مجله علوم و مهندسی زلزله
سال هفتم شماره 4 (پیاپی 25، زمستان 1399)

The Zagros fold Thrust belt is approximately 1,500 to 1,600 km long and 200 to 300 km wide, formed from western Iran to the Strait of Hormuz under the influence of the convergence of the Arabian and Eurasian plateaus. The faults in this belt are mainly parallel to the folds and are of the thrust type with slopes to the north to the northeast. The Zagros zone is divided into five units based on morpho-tectonic, seismic, and structural data, and it is separated by deep faults called the High Zagros Fault (HZF), Mountain Front Fault (MFF), Zagros Foredeep Fault (ZFF), Main Zagros Reverse Fault (MZRF) and Kazerun Fault (KF). The earthquake of November 12, 2017, which occurred in the Zagros region near to Iran-Iraq border (MW = 7.3), is the only earthquake with a magnitude of more than 7 recorded in the northwestern part of the Zagros Thrust Zone (ZTZ). The largest earthquake in the region near the source of the recent earthquake was the January 11, 1967 earthquake (M = 6.1), located about 100 km south of the epicenter of the 2017 earthquake. The closest city to the epicenter of the recent earthquake is the city of Azgeleh with a distance of approximately 5 km. The depth of the earthquake was also reported to be about 11 km. In this study, using seismic data recorded in the Global Digital Seismic Network (GDSN), the seismic moment of this earthquake was calculated using the Kelis-Borok method and its average value obtained was 1.45E+27 dyne-cm. According to the source time function of this earthquake and obtaining the peak value of the rise time, duration of the most fracturing and fall time, the total rupture time is calculated that is 4.998 seconds.

Rayleigh waves can be more destructive in comparison with other types of waves during earthquakes. Identification and extraction of Rayleigh waves from seismic records can be more exactly performed in time-frequency domain. In the present study, Continuous Wavelet Transform and Stockwell Transform have been used to transfer three-components of seismic signals to time-frequency domain. Rayleigh waves can be extracted based on elliptical characteristics of these waves. On the other hand, Retrograde and Prograde Rayleigh waves and their propagation azimuth can be separately extracted with phase transferring of vertical component as well as normalized inner product technique. In the present study, the mentioned two algorithms have been programmed. Accuracy of the methods has been verified with using synthetic and Chi-Chi earthquake data. Finally, Rayleigh waves and their seismic parameters have been extracted from Tabas, Bam and Manjil earthquake records. Rayleigh waves of the earthquakes have been analyzed and compared with using different seismic parameters. According to the results, Retrograde Rayleigh waves are more predominant in comparison with Prograde Rayleigh waves in earthquake signals.IntroductionExtraction of Rayleigh waves is one of the important tasks in seismology and applied geophysics due to their destructive effects. A lot of study has been conducted in order to develop an effective method for the extraction of Rayleigh waves. Most of these methods have been presented based on the ellipticity characteristics of Rayleigh waves. Each of them has their advantages and disadvantages. In this study, two of them have been investigated, programmed and compared with each other.Methodology and ApproachesRayleigh waves have been extracted from three-component signals in time-frequency domain in both of the investigated methods. In algorithm 1, continuous wavelet transform has been used to transfer three components of signals from time to time-frequency domain. However, Stockwell transform has been used to perform this approach in algorithm 2. Stockwell transform retain the absolute phase of each localized frequency component, the characteristic of which is required in algorithm 2 for time-frequency transform. On the other hand, Rayleigh waves have been extracted based on the instantaneous reciprocal ellipticity and phase difference in algorithm 1 and 2 respectively. The most important advantages of algorithm 2 is that retrograde and prograde Rayleigh waves and their propagation azimuth can be separately extracted while it is not possible in algorithm 2.In this study, both of these algorithms have been clearly illustrated by flowcharts and programmed in Matlab. Synthetic signals and Chi-Chi earthquake signals have been used for verification. After verification, results of both algorithms have been compared, and finally they have been employed in extracting Rayleigh waves of Tabas, Bam and Manjil earthquakes. Finally, maximum displacements of Rayleigh waves and total earthquake signals have been investigated and compared in these three earthquakes. In addition, energy discharge of extracted Rayleigh waves during the earthquakes have been investigated. Results and ConclusionsResults of the study showed that algorithm 1 can be effectively used if there is not required to extract retrograde and prograde Rayleigh waves and their propagation azimuth separately because the algorithm is simpler and faster in comparison with algorithm 2. However, algorithm 2 should be employed on the condition that the extraction of retrograde and prograde Rayleigh waves and their propagation azimuth have been required. In addition, it has been concluded that Rayleigh waves have more energy in earthquake signals compared to other types of waves. Consequently, elliptical polarization is more than linear polarization in earthquakes. In comparison of retrograde and prograde Rayleigh waves, it has been concluded that retrograde Rayleigh waves have more energy and therefore they can be more destructive. Finally, it has been resulted that ratio of Rayleigh waves are different in earthquakes. Induced displacement by Rayleigh waves in Manjil earthquake was more than Bam and Tabas earthquakes.

The artificial neural network (ANN), one of the most powerful tools of artificial intelligence, was used to control the seismic responses of the cross section, and to predict earth dam seismic responses rather than utilizing dynamic time-based analyses. In terms of the artificial neural network, apart from the aforementioned ANN-based applications in various engineering problems, an increasing number of articles have been published over the last decade where the efficient implementation of ANNs in geotechnical earthquake engineering is presented. Most of these studies focus on liquefaction potential under seismic excitations, which is an extremely computationally intensive issue and therefore suitable for ANNs. In some of the studies in this field, the applicability of ANNs in soil dynamic analysis was examined. In addition to the generation of spectrum compatible accelerograms, prediction of earthquake parameters, wave propagation approximations, estimation of peak ground acceleration using microtremors, and the detection of earthquake electric field patterns. Not much has been done into predicting the seismic responses of embankments and earth dams using artificial neural networks tools. The first study on the domain was carried out by Tsompanakis et al. [1] in 2009 in which using the artificial neural network, a seismic response (maximum horizontal acceleration) of homogeneous and symmetric embankment in different locations was predicted. Although being a case study on a simple embankment, earthquake records (accelerograms) were used, which contained a variety of frequency concepts, simplicity or complexity, and various PGA levels, in addition to a general neural network that could be used in all PGA levels the seismic response was not accurately predicted. The objective of the present study was to investigate the ability of the artificial neural network to reach the dynamic response of the embankment and earth dams under seismic loads and different levels of intensity. In other words, it was planned to reduce the heavy cost of calculating the applicable issue in seismic geotechnical engineering. To fulfill the aim, a comprehensive study was carried out and various parameters were evaluated. According to the results, the artificial neural network could reach a good approximation of the seismic response of embankments and earth dams. Although comparing the results of artificial neural networks segmented from generic artificial neural network showed the accuracy of the estimated artificial neural network structured for each PGA level, especially in the linear region, the study was successful in eliminating the defects of previous research, making a perfect arrangement for a generic artificial neural network to be used for this type of neural network to achieve a favorable estimation of all PGA levels. Finally, it can be concluded that considering the complexities of the problem and the performance of the artificial neural networks in solving them, as well as the history of successful performance reports on this tool in other similar applications, the application of Artificial Neural Networks proved useful for estimating and predicting seismic responses of embankments and earth dams. Using artificial neural networks to meet this purpose would reduce the cost of calculating the seismic response assessment of embankments and earth dams. Therefore, applying this tool in the complicated field of seismic geotechnical engineering may be highly promising. Overall, future developments in this research field and promotions in artificial neural network training in other situations by changing the dimensions and properties of materials and load conditions would realize this idea and create a more general neural network for this application. 

Failures caused by surface rupture in recent earthquakes emphasize the need to investigate fault-structurefoundation interaction while designing engineering structures and life lines in fault zones. Many civil projects, especially buildings on problematic soils (e.g. loose soils with low carrying capacity, high settleability and liquefaction, made grounds, and others), use micropiles in the sub-base to improve the soil and increase its carrying capacity. If such structures are positioned in fault zones, however, the interaction between the faulting and the micropiles located at the faulting zone is often neglected. Therefore, this paper used numerical modeling in Abacus to investigate the reverse fault-micropile interaction, and modeled and analyzed the three-dimensional soil, foundation and micropile system using the finite element method in Abacus. In this regard, a footing with an 11m length and width and a 91 kPa load (equivalent to a 9-floor building) was modeled atop a 15m-thick sandy soil layer. S set of 36 concrete micropiles with 20cm diameter, 10m length and 2m distance to each other were also used for investigating the base-fault-micropile interaction. The numerical analysis was validated using laboratory results. The ratio of the distance between the faulting and sub-base contact to base width (S/B) was the most important criterion used in this study, and the parametric study investigated the effect of the micropile foundation’s location relative to faulting after selecting the S/B parameter. Results show that the foundation and micropile response depends on their position relative to the fault, and responses to deviation or rupture propagation around the structure differ by structure position. The overall results show that the presence of micropiles does not significantly affect horizontal displacement control, resulting in a 10- 15% reduction in horizontal displacement compared to the no-micropile state for S/B>1.0. Micropiles are consistently effective for controlling vertical foundation displacement and reduce vertical displacement by about 60% compared to the non-micropile state. This is caused by the faulting’s contact with the micropile-foundation block, which makes it act as a retaining wall and leads to rupture diffraction, distribution, and diversion. However, results differ by foundation rotation in different conditions. Due to the residual stress between micropile rows in 0.9<S/B<1.5, the faulting-micropile interaction increases the foundation rotation in the micropile state by approximately 40% compared to the non-micropile state, that points to the inefficiency of micropiles in controlling foundation rotation when put in such a condition. Therefore, if the objective, importance, limitations and other criteria affecting engineering structure placement call for a mixed “foundation and micropile” structure adjacent to the fault, the position of the foundation with the micropile set relative to the faulting is a determinant factor of soil-faulting-micropile and foundation interaction results. Accordingly, investigating horizontal and vertical displacement as well as foundation rotation with S/B ratio changes can determine the optimal structure position relative to the fault and fault path and prevent damages.

In most cases of design or assessment of the buildings, the fixed foundation assumption is used and the effect of soil-structure interaction (SSI) is neglected. In fact, considering the SSI effects in analysis leads to substantial changes in the dynamic properties and the dynamic responses of the structure. On the other hand, the shear wall-frame lateral load resisting system is one of the most popular systems for resisting the earthquake and wind loads because of its ability to provide high energy dissipation, high lateral stiffness and high lateral capacity in comparison with shear wall or bare frame only. In this paper, a comprehensive study of the seismic behavior of the RC wall-frame system with and without SSI effects is performed. For this aim, three 5-, 10- and 15-story buildings were simulated in OpenSees software so that the wall-frame dual system for each of them is located at the perimeter of the structure. The shear wall modeling was performed using the shear flexure interaction (SFI) model, which was developed by (Kolozvari et al., 2015). For nonlinear modeling of beams and columns, the beamWithHinges and nonlinearBeamColumn elementswere used, respectively. Also, by using the beam on nonlinear Winkler foundation (BNWF) theory the SSI phenomena was simulated. Modal and pushover analyses were carried out in two cases, namely, fixed and flexible bases. The lateral load capacity, structure period, base shear contribution ratios of the shear wall and moment frame and the story drifts were studied in details. Also, for different performance levels, the base shear, story shear force and the development of the plastic hinges were investigated. The results of the modal analyses show that the SSI effects increase the first mode period of the structure and the percentages of increase in the first mode period for the 5-, 10- and 15-story models are 75.2, 23.8 and 13.2%, respectively. In addition, considering the SSI effects in eigenvalue analysis leads to change in the first mode shape of the 5-story model, so that it will be close to linear case. Also, the SSI effects lead to decrease in the base shear capacity of the 5-story model and by increasing the height of structure its effects on the base shear capacity are negligible. The results indicate that the foundation of the shear wall under SSI effects has a rotational movement that decreases the shear wall contribution in bearing the lateral loads and increases the frame base shear contribution at the linear stage. Besides, by increasing the lateral load level, the shear wall base shear contribution ratio increases and more nonlinearity will take place at this stage. In case of flexible base, the story drifts increase, especially in the lower stories and at the high damage stage the base shear contribution ratios of the shear wall and frame will be equal. Also, the SSI effects lead to increase in the seismic demands of the structural components in the lower stories and decrease them at the higher stories.

Identification of the modal parameters of the damaged structure by signal processing of vibration based on changes in dynamic properties such as frequency, stiffness, and mode shape of structures. Some of these approaches fail when applied to civill engineering structures, the main reasons are the low sensitivity of the structural response to the damage location, or the low accuracy of structural response obtained by installed sensors. However, due to the rigorous evaluation and low cost of signal processing methods, this method has made a great progress. Signal processing methods have been extensively employed to examine the measured system responses and determine system variations. These methods include Fourier analysis, wavelet transform and Hilbert-Huang transform.During the last decades, the number of vibration-based damage detection methods has been greatly developed and has influenced much of the research. The purpose of these methods is to determine the resulting changes in the modal characteristics of the structure including its natural frequency, mode shape, and damping ratio. For example, the basis of the Fourier transform method is to determine the structural modal parameters from the random vibration data in the frequency range. However, time-frequency analysis is introduced to overcome the limitations of the Fourier method, the most important of which is not providing a frequency-time range of a signal. The first case of frequency-time analysis was the short-time Fourier transform method based on the Fourier transform of the data divided by the time window function. According to this method, the interaction between time and frequency is difficult due to the existence of the time window function. If the windows are smaller in the time segment, its accuracy increases, and in the frequency domain it becomes less accurate.Several damage detection methods have been proposed based on the vibration signal of structures. In most of them, a damage index has been described as the difference between damage and undamaged structure. This paper intends to propose a damage detection method based on the amplitude coefficient correlation of damaged and undamaged responses of structures, while a signal decomposes to IMFs and the changes appear in the first IMF. Therefore, every change on the original signal can be revealed on IMFs, since the original signal depends on IMFs. Also, these changes have an effect on the analytical signal and the Hilbert transform. The instantaneous frequency is measured joints on the structure is calculated by the Hilbert transform of the first IMF of response. Then, by introducing the instantaneous frequency energy (EDI), the location of damages are detected. To assess the feasibility and reliability of the proposed method, the ASCE benchmark problem has been used. To consider the robustness of the proposed method, contamination of signals during the data acquisition process is investigated. The ASCE benchmark study is carried out by the International Association (IASC) ASCE Structural Health Monitoring Task Group. The dynamic responses of the structure have been obtained by numerical analysis under random vibration loading. To evaluate the HHT method, it is required to attain the damaged responses of the ASCE benchmark. The first five damage patterns of the ASCE benchmark building is used. Then the damage on the structure is detected with a comparison of damage and undamaged dynamic responses. According to the measured noise levels, dynamic responses to noise values have been contaminated and the results have been evaluated. According to the results, this method can trace the location of the damage by the energy of instantaneous frequency. Therefore, the locations of damages in different scenarios were located with the EDI index and velocity vector. The results show that the proposed method determined the location of damage with the acceptable accuracy for low and moderate damage in damage scenarios.

In the present study, the FLAC Software is applied to investigate the different models of Daikai Subway Station, which was heavily damaged in the 1995 Kobe earthquake. The responses of the ground surface were analyzed with and without the presence of the structure to examine the phenomenon of amplification. The maximum amplification of 2.2 was obtained in the middle point of the structure, while the maximum acceleration was obtained in the point adjacent to the edge of the structure. The points located on the contact surface of the structure were experienced the highest amplification in comparison with the case of without structure. In the next step, different models of the structure with the central column thicknesses of 10-70 cm were developed to investigate the lateral displacements of the structure. The results indicated that the maximum lateral displacement of the structure between the maximum and minimum thicknesses was smaller than 10 mm.1. IntroductionToday, the importance of underground structures such as subways is more than ever because of their no doubt critical roles in solving traffic problems in metropolises. Since subways are typically constructed in populated cities and pass beneath major commercial and economic centers, they can impose considerable casualties and economic losses if damaged or destroyed [1]. For many years, the performance of underground structures was believed to be better than that of ground structures when subjected to the earthquakes. However, the experience of some earthquakes demonstrated that underground structures might undergo large deformations or even major failures [2-3]. The present study models a real underground structure, which experienced serious damages during the earthquakes, by the finite difference method (FDM) using the FLAC Software. The present study aims to investigate the effects of the structure on the scattering of the seismic waves reaching the ground surface. The quantity and pattern of relative displacements for different sizes of the central column (destroyed by the earthquake), the relationship between the recorded accelerations on the ground surface and the effects of the presence of the structure on the acceleration response are investigated.2. Numerical ModelingThe numerical modeling was performed by the Finite Difference Method using FLAC 3D software. The static mode of the gravity loading was employed to define stresses induced by the soil weight. Roller supports were used for vertical boundaries, while pinned supports were applied to the bottom of the model. These boundaries reflect waves into the model and thus they cannot be applied to the dynamic mode.ConclusionThe present study employed the FDM to investigate the seismic responses of a real subway station. The obtained results are as follows: 1. Comparison of the amplification induced by the effects of the structure on the scattering of seismic waves indicated that the scattering effects increased near the edges of the structure, leading to the large amplification factors at the end area.2. The maximum amplification of 2.2, and a 25 percent increase was obtained in the maximum acceleration of Daikai under the 1995 Kobe earthquake.

Structures may undergo unexpected changes during the construction and assembly of materials and their sections, or may be affected by earthquakes of various magnitudes and mechanisms during their life time. Each of these changes may affect the possibility of structural failure. These variable parameters are examined as reliability. By the theory of reliability, uncertainties of the structural and non-structural parameters such as material properties, geometrical dimensions, and seismic inputs, etc. are taken into account. The theory of reliability with fragility curves shows which part of the model is more affected by uncertainties. These curves show the probability of passing a certain level of damage against seismic parameters.This paper presents a reliability analysis of the steel moment resistance structure equipped with viscous dampers applied by uncertainties in seismic input and structural properties such as ductility and fragility of beam and column, damping coefficient of dampers by obtaining fragility curves. Given the intensity and frequency of near-field earthquakes that cause large displacements in the structure and it is possible that the braces on which the dampers are mounted enter the nonlinear domain before activating or collapse of viscous damper, the analysis are examined only on the basis of far field earthquakes. Nonlinear incremental dynamic analysis (IDA) is applied to the 5 and 10-story frames modeled in OpenSees software under a set of 22 pairs of acceleration records. Also, to reduce the time and number of analyzes, the response level method has been used in this study. The number of scenarios required for the response level method is obtained from the box-Wilson method. According to the results obtained from the analyzes (mean and deviation from the failure level criterion), a second-degree surface procedure for mean and standard deviation has been internalized. The Monte Carlo method also simulated 10,000 fragility curves to determine the final fragility curve. The results show that increasing structural story has a significant effect on the uncertainty in the response of structures. Based on the results, considering uncertainties in 5 and 10-story structure reduce failure spectral acceleration (Sa) by 7.1% and 9.3% respectively. Besides, in spectral values corresponding to the first mode of 5 and 10-story structures, with considering the uncertainty, probability of failure increase about 52.4% and 74.7%, respectively. The results showed that considering the uncertainty in some parameters increases and others reduce the capacity of the structure; however, by considering uncertainty in all mentioned parameters simultaneously, it will reduce the capacity and increase the probability of collapse.

Determining the modal characteristics of structures such as natural frequencies and damping ratios is one of the most important issues in structural engineering. In this regard, providing a low-cost and robust experimental method against all types of noises is very important. In the present paper, a new algorithm for determining the natural frequencies and damping ratios of structures under impact loads using the proper orthogonal decomposition technique is presented. This method uses the vibrational response of the structure to impact loads, without the need to calculate the impact magnitude. One of the strength points of the proposed methodology is the accumulation of laboratory noises in the latest modes. In other words, in the process of calculating the frequencies related to the first few modes, laboratory noise does not enter the calculations and will be aggregated in higher modes that are less important. The feasibility and efficiency of the new method was evaluated using numerical simulations as well as laboratory validation. In this research, four pure numerical models were used to evaluate and validate the accuracy of the proposed algorithm. These models include a simply supported beam, a two-dimensional portal frame, a three-dimensional truss and a clambed-clambed beam. The natural frequencies and damping ratios of the mentioned models were calculated using the new method, then the results were compared with those that obtained from the finite element method. Very good agreement was observed between the results of the two methods. For further investigation, various states such as the effect of the noise on the results, the effect of multiple impact loads and the effect of the number of sensors were also studied. The results of these numerical studies showed that the proposed method is very stable and robust against laboratory noises and has small errors. Acceptable results can be obtained in using a few numbers of sensors (even one sensor) and repetition of the test. Also, the results for multiple impact loads are almost similar to the results obtained from the excited state with a simple impulse load. In this study, in order to further investigate the efficiency of the POD based method, a small-scale laboratory model developed at Shahid Rajaee Teacher Training University and whose modal information has already been calculated using the closed image processing method, was studied. Good agreement was observed between the results of the two methods, so it can be another reason for the efficiency and capability of the new method.Based on various studies, it was concluded that for structures with low damping, the proposed method has acceptable accuracy and with increasing damping, the accuracy of the results gradually decreases. Since conventional structures have a relatively low attenuation, the proposed algorithm is very suitable for determining their modal information. The proposed method due to the availability, cheapness and no need for complex experimental tools can be used as a useful algorithm to determine the modal information of a structure as well as control the results obtained from other experimental methods.

The footing of all buildings will be located on the ground. Thus, civil engineers are always faced with the problem of estimating the bearing capacity of shallow foundations. So far, various analytical and numerical methods such as stress characteristic lines method, limit equilibrium method, lower and upper boundary limit analysis methods, as well as finite difference and finite element methods, have been utilized to evaluate the bearing capacity of shallow foundations.In the meantime, the stress characteristic method, also known as the “slip-line method”, has been shown to be an efficient and useful technique for solving the bearing capacity problem due to its simplicity, high speed in calculations, and no need for neither meshing nor complex soil behavior models. These advantages attracted the attention of many researchers over the past three decades to the stress characteristic lines method.Obviously, the bearing capacity of footings would be reduced, if they are subjected to seismic loadings or if they are located on slopes or adjacent to them. On the other hand, experimental as well as numerical studies reported in the literature revealed that the roughness of the footing would have an important effect on its bearing capacity. Hence, estimating the bearing capacity of a footing taking into consideration the roughness of its contact surface constitutes one of the important issues in foundation engineering, which of course has been studied only by a small number of researchers.This paper intends to review the most important works reported in the literature concentrated on the estimation of the bearing capacity of rough shallow foundations by the stress characteristic lines method. We wanted to find the answer to the question that how different researchers considered the roughness factor in evaluating the bearing capacity of footings by the stress characteristic lines method? For this purpose, firstly the mathematical formulation of the stress characteristics method was briefly reviewed. After that, the numerical algorithm of implementing this method to estimate the bearing capacity of smooth as well as rough footings, the corresponding boundary conditions, and calculation procedure for determining the bearing capacity coefficients 𝑁𝑐, 𝑁𝑞, and 𝑁𝛾 were discussed. Various techniques for taking into consideration the roughness factor, especially their assumptions, were investigated and compared with the smooth footing case.An important point to keep in mind when calculating the bearing capacity of rough footing is the non-uniformity of the contact pressure inclination. Various researchers employed different assumptions and obtained quite different results. Some researchers considered uniform roughness by regarding different inclination for the contact pressure along the soil-footing interface or by placing a non-plastic curved wedge immediately beneath the footing. Some other researchers adopted the non-uniform roughness and solved the roughness problem by assigning a non-plastic curved wedge under the footing, or by considering it as a line and two types of parabolic equations, or even without regarding any pre-assumption failure pattern. The influences of these assumptions and methods on estimating the bearing capacity of rough footings were investigated in detail and discussed. Finally, the most important challenges ahead in evaluating the bearing capacity coefficients of rough footing by the stress characteristic lines method were summarized and explained.