arXiv subjects

Mojtaba Harati

Publications and source records attributed to Mojtaba Harati.

10 recordsLinked to original sources

arXiv2021

On the reduction of required motions in the dynamic analysis using an optimization-based spectral matching

This study aims to show the efficiency of a proposed spectral matching technique for the reduction of required ground motions in the dynamic time history analysis. In this non-stationary spectral matching approach, unconstrained optimization is employed to adjust the signal to match a target spectrum. Adjustment factors of discrete wavelet transform (DWT) coefficients associated with the signals are then considered as decision variables and the Levenberg-Marquardt algorithm is employed to find the optimum values of DWT coefficients. This matching algorithm turns out to be quite effective in the spectral matching objective, where matching at multiple damping ratios can be readily achieved. First, the efficiency of the spectral matching procedure is investigated in a case study earthquake record and then compared with two conventional spectral matching methods. Results show considerable improvement in the matching accuracy which is accompanied by minimal changes in shaking characteristics of the original signal. In addition, it is shown that earthquake records matched with the proposed method can noticeably reduce the essential number of ground motions that are normally required for the dynamic analysis of a case study model. In this regard, it has been found that we can reduce the number of required motions by more than 80% when matched motions are selected to be used as the seismic inputs for the dynamic analysis.

physics.geo-ph↗

arXiv2020

A state-of-knowledge review on the Endurance Time Method

Endurance time method is a time history dynamic analysis in which structures are subjected to predesigned intensifying excitations. This method provides a tool for response prediction that correlates structural responses to the intensity of earthquakes with a considerably less computational demand as compared to conventional time history analysis. The endurance time method is being used in different areas of earthquake engineering such as performance-based assessment and design, life-cycle cost-based design, value-based design, seismic safety, seismic assessment, and multicomponent seismic analysis. Successful implementation of the endurance time method relies heavily on the quality of endurance time excitations. In this paper, a review of the endurance time method from conceptual development to its practical applications is provided. Different types of endurance time excitations are described. Features related to the existing endurance time excitations are also presented. Particular attention is given to different applications of the endurance time method in the field of earthquake engineering.

cs.CE↗

arXiv2020

Influence of ground motion duration on the structural response at multiple seismic intensity levels

This paper aims to investigate the effects of motion duration on the structural seismic demands, seeking potential correlations between motion durations and structural responses at several seismic intensity levels. Three seismic intensity levels with 100years, 475years, and 2475years earthquake return periods (RPs) are first considered for correlation computations. Spectrally matched ground motions are employed to isolate the contribution of duration from the effects of ground motion amplitudes and response spectral shape. Four single degree of freedom systems derived from four real reinforced concrete structures are studied, where both degrading and non-degrading equivalent SDOF systems are included for structural modeling. Results show a low positive correlation between motion duration and structural displacement demand, but this correlation increases with an increase in earthquake RP. It is also investigated whether or not this insignificant positive correlation has an impact on the incremental dynamic analysis curves. The spectrally matched ground motions are divided into two distinct groups in this case: short and long duration ground motions. The comparison of incremental dynamic analysis of these two groups at the collapse limit reveals that long-duration ground motions can cause up to a 20 percent decrease in the spectral acceleration demand of considered structural systems.

physics.geo-ph↗

arXiv2020

Estimating the duration effects in structural responses by a new energy-cycle based parameter

Strong-motion duration and number of cycles of motion are the well-known parameters with which earthquake engineers currently use to take into account the influence of motion duration for the dynamic analysis of structures. In addition to these prevalent means, using the number of nonlinear cycles, which a structure experiences during the motion, is also one another method to find the influence of the motion duration in the response of structures. In this study, a new approach is proposed to determine the number of nonlinear cycles. Moreover, a parameter based on both hysteretic energy and number of nonlinear cycles is likewise suggested to precisely reflect the shaking characteristics of the motion. The proposed parameter is expected to demonstrate a high correlation with the response of structures even for the structures having deteriorative and degraded behaviour during the motion. This matter is examined and confirmed with the application of this parameter to predict the influence of motion duration in the response of several degrading RC frames. It is revealed that the proposed parameter is able to well predict the influence of motion duration and can be used as a criterion to select ground motions for dynamic analysis.

physics.geo-ph↗

arXiv2020

Incorporation of Strong Motion Duration in Incremental-based Seismic Assessments

This study proposes a new approach to incorporate motion duration in incremental dynamic assessments. In the proposed methodology, at each intensity level, a simulation-based approach, which is verified with actual data, is employed to determine the median duration and the median acceleration spectra of ground motions expected to occur at the site. Afterward, at each intensity level, artificial or spectrally matched motions are produced based on the median acceleration spectra and the median duration, indicating that different intensity levels are directly covered by the generated artificial or adjusted motions rather than just scaling up and down a set of recorded ground motions. In the proposed methodology, duration and acceleration spectral shape changes against intensity level while they remain the same for different intensity levels in approach where responses are derived by scaling up and down of a set of ground motions. The functional relationship between duration and seismic intensity level, which is vital for the estimation of median duration at each intensity level, is firstly investigated for the sites with different soil conditions and rupture distances. Not only is it demonstrated that the data can fit into exponential functions, but the sensitivity of the functions against different parameters is also explored as well. The proposed duration-consistent incremental seismic assessment is used in nonlinear seismic assessment of two single degree of freedom structures, with and without a degrading behavior capability. It is revealed that when changes in duration and spectral shape of the motions at different intensity levels are considered in the nonlinear dynamic analysis, an impactful influence that cannot be easily ignored is witnessed in the structural responses of incremental analyses.

physics.geo-ph↗

arXiv2020

Correlation of ground motion duration with its intensity metrics: A simulation based approach

There are different kinds of intensity measures to characterize the main properties of the earthquake records. This paper proposes a simulation-based approach to compute correlation coefficients of motion duration and intensity measures of the earthquake ground motions. This method is used to investigate the influence of the ground motion data set selection in resulting duration-intensity correlation coefficients. The simulation procedure is used to tackle the problem of inadequate available ground motions with specific parameters. Correlation coefficients are investigated in three different cases. In case one, simulated ground motions differ in terms of earthquake source parameters, site characteristics, and site-to-source distances. In case two, ground motions are simulated in a specific site from probable earthquake events. In case 3, ground motions are simulated from a specific event in different sites. The first case doesnt show a significant correlation, while the second and the third case demonstrate significant positive and negative correlations, respectively.

physics.geo-ph↗

arXiv2020

Introducing a response-based duration metric and its correlation with structural damages

This study proposes a response-based parameter for strong motion duration which is computed for structures and is the total time they are nonlinear during an earthquake. Correlation between structural response and duration for structures, subjected to a set of spectrum matched ground motions, is employed to examine the efficiency of the proposed method. The spectral matching procedure ensures that the influence of amplitude and frequency content of motions on structural response variability is significantly removed. Four concrete building type systems are studied and correlation coefficients of structural response with the proposed duration definition are examined. Comparison of the proposed method with other existing definitions, the record-based and response-based metrics, shows about 15-20% improvement in the correlation values.

physics.geo-ph↗

arXiv2020

Nonlinear behavior of RC shear walls: From experiments to the field reports

Post-earthquake damage evaluation has indicated that although the buildings with shear walls exhibited an appropriate overall performance in the recent severe earthquakes, in some cases, however, the columns and the shear walls were damaged, due to the presence of short structural elements and inadequate transverse reinforcement. Such memories amongst engineers have promoted this attitude that the shear walls structures exhibit a brittle overall performance. So the conviction that shear walls are inherently brittle still prevails amongst engineers, and therefore, they usually prefer to choose moment-resisting RC frames without shear walls. For getting an insight into the nonlinear behavior of RC shear walls, we have reviewed the experiments conducted on the RC shear walls. Besides, through using the post-earthquake reports written after major earthquakes, we have checked and scrutinized the seismic performance of RC frames equipped with shear walls too. Investigations performed on the nonlinear behavior of the shear walls have demonstrated that slender shear walls, designed to fail in flexural mode, could safely dissipate excitations energy of the earthquakes. These studies also revealed that even squat shear walls can be designed in a way that their behavior would be similar to those of the slender walls. Considering these matters, designed shear walls according to the recent codes maintain sufficient shear strength and respectable energy dissipation capability.

nlin.PS↗

arXiv2019

On the optimal parameters of a PSO-based algorithm for simulation of Endurance Time Excitation Functions

This paper presents a particle swarm optimizer for production of endurance time excitation functions. These excitations are intensifying acceleration time histories that are used as input motions in endurance time method. The accuracy of the endurance time methods heavily depends on the accuracy of endurance time excitations. Unconstrained nonlinear optimization is employed to simulate these excitations. Particle swarm optimization method as an evolutionary algorithm is examined in this paper to achieve a more accurate endurance time excitation function, where optimal parameters of the particle swarm optimization are first determined using a parametric study on the involved variables. The proposed method is verified and compared with the trust-region reflective method as a classical optimization method and imperialist competitive algorithm as a recently developed evolutionary method. Results show that the proposed method leads to more accurate endurance time excitations.

eess.SP↗

arXiv2019

An investigation on the effect of the near-fault earthquakes on the seismic behavior of RC Moment Resisting Frames (MRFs) designed based on Iranian seismic code (standard no. 2800)

Past severe earthquakes, such as Bam earthquake of 2003 and Tabas earthquake of 1978, have demonstrated that many cities in Iran are prone to be struck by near-fault earthquakes. Such earthquakes are impulsive in nature, and therefore, they are more destructive than the ordinary ground shaking. In the fourth edition of Iranian seismic code (Standard No. 2800), some changes, including a modification factor for the elastic acceleration response spectrum (EARS) have been recently recommended to reflect the effects of such probable near-fault earthquakes for the designing procedure. In this study, a numbers of 2D RC moment resisting frames (MRFs), from four to twelve story buildings, are designed linearly based on Iranian National Building Code (INBC) and Standard No. 2800 as well. Subsequently, their nonlinear models are reproduced for conducting nonlinear dynamic time history (NDTH) analysis. For this purpose, twenty impulsive ground motions are selected and scaled to be compatible with the design basis earthquake (DBE) spectrum of the abovementioned code. It is concluded that the seismic performance of the analyzed structures are not satisfactory at all; no buildings are successful to satisfy the life safety (LS) performance level posed by guidelines such as ASCE41-06 or ASCE41-13. Moreover, it is worth mentioning that even collapse prevention (CP) limit states are not also met in some cases. Therefore, the recently added modifications in the Standard No. 2800 may be inadequate to incorporate the near-fault earthquakes' effects.

physics.geo-ph↗