جستجوی مقالات مرتبط با کلیدواژه "collapse capacity" در نشریات گروه "مهندسی زلزله"

The seismic collapse capacity of a structure is a critical factor in earthquake risk assessment within engineeringpractices. Conventionally, evaluating this capacity involves intricate and time-consuming incremental dynamicanalyses. However, recent progress has brought forth alternative, streamlined methodologies grounded in the use ofstructural behavior curves. This study employs the application of these innovative approaches to comprehensivelyassess the seismic collapse capacity of structures. Embracing advancements, it strives to enhance the efficiency andprecision of seismic risk assessments in engineering practices.In addition to the efficiency of assessment methodologies, it is imperative that the calculation of seismic collapsecapacity aligns with the specific demands of the construction site. This ensures that the seismic risk falls within theestablished allowable limits. This consideration becomes particularly critical for construction sites located in closeproximity to fault zones. In such areas, the presence of directivity pulses heightened attention to seismic collapsecapacity. Recognizing that structural ductility and the pulse period ratio in the near-fault are primary factorsinfluencing seismic collapse capacity, which is demanded in site, this study delves into a detailed numericalinvestigation of these critical elements. Subsequently, the seismic collapse capacity demanded in the near-fault ismeticulously estimated based on these considerations.The extensive investigations undertaken in this study yield insightful revelations. It is evident that heightenedstructural ductility correlates with an augmentation of seismic collapse capacity, both in the near-fault and far-faultscenarios. Conversely, a reduction in seismic collapse capacity in the near-fault is discerned as the pulse period ratioincreases concerning the fundamental period of the structure. To conduct a comprehensive evaluation, the ratio ofseismic collapse capacity in the near-fault to that in the far-fault is calculated, taking into account both ductility andpulse period ratio. This derived parameter, denoted as γ, is then employed to estimate the seismic collapse capacitydemanded in the near-fault. This analysis contributes valuable insights to the understanding of seismic behavior inboth near-fault and far-fault regions.For the assessment of seismic collapse capacity demand at construction sites, the study recommends theutilization of a lower bound of the ratio of near-fault to far-fault seismic collapse capacity. This lower bound,associated with lower ductility and a higher pulse period ratio, is not just conservative but also robust. Importantly,this cautious approach ensures that an increase in this parameter does not significantly escalate the demand at theconstruction site. Such a calculated and conservative estimation of seismic collapse capacity demanded contributesto a more resilient seismic risk assessment for structures situated in near-fault zones.In conclusion, the results indicate that for the assessment of seismic collapse capacity that is demanded atconstruction sites in near-fault zones, utilizing a lower bound of the ratio of near-fault to far-fault seismic collapsecapacity, associated with lower ductility and higher pulse period ratio, is sufficiently conservative. Moreover, anincrease in this parameter does not significantly escalate the demand at the construction site.This approach ensures a cautious estimation of seismic collapse capacity demand, contributing to a more robustseismic risk assessment for structures in near-fault zones.

Performance-based design is a new approach to topics of the seismic design of structures, which unlike the traditional methods of force-based design, is based on changing the location of the structure. The use of this approach in the process of structure design results in the access to structures with proper performance and an acceptable level of reliability. The main goal of this contribution is to investigate the impact of near- and far- field earthquakes on the collapse capacity and fragility of performance based optimization of RC moment frames using the center of mass meta-heuristic algorithm. Push over analysis has been utilized in the optimization process to control the responses of the studied frames at functional levels and incremental dynamic analysis has been used to evaluate the fragility of the obtained optimal frames. According to the results for the collapse margin ratio and the adjusted collapse margin ratio for the 3-, 6-, and 12-story frames, it is indicated that the collapse margin ratio and therefore the seismic safety under far-field earthquakes are 7%, 16%, and 8% higher than those of the near-field earthquakes, respectively. In other words, the optimized frames in this study against near-field earthquakes have low seismic safety and more fragility than far-field earthquakes.

In the present research, the seismic fragility and collapse capacity of concrete moment frames have been investigated by considering different ratios for the weak beam-strong column rule in the optimization process in the performance-based design framework. In order to implement performance-based optimization, the center of mass metaheuristic algorithm has been applied in this research. The philosophy of design approach based on performance and even traditional design methods allows the structure to suffer damage facing strong and relatively strong earthquakes. Therefore, in order to estimate the level of safety of the structure against earthquakes, it seems necessary to use quantitative indicators of seismic safety and the collapse capacity of the structure. In order to predict the collapse capacity of each optimal structure, using incremental dynamic analysis, the modified collapse safety margin ratio under far and near fault earthquakes has been calculated. Two examples, 3-span three and six floor frames have been studied in this research, which are designed in the performance-based optimization framework and considering the coefficients of 0.8, 1.2 and 1.6 to control the weak beam-strong column rule in the optimization process. The results indicate that increasing the rigidity of the column compared to the beam in this research actually affects the ductility of the structure, and by choosing structures with greater rigidity of the column compared to the beam, it leads to an increase in the collapse capacity and a decrease in the fragility of the structure.

The steel braced frame system is one of the lateral load resisting systems which is used extensively for low- to mid-rise buildings. In this structural system, the braces can be arranged in different forms along the building height due to different reasons such as architectural and structural limitations or design considerations. The bracing arrangement affects the seismic performance of the structural system and each of the elements. In this study, the impact of bracing arrangement along the building height on ultimate failure capacity and collapse fragility curves of steel CBFs is investigated. For this purpose, 4 and 8-story steel CBF buildings with 6 different arrangements of braces were selected and modeled in PERFORM-3D software. The models were then analyzed using the incremental dynamic analysis (IDA) method. Afterwards, the collapse capacity of the models and the uncertainty index were calculated, and the collapse fragility curves were generated. The results show that, by modifying the arrangement of braces without significant changes in lateral stiffness and fundamental period of structure, it is possible to increase the collapse spectral acceleration and decrease the probability of collapse at the maximum considered earthquake intensity.

One of the situations that distorts the safety of concrete moment frame structures is the rare seismic events. Bam earthquake in 2003 was one of the rare seismic events that caused damages to many newly built structures. In this paper, the capacity of structures was evaluated according to the standard record sets of FEMA P695 and maximum considered earthquake (MCE). A comprehensive method should be used to express the seismic behavior of structures for assessing the collapse capacity. Incremental dynamic analyses proposed by Vamvatsikos and Cornell in 2002. This method is used for assessing the collapse capacity of structures in this study. The proposed methodology is used for collapse assessing of an individual as well as a group of buildings with due attention to rare seismic events and incremental dynamic analyses method. Illustrative results show that, if structures provide minimum acceptable requirements of FEMA P695, they would have been secured against rare seismic events.
Development of nonlinear models for collapse stimulation is the first step of collapse assessing methodology. All of the structures have been designed according to ASCE 7-05 code, and for expressing of nonlinear behavior of materials, Mander and Menegotto-Pinto model has been considered. Selection of ground motion record sets for collapse assessment of building structures is very important. Both far-field and near-field records have been considered in FEMA P695, but in this paper, the far-field records were used. Three analyses have been considered in assessing the collapse capacity. Eigenvalue analyses, incremental dynamic analyses and static pushover analyses are required for assessing the collapse capacity. Incremental dynamic analyses is one the suitable methods for expressing of seismic behavior of structures. The basic idea of this analysis was described by Bertero in 1997. In 2002, this method was accompanied with big progress by Vamvatsikos and Cornell. Illustrative results show where the incremental dynamic analyses curve slope is equal to 20% of the elastic while the point also belongs to softening branch defined as collapse point. Additionally, another candidate point is displacement ratio of 10%. Illustrative results show that where the incremental dynamic analyses curve lining to infinity is being defined as collapse point. The incremental dynamic analyses curves show record to record variability, thus it is essential to summarize such data. The fragility fitting approach has been used widely for defining the median collapse acceleration. Adjusted collapse margin ratio is the most important parameter for assessing the collapse capacity of structures. According to FEMA P695, the acceptable value of the adjusted collapse margin ratio for each individual model within a performance group should exceed ACMR (20%). Additionally, the average value of adjusted collapse margin ratio for each performance group should exceed ACMR (10%).
Finally, collapse capacity of 5 and 10 story concrete moment frame structures are defined. Both structures have acceptable adjusted collapse margin ratio and both of them have acceptable safety according to rare seismic events. Structures that could not satisfy the FEMA’s conditions must increase their lateral strength and re-evaluate.

From the various intensity measures that may be applied to evaluation of the seismic risk of structures, the acceleration response spectrum, Sa(T), is the most famous. As a key assumption in usual risk assessment procedures, like as PEER methodology, the structural response depends only upon the applied intensity measures, and not on any other properties of the ground motion. This required condition has termed “sufficiency” of used intensity measure. The limited “sufficiency” of Sa(T) has been emphasized in the recent researches and as a result, different methods have been proposed to modify the structural response analysis. In this paper, the problem has been re-defined and then the recent studies have been surveyed. This paper is mainly focused on the spectral shape concern. It has been discussed how the spectral shape of a ground motion affects the structural nonlinear response. Epsilon, as a well known seismological parameter is introduced as a convenient indicator of spectral shape. Epsilon as an indicator on the spectral shape has significant influence in the structural collapse risk assessment. The epsilon is defined as a measure of the difference between the spectral acceleration of a record and the mean value obtained from a ground motion attenuation model for a given period. As a direct approach for the consideration of the spectral shape in the record selection, a target ε value, associated with a selected hazard level, is first obtained from the hazard disaggregation procedure, and then records with a closer epsilon value to the target value can be chosen. The major challenge in considering the spectral shape for the selection of records lies in the finding of different sets of ground motion records for each level of hazard for calculation of the MAF of a limit-state for a given structure. Due to the dependence of epsilon on period, it may not be practical to select different specific ground motion sets for any specified period (T1) corresponding to a given site with a particular hazard level. A simple alternative has been proposed in the ATC63 project which could be used instead of the direct selection approach. In this approach, a general set of ground motion records could be used for assessment of the collapse fragility of any structure, without considering the spectral shape of the records. Theresulting mean collapse capacity can then be adjusted to meet the hazard-related target epsilon value. The major challenge of this method is that the objective structure shall be necessarily analyzed via a huge number of ground motions which is unfavorably time consuming task. Another more straight-forward approach is defined in this paper for the mentioned issue. In this method, an efficient range of epsilons is proposed to select a limited number of ground motions for collapse risk assessment of the objective structure. The proposed efficient range depends on the structural major parameters; period and ductility as well as on the seismicity level of the site. The initial results confirm the validity of this simple approach.