optimization process. In the proposed neural network model, an ensemble of parallel neural networks is used to provide excellent prediction accuracy. In addition, a new repairability constraint is proposed to check the seismic damage level of structures during the optimization process with the aid of the proposed neural network model. Moreover, an efficient metaheuristic algorithm is used to achieve the optimization task. Two numerical examples are illustrated to demonstrate the efficiency of the proposed methodology. The results show that the proposed neural network model outperforms the existing standard models in terms of prediction accuracy. Furthermore, it is shown that by using the proposed methodology, the optimal seismic total cost of steel frames increases by less than 2.5%, yet their seismic collapse capacity increases by at least 30%.

Influence of earthquake duration on structural collapse capacity: Application to self-centering shear walls

This study investigates the influence of earthquake duration on the structural collapse capacity of self-centering shear walls (SCSW). The assessment process employing different response indicators is examined when considering the duration effect, including the inter-story drift ratio (IDR) and damage index (DI) quantified through a modified damage model. SCSWs with varying self-centering parameters λ are designed and modeled, and incremental dynamic analyses are performed using two record sets with distinct effective durations and equivalent frequency contents. The assessment criteria outlined in FEMA P695 are utilized to quantify the collapse capacity of SCSWs considering the duration effect. Meanwhile, the duration effect on structural performance corresponding to multiple damage states are evaluated. Results show a minimal correlation between the duration effect and collapse capacity when employing IDR during evaluations. Conversely, earthquakes with longer durations may impose higher collapse risks to SCSWs when assessing collapse capacity using DI. Meanwhile, enlarging λ can effectively mitigate damage development and reduce collapse risks for SCSWs. The results further demonstrate that damage may develop more rapidly within SCSWs under long-duration earthquakes, potentially leading to inadequate redundancy during intense seismic events. Additionally, the result suggests that the current performance assessment process only employing deformation-based response indicators may lead to inadequate evaluation of collapse risks for SCSWs, especially when accounting for the effect of earthquake duration.

Seismic collapse fragility analysis of steel moment-resisting frames (MRFs) with stiffness and strength deterioration considering the spectral shape of ground motion records

In this paper, the seismic collapse fragility analysis of special steel moment-resisting frame (MRF) buildings with strength and stiffness deterioration is comprehensively evaluated, taking into account the effect of record selection based on the spectral shape parameters. For this purpose, three special steel moment-resisting frame buildings, including 3, 10, and 20-story structures as representatives of low-rise, mid-rise, and high-rise buildings, are investigated. The modified bilinear Ibarra-Medina-Krawinkler (IMK) relation is used for modeling the concentrated plasticity and considering cyclic degradation in beams and columns under earthquake effects in Incremental Dynamic Analysis (IDA). The ground motion records are selected and categorized based on the three parameters, including the epsilon, SaRatio, and Np from a large database consisting of 2000 earthquake records. The records are divided into 7 groups using the epsilon parameter. The SaRatio and Np parameters further divide the records into 5 categories. Additionally, a randomly selected set of 200 earthquake records is used. The effects of hysteretic deterioration and spectral shape parameters of earthquake records on the median IDA curves, collapse fragility curves, and collapse capacity of the steel MRF structures are investigated. In addition to the hysteretic deterioration of steel structural members based on the results of the previous experiments, different values of the deterioration parameter (λ) are used in the nonlinear time history analyses to parametrically study the effect of deterioration on the collapse capacity. Finally, the effect of various parameters on the seismic collapse capacity of the structures in the form of Tornado diagrams is illustrated. The results demonstrate that the randomly selected records yield results close to the records sets with Np = 1, SaRatio = 1, and ε = 0. Furthermore, for a constant deterioration level, increasing epsilon and SaRatio, and decreasing Np lead to an increase in the collapse capacity of the structures. The results also show that the effect of deterioration on the collapse capacity of the buildings, especially the low-rise 3-story building, is significant.

Seismic performance investigation on self-centering friction frames: Collapse capacity and post-earthquake recovery

Self-centering technology is a promising approach to enhance seismic resilience of frame structures. Despite extensive experimental and theoretical research on various self-centering components, there has been relatively limited quantitative study on the collapse and post-earthquake recovery capabilities of self-centering frame structures. Therefore, this paper introduces a self-centering friction beam-column joint (SCFJ) and develops a simplified analytical model for the joint. Then a series of RC frames with SCFJ are designed, and their seismic resilience and post-earthquake recovery performance are conducted. Finally, this study quantitatively investigates the contribution of SCFJ to the seismic safety and post-earthquake recovery capabilities of RC frames with varying stories. The results indicate that under maximum considered earthquake, the peak floor accelerations and inter-story drift ratio of self-centering friction frames (SCFF) are comparable to those of ordinary RC frames (ORCF). However, the overall residual deformation of the SCFFs can be reduced by 85%–94%. Furthermore, their seismic collapse resistance is improved by 8%–23%. Immediate occupancy and reparability of SCFFs are enhanced by 41%–72% and 29%–35%, respectively. Notably, the enhancement in collapse capacity and post-earthquake recovery performance of SCFFs becomes more pronounced as the number of stories increases.

Deterioration-induced asymmetry-insensitive collapse behavior in low-rise torsionally-stiff buildings

This study investigates a significant point in the collapse behavior of asymmetric torsionally-stiff buildings, which is ignored by seismic design codes. The aim is to show how the effect of asymmetry on the collapse capacity depends on the deterioration characteristics of structures. To this end, the collapse performance evaluation of low-rise moment-resisting frame buildings is performed according to the FEMA-P695 methodology, emphasizing the drawbacks of ignoring degrading properties of structures in the collapse assessment. Then, the importance of deterioration is measured by a novel criterion, which is able to resolve the deficiency of prior justifications for the effect of asymmetry on the collapse performance of structures. Accordingly, there is a limit state for the proposed measure above which we may have an asymmetry-insensitive collapse performance for low-rise torsionally-stiff buildings. A two-step verification process is employed to confirm the validity of the results.

A collapse capacity prediction model based on ground motion duration

A prediction model for the seismic collapse capacity, which explicitly considers the influence of strong motion duration, in addition to key structural properties, is presented in this study. The model is developed based on incremental dynamic analyses of single-degree-of-freedom systems, employing a set of 67 earthquake records. The selected ground motions are matched by scaling to a target Eurocode 8 response spectrum, to avoid spectral shape bias in the results, and have varying 5–75% significant duration from about 5 s up to nearly 70 s. The structural models exhibit vibration periods in the range of 0.2 s to 3.0 s, varying negative slope in their post-capping branch from 0.02 to 0.30, and ductility between 1.0 and 6.0. Each oscillator is assigned bilinear and pinching hysteresis. Correlation coefficients are computed between the collapse capacity and the structural properties examined, as well as three ground motion parameters, which include the duration, Arias Intensity, and Husid plot slope. The period, post-capping slope, and ductility are all shown to be strongly correlated to collapse resistance while, among the ground motion variables, the duration exhibits the highest correlation. Based on iteratively reweighted least squares, predictive models are fitted to the collapse capacity data, providing estimates for the median collapse capacity and the variance. The predicted collapse capacity associated with the maximum duration considered is found to be up to 67% lower than that corresponding to the minimum duration. The rate of collapse capacity reduction with duration is shown to depend on the period, P-Δ level, hysteresis type, and the duration value itself. The proposed model is compared to those from previous studies and is shown to provide a simple and computationally efficient method to obtain collapse capacity estimates for specific target levels of strong motion duration.

Performance‐based optimum seismic design of self‐centering steel moment frames with SMA‐based connections

AbstractShape memory alloys (SMAs) have found several applications in earthquake‐resilient structures. However, because of high material costs, their implementation on industry projects is still limited. Developing design approaches that minimize the use of expensive SMAs is critical to facilitating their widespread adoption in real structures. This paper proposes a performance‐based seismic design optimization procedure for self‐centering steel moment‐resisting frames (SC‐MRFs) with SMA‐bolted endplate connections. The topology optimization uses a metaheuristic algorithm to minimize the frame's total cost, including the initial construction and expected repair costs. The design variables are the steel beam and column sections, SMA connection properties, and the topology of the SMA connections. Different constraints are considered, such as the constructability of the chosen steel sections, member strengths, performance‐based design, Park‐Ang damage index, and strong‐column weak‐beam requirements. Furthermore, the seismic safety of optimal designs is assessed by calculating adjusted collapse margin ratios according to FEMA‐P695. An illustrative optimization study using three‐ and nine‐story SC‐MRFs is presented. The optimal SC‐MRFs are then assessed in terms of cost and seismic performance. The results confirm the effectiveness of the proposed optimum design, which minimizes the use of SMAs while ensuring improved seismic performance. The case studies show that the optimal placement of SMA connections can reduce the total cost by up to 71% and 61% for the three‐ and nine‐story SC‐MRFs, respectively, compared to nonoptimal frames. Moreover, the optimal SC‐MRFs exhibit more uniform drift distributions, lower residual story drifts by up to 96%, and increase collapse capacity by up to 102%.

Seismic strengthening of isolated RC framed structures through orthogonal steel exoskeleton: Bidirectional non-linear analyses

The catastrophic effects of recent earthquakes around the world have highlighted the vulnerability of civil buildings; in Europe, most of the existing buildings were built after World War II and since they have largely exceeded their useful life, are characterized by a high vulnerability linked both to the durability of the materials and to the lack of seismic provisions in design. The current trend in constructional field is to design interventions aimed at the seismic and energy improvement of these structures jointly, in order to achieve a sustainable and integrated retrofit intervention. To this aim, the use of 2D orthogonal steel exoskeletons for the seismic strengthening represents a suitable solution, providing at the same time an enhancement in thermal insulation, through a ‘double-skin’ solution. Due to the growing interest in the design of steel exoskeleton interventions, a detailed dynamic characterization of this external strengthening solution is needed. In this framework, the present work, aims to investigate the effectiveness of the investigated strengthening solution by means of bi-directional non-linear dynamic analyses. Extensive numerical analysis was carried out. The selected case-study is an Italian existing pre-1980 s RC frame building located in Mugnano di Napoli (NA, Italy), which is a medium-high seismic hazard site in Italy (peak ground acceleration equal to 0.156 g). Two alternative strengthening solutions were numerically investigated by means of non-linear time-history and incremental dynamic analyses performed applying fifteen bidirectional ground motions on three-dimensional numerical models. The results of the dynamic non-linear analyses allow to quantify more appropriate performance indicators, both in terms of expected demand and overall collapse capacity, useful for the development of optimization strategy and design of exoskeleton system. The results highlighted that both the investigated strengthening scenarios allow to increase the stiffness of the existing building and allow it to meet the current code safety requirements reducing also the residual inter-storey drift. The obtained results are not strictly related to the investigated case study, but can be extended to all the structures retrofitted that follows the presented design procedure.

Effect of aftershock characteristics on the fragility curve of post-mainshock RC frames

Earthquakes are not isolated events but are often followed by aftershocks. While structures are typically designed to withstand the mainshock, the presence of aftershocks poses additional challenges. In some cases, a damaged structure could collapse under a strong aftershock because of the damage evolution and cumulative damage as well as strength and stiffness degradation. Thus, it is essential to understand the structural behavior for post-mainshock decision-making and seismic assessment. The current study explores the impact of aftershock characteristics on the collapse capacity of buildings in seismic zones in accordance with a probabilistic point of view. In this regard, two non-ductile reinforced concrete (RC) frames with 3 and 6 stories were investigated and analyzed using 62 real aftershock records with varying characteristics. Frames were modeled in OpenSees software using Beam With Hinges Element with appropriate plastic hinge length and fiber section to provide suitable accuracy for dynamic analysis. The incremental dynamic analysis (IDA) was used to generate collapse diagrams and fragility curves. Some important seismic parameters including the mean period (Tm), predominant period (Tp), bandwidth frequency (Ω), significant duration of aftershock (Ds) and site characteristics were considered. The results revealed that the probability of collapse would increase in aftershocks with longer periods. For example, for Sa (T1, 5 %) of 1 g and 3-story frame, the collapse probability is increased ≈25 % in Tm > 0.5s compared to Tm < 0.5s which is remarkable. Also, site characteristics could increase the probability of collapse by ~ 6 % in the 6-story frame. The same results can also be seen for aftershocks with a larger predominant period (Tp > T1), greater significant duration (Ds > 11 s) and lower bandwidth frequency (omega <0.5). The findings emphasize the significance of aftershock characteristics on structural collapse capacity, necessitating careful selection of seismic records for accurate evaluation of seismic behavior.

Elevated collapse risk based on decaying aftershock hazard and damaged building fragilities

This article proposes a framework to support postearthquake building safety and reoccupancy decisions by quantifying the change in building collapse risk following a mainshock earthquake event. This risk may be exacerbated by both an increase in seismic hazard due to aftershock activity and a reduction in building collapse resistance due to structural damage. To address these factors, the framework is based on a hazard that includes (1) both the steady-state and the aftershock occurrence rates, that is, the elevated hazard that accounts for the dependence on the mainshock magnitude and the aftershock rate that decays over time, and (2) revised collapse fragility functions that account for structural damage sustained during the mainshock. The framework is capable of addressing region-specific questions such as (1) What are the mainshock magnitudes for which aftershocks pose a life-safety concern? (2) How long does it take for the elevated risk due to aftershocks to dissipate? and (3) What gaps in current knowledge deserve further attention from the earthquake engineering and seismology communities? The framework addresses these questions for a 20-story building in San Francisco, assuming three different, hypothetical mainshock events of magnitudes 7,7.5, and 8 M W on the San Andreas fault. This is followed by a parametric study that considers a range of buildings and provides a graphical representation of the elevated risk to inform building evaluation (tagging) decisions, based on the intact building’s collapse capacity, the amount of structural damage, and the length of time after the mainshock.

Finite element analysis of JCO-E fabrication process and its influence on the collapse capacity of offshore pipelines

In the present work, the JCO-E fabrication process is numerically simulated using advanced finite element tools for the case of a thick-walled 30-inch-diameter pipe, which is candidate for deep water installation. Subsequently, the structural performance of the line pipe under external pressure is investigated in a unified approach. Uniaxial tests are performed to obtain the material properties used in the finite element model, which are representative of the loading history that the plate is subjected during the process. The finite element model is validated with geometric measurements from the actual pipe fabricated in the pipe mill. The effect of the expansion level on the properties of the final product is investigated through extensive numerical studies. The results have shown that there exists an optimum expansion range for achieving the minimization of the geometric imperfections of the final product and the maximization of the collapse capacity of the pipe.

Seismic collapse assessment of intermediate RC moment frames subjected to mainshock-aftershock sequences

This paper assessed the effects of mainshock-aftershock (MS-AS) sequences on the seismic vulnerability of intermediate reinforced concrete moment frames (IRCFs), considering various mainshock intensity scenarios. Moreover, given the limited library of real MS-AS ground motion sequences in the Iran plateau, a MS-AS sequence-selection method is proposed to generate artificial sequences that incorporate the magnitude, seismotectonic region, fault mechanism, and site condition of a target area. Three 4-, 7-, and 10-story IRCFs, modeled in OpenSees software, are subjected to real as well as artificial MS-AS sequences and collapse fragility curves are derived using the incremental dynamic analysis (IDA) results. The results show that the occurrence of mainshock can significantly increase the vulnerability of buildings during the aftershock. For various mainshock intensity scenarios, a general decrease in the collapse capacity is observed across all mainshock-damaged buildings, with a more significant decrease for taller buildings. Hence, it is essential to consider the effects of aftershock in the seismic design of IRCFs to mitigate the risk of collapse. Furthermore, by comparing the seismic responses caused by artificial sequences with those caused by real sequences, the validity of artificial sequence-selection method is confirmed; thus, artificial sequences can be used as a substitute for real sequences. Regression equations are also presented to predict the collapse capacity reduction of frames with different story numbers in presence of various mainshock intensity scenarios.

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.

Collapse Resistance of Composite Structures with Various Optimized Beam–Column Connection Forms

Steel–concrete composite structures are widely used in composite frame structures and super high-rise buildings. However, the lack of relevant building design standards to ensure their structural stability under extreme conditions has led to potential failures in beam–column connections due to excessive loads. These failures can trigger the progressive collapse of high-rise buildings, resulting in severe casualties. In this study, a comparative numerical analysis was conducted to evaluate the collapse resistance of composite structures in the event of a middle-column loss scenario, focusing on six commonly used beam–column connections. The results show that while the six connections exhibit minimal differences under normal operating conditions, they display significant variations when subjected to extreme loads. Furthermore, a design concept is proposed to enhance the collapse capacity of these structures, and its effectiveness is validated via analysis.

Uncertainty and bias in fragility estimates by intensifying artificial accelerations

To comply with the objectives of Performance-Based Earthquake Engineering and gain a comprehensive understanding of a structure’s seismic response, the utilization of multiple ground motion records selected or scaled at various hazard levels is often necessary. Incremental Dynamic Analysis (IDA) and cloud analysis methods represent wide-range probabilistic approaches employed for this purpose. However, due to their computational complexity, researchers and engineers have sought alternative solutions. One such solution is the application of intensifying artificial accelerations (IAAs). By employing IAAs, the structural response can be recorded from linear elastic to nonlinear, and ultimately up to the collapse capacity, all within a single simulation.This paper presents a comprehensive summary of various generations of IAAs and their unique characteristics. To assess their performance on nonlinear single degree-of-freedom systems, the IAAs are compared against IDA results. As IAAs are typically generated to be independent of specific hazard levels, a series of generic (i.e., standardized) ground motion sets are used for IDA. Our objective is to quantitatively evaluate the bias in the mean response estimation by employing different generations of IAAs, developed based on various base response spectra and optimization objectives. Additionally, this paper provides a simplified estimation of fragility curves based on IAAs, and compares the resulting uncertainty with those generated through IDA. The findings aim to enhance the understanding of IAAs’ capabilities and limitations in capturing the seismic response characteristics of structures.

The effect of the ground motion frequency content on seismic performance and fragility curve of special concentric braced frames

Several articles have investigated the effect of various seismic parameters on the response of the structure. However, there has been limited discussion on the impact of seismic frequency content on the brace failure modes and the collapse mechanism of this system. This study investigated the effect of this parameter, considering three special concentric bracing frames of 2, 4, and 6 stories. For the frame dynamic analysis, the open system for earthquake engineering simulation software (OpenSees) is used. To evaluate the effect of frequency content, 60 earthquake records with high, medium and low frequency content are selected from the pacific earthquake engineering research center ground motion database, considering the peak ground acceleration divided by the peak ground velocity ratio. Incremental dynamic analysis is conducted to draw fragility curves and examine the response of structures.The results indicated that the collapse capacity of structures decreased as the frequency content was reduced. The percentage of reduction in collapse capacity under earthquakes with high to the low frequency content in 2, 4 and 6 story structures were 68, 67 and 71, respectively. In fact, reducing the frequency content increased the probability of occurrence of low cycle fatigue mode in the brace.