incremental dynamic analyses

Steel Beam - Reinforced Concrete Column (RCS) frames are a new seismic load-bearing system. However, there has been limited research comparing the behavior of RCS frames to steel and concrete frames. This study aimed to fill that gap by examining moment frames in composite (RCS), steel, and reinforced concrete forms in three different height categories: 5, 10, and 20 stories. The frames were seismically loaded according to ASCE7-22 code and then designed and optimized based on AISC360-22 and ACI318-19 codes. Two-dimensional frames were extracted from the designed three-dimensional structures and subjected to nonlinear modeling in the finite element software OpenSees. Initial static nonlinear analyses were conducted to evaluate preliminary seismic responses in the post-yield regions, followed by incremental dynamic analyses on all models. The collapse states of the models were compared using fragility curves. The study found that the seismic intensity index for steel frames is roughly 1.27 times greater than that for RCS frames. Similarly, the intensity index in RCS frames is about 0.9 times that of concrete frames. Additionally, the collapse-related behavior in RCS frames was found to be more desirable than in concrete frames as the number of stories increases. At a given seismic intensity, the probability of collapse is lower in RCS frames than in concrete frames. The modified collapse margin ratio in ten-story frames and twenty-story RCS frames, compared to the concrete frame, is about 1.1 and 1.26, respectively. In conclusion, the study shows that RCS frames have a more favorable seismic performance compared to steel and concrete frames, especially as the number of stories increases. This research contributes to a better understanding of the behavior of RCS frames in seismic conditions.

Experiences of previous earthquakes shows the effects of soil-structure interaction and behavior of beam-column connections on the seismic behavior of the building structures. In this research, seismic vulnerability assessment of RC precast Frames was investigated by consideration of the effect of soil-structure interaction and nonlinear behavior of beam-column connections. The RC precast building represented in this study, damaged in Bojnord earthquake 2017 and located on the soil type II of Iranian seismic design code. The soil-structure interaction is modeled using Beam-on-Nonlinear-Winkler foundation. In this procedure, an array of vertical q–z springs is used to capture vertical and rotational resistance of the foundation, while two springs, namely p–x and t–x, are placed horizontally to capture the passive and sliding resistance of the foundation, respectively. The seismic vulnerability and performance of RC precast frames are evaluated using nonlinear static pushover, nonlinear dynamic time history analyses and incremental dynamic analyses (IDA). The numerical models are developed using OpenSees software by consideration of nonlinear behavior of the beam-column joint. The numerical results are shown the significant role of soil-structure interaction and beam-column connections on the seismic vulnerability and performance of RC precast buildings. In fact, seismic vulnerability of RC precast buildings are increased by considering soil-structure interaction and beam-column connections effects.

Nonlinear time history analysis (NL-THA) is the most accurate method to estimate the seismic demand of structures and predict their failure. To that end, extensive efforts have been made to develop fast and convenient methods to carry out nonlinear static analyses. In recent years, pushover methods have been widely used as a suitable tool to evalute the seismic performance of structures. Also, various advanced pushover procedures have been proposed to take into account the effect of higher modes and the change in the dynamic properties of structures in the nonlinear phase. Therefore, different pushover procedures have been further developed for this purpose. The nonlinear static analysis  has been widely employed to evaluate the nonlinear behavior of structures. The pushover analysis was first expanded in a number of studies to investigate buildings. Not many studies have been conducted on the seismic demand of latticed space structures. In the present work, therefore, an optimization procedure has been employed to refine the performance of the pushover analysis in estimating the seismic response of double-layer barrel vault roofs with vertical double-layer walls. In the  method proposed herein, the coefficients of the modal load combinations of the studied structures have been optimized using the simplex algorithm to find the optimum load pattern. Fifteen models with various rise-to-span and height-to-span ratios were considered to assess the accuracy of the proposed method in predicting the seismic demand of these structures. The models were analyzed using the OpenSees software. In order to model the buckling behavior of the members, each member was divided into two nonlinear beam-column elements with an initial imperfection of 0.1% at its mid-node.  The models were designed with the dead, snow, temperature, and earthquake loads having been considered. All of the mentioned loads, with the exception of snow load, were applied to the structures as concentrated nodal loads. The snow load, by contrast, was applied to the structures in two symmetric and asymmetric patterns in accordance with the sixth volume of the Iranian national code of buildings. For earthquake loads, the 4th edition of the Iranian code of practice for seismic resistant design of buildings was used. It should be noted that the seismic mass of the roof of each model was calculated by considering the entirety of the dead load in addition to 40% of the snow load. In the design process of each model, the dead, snow, temperature, and earthquake load combinations were formulated based on the AISC-ASD89 standard. The sections of the members of the structures were chosen from hollow tubular sections, with  their slenderness ratios limited to 100. Afterwards, pushover analyses were performed using the optimized load pattern. The obtained results were compared to those of the incremental dynamic analyses (IDA) and two other well-known pushover methods, namely the MPA and the conventional first-mode pushover analysis. The results revealed that the proposed pushover method can provide a good estimation of the base shear and intial stiffness of the structures when compared to dynamic analyses. In addition, an increase in the rise-to-span ratio of the roof causes an improvement in the accuracy of the proposed pushover method. Also, in comparison with the  MPA and conventional pushover procedures, the responses produced by the proposed method are closer to those generated by dynamic analyses. In addition, a comparison of the obtained drift patterns reveals that the results of both the pushover and incremental dynamic analyses along the longitudinal direction of the wall are quite close to each other. Another advantage of the proposed pushover method is that it also produces acceptable results on the nodes on the roof of the space structure. Also, along the transverse direction, the proposed method yields better results.

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.