مجله علوم و مهندسی زلزله
سال هفتم شماره 3 (پیاپی 24، پاییز 1399)

Attenuation of seismic waves is considered a very important characteristic of the wave propagation path because this physical parameter affects how the seismic waves propagate, and consequently knowing the amount of it is essential in accurate calculation of seismic source parameters, modeling and reducing seismic risk in seismic areas. Attenuation of seismic waves in an environment is the result of two physical processes, elastic and inelastic. In the elastic process, energy remains constant in the propagation medium; however, the amplitude of the waves may increase or decrease due to the geometrical spreading, multipath, and scattering. These factors depend on the type of wave, frequency, degree of heterogeneity and characteristics of the propagation medium. In the inelastic process, part of the energy of the seismic waves is converted to heat and the amplitude of the seismic waves decreases due to the loss of a part of the energy. In the inelastic process, factors such as inelastic properties of the environment and physical properties of the environment (wave velocity, density and temperature) play an important role. Not only the scattering and the intrinsic absorption are important factors in reducing the amplitude of direct waves, but they also affect the appearance of a seismogram.Decreasing the amplitude of seismic waves by increasing the propagation distance from the source and the frequency changes made in the time history of the earthquake is called attenuation. In general, the factors that weaken the waves and the energy emitted from the seismic source are reflection and passage of waves through the boundaries of the layers, multi-path, geometrical spreading, wave diffraction, attenuation and inherent absorption of waves due to heterogeneity in the propagation path. The attenuation of seismic waves is described by the dimensionless quantity of the quality factor Q, which indicates a decrease in the amplitude of the wave along the propagation path. This parameter is a function of frequency, seismic wave type, time window intended for seismic analysis, geological characteristics below the seismic recording site and tectonic activity of the region. Physically, Q is the ratio of wave energy to energy wasted in each cycle of oscillation. By examining and calculating the range of Q changes from the data processing of seismic waves emitted in the Earth's crust, the characteristics of its various parts can be realized. Parts of the Earth's crust that have very low attenuation have very high Q values, and parts with severe attenuation have very low Q values. Tectonically active regions have a relatively high heat flux, so they are more strongly adsorbed than other regions, so they have a lower Q value.The attenuation of coda waves, Qc, has been estimated in Fariman region, NE-Iran, using a single back-scattering model of S-coda envelopes. For this purpose, we used the time histories of 124 aftershocks of Do-Ghaleh Fariman earthquake (Mw6, 2017), recorded by local seismic network belong to International Institute of Earthquake Engineering and Seismology (IIEES). In this research, the frequency-dependent Qc values are estimated at central frequencies of 2, 4, 8, 16 and 24 Hz using different lapse time windows from10 to 60 s and the frequency-dependent relationships obtained for 30 s is Qc=(73±11) f (0.89±0.06). It is observed that the exponent n decreases and Qo increases as lapse time increases. Any increase of Q with depth or with distance from the seismic source to receiver would cause the increasing of coda-Q with lapse time. The average Qc values estimated and their frequency dependent relationships correlate well with a highly heterogeneous and highly tectonically active region. Our results regarding QC factor is the first study in this area and would be significant for reassessment seismic hazard and risk management in southern part of Khorasan Razavi.

Fault displacement can affect structures along their outcropping on the ground surface and cause varying levels of damage to buildings. In this regard, the interaction of different types of foundations, including shallow, buried and pile ones, in the interaction with this phenomenon have been studied. In this study, a series of centrifuge tests have been conducted considering the kinematic effect of the foundation in order to investigate the effect of foundation location and type of fault mechanism on the interaction of dip-slip faults and shallow foundations. The dip-slip fault rupture at a dip angle of 75° propagates in a moderately dense sand layer and interplays with the shallow foundation. A summary of conclusions is as follows:- For the reverse fault, the formation of tension cracks on the ground surface is due to the soil moisture and the apparent cohesion of the wet sand. Also, the fault-zone width is greater in wet sand than dry sand due to the formation of tension cracks. Therefore, these cracks should be considered in specifying the width of the set-back zones.- For the reverse fault, changing the position of the foundation from the foot wall to the hanging wall increased the rotation and displacement of the foundation, and the interaction mechanism was changed. The behavior of foundation and the development of rupture mechanisms are fully dependent on the location of the foundation relative to the fault rupture and the magnitude of the fault offset.- The formation of a graben due to normal faulting is one of the results of this study.- By changing the fault mechanism from reverse to normal, it was found that the foundation and superstructure are damaged at smaller fault-induced dislocation. In a normal fault, the foundation will experience rotation for small amounts of the fault displacement because of the nature of stress in normal faulting and the weakness of the soil in tension.- The superstructure did not have a significant effect on the interaction mechanisms of the foundation and the fault, but it certainly had an effect on the foundation rotation.

In recent decades, the recognition of seismic ground motions and damage investigations in the presence of subsurface heterogeneities including cavities and inclusions during an earthquake have been considered among the seismologists. This issue is more significant for subsurface inclusions because they can change the initial nature of incidence waves and amplification/de-amplification on different zones of the surface. Therefore, evaluating various effective factors including geometry and type of features, site conditions, type of incident waves and paths of wave motion requires appropriate methods for their analysis and detailed understanding. Using these approaches allows modeling the problems of wave scattering and predicting the real seismic responses. Technically, researchers have proposed various approaches for seismic analysis. These methods can be divided into analytical, semi-analytical, experimental, and numerical ones. Despite the high accuracy of analytical methods, their lack of flexibility in the modeling of complex features has forced the researchers to use alternative approaches such as numerical methods. In recent years, increasing the power of computers has helped to solve complex engineering problems using numerical methods. In the use of numerical methods, one can never claim that the obtained results are completely exact; rather, the main purpose is to move toward accurate responses as close as possible. The numerical methods are divided into two general categories known as the domain and boundary methods. The common domain methods include the finite element method (FEM) and finite difference method (FDM). Moreover, the boundary methods are separated into two categories including full-plane and half-plane, in which each part is developed in the transformed and time domains as well. In the use of boundary element methods (BEM), one dimension of the model is reduced and the radiation conditions of waves at infinity are satisfied. The advantages of the BEM compared to the domain approaches are include the concentration of meshes only around the boundary of desired features, the satisfaction of wave radiation conditions in far boundaries, low volume of input data and memory seizure, a significant reduction in analysis time and high accuracy of the results.In this study, step-by-step transient analysis of arbitrarily shaped twin elliptical inclusions are presented subjected to propagating obliquely incident plane SH-waves using the direct half-plane time-domain BEM approach. Based on the sub-structuring process, the model of twin subsurface inclusions was decomposed into a dual pitted half-plane and twin closed filled solids. By determining all the related matrices and applying the continuity conditions of the displacements and tractions at the interfaces, the coupled matrix was achieved to obtain all unknown boundary values. After developing the method to analyze the problem of twin inclusions, it was implemented in the general algorithm previously called DASBEM and its validity was evaluated by some practical examples. The key parameters of the shape ratio of inclusions and incident wave’s angle were considered to sensitize the response behavior. In order to complete the numerical results, some synthetic seismograms and 3D (three-dimensional) blanket amplification patterns were presented to illustrate the time and frequency-domain responses in the presence of twin inclusions. The results clearly demonstrate the significant role of the elliptical twin inclusions on the seismic response of surface and show that the maximum scattering and amplification are achieved in minimum shape ratio for vertical incident waves. It should be noted that the main objectives of the present study are presenting the ability of the proposed method in preparing simple twin inclusions models, transient analysis of complex engineering problems, obtaining high accuracy results, and illustrating a better view of subsurface irregularities interactions in the field of geotechnical earthquake engineering.

Despite extensive studies on the microtremor processing technics and horizontal to vertical (H/V) spectral ratio analysis, limited studies have been tried to interpret the observations and experimental results and confirming them via two and three-dimensional numerical modeling. The present article deals with this issue by selecting the Urmia alluvial basin. Previous seismic microzonation studies in this city show a broad peak or two neighboring peaks, close to each other at some points near to the edge of sedimentary basin. By examining the transfer functions obtained from one-dimensional and two-dimensional numerical analysis performed on an east-west section of the Urmia sedimentary basin, it was found that this issue could be due to a sudden change in thickness of the soil profiles on either side of measuring point and coupling of shear waves passing through these two different environments. This study shows a very good agreement between the frequency position of the two neighboring peaks between the H/V curve and two-dimensional transfer function. In addition, this study shows that when two-dimensional site effect is predominant, the resonance frequencies are higher compared to one-dimensional conditions. This study also suggests that for recognition of the real peaks in H/V curves, the variation trend of dominant frequencies between the different measuring points in study area should be considered and each curve should not be evaluated individually.

A special class of ground motions near the fault regions has distinct characteristics that are different from far-field ground motions. One of the critical features of these ground motions is the existence of a velocity pulse in the ground motion record in the direction perpendicular to the fault rupture. These pulse-like ground motions generally occur when the fault rupture propagates towards a site located near the fault. The accumulation of energy in the seismic wave front results in a velocity pulse with a relatively long period in the direction perpendicular to the fault line. This phenomenon could adversely affect the seismic performance of tall buildings with relatively long fundamental periods.In this paper the effect of velocity pulse on seismic response of a 42-story reinforced concrete building with a central core wall structural system is evaluated. The building has already been studied in the TBI project of the Pacific Earthquake Engineering Research Center (PEER). The structural model of the building is developed based on the information contained in the PEER report. The model is first verified by conducting a nonlinear response analysis using one of the ground motion records in the PEER report and comparing the results with that report. After verification, the model is subjected to three earthquake records each containing a velocity pulse (i.e., Northridge 1994, Cape Mendocino 1992, and Chuetsu-Oki, Japan 2007). Subsequently, the velocity pulse of each record is removed using a recently proposed wavelet-based signal processing approach and the building is analyzed again to determine the impact of the velocity pulse. The results of the analyses show that the velocity pulse significantly affects the seismic response of the building. By removing the velocity pulse, the lateral drift and rotation of the coupling beams decrease by about 50% and 60%, respectively. Also, the forces in the structure (shear and bending moment) are reduced by about 40%. The dominant effect of the velocity pulse on the seismic response of the building is due to the high energy content in the frequency range of the pulse velocity. However, due to the different frequency content of each record, the effect of the pulse period cannot be accurately assessed. Therefore, the study of buildings with the same lateral bearing system and different natural periods (i.e.,

Structures are subjected to different loadings during their lifetime. Most of these loads are time dependent and change over the time. Therefore, it is important to evaluate structures under dynamic loads. On the other hand, dynamic response of structures is affected by several factors that results in many complexities to structural analysis. Thus, numerical methods are used for seismic analysis of structures. In this article, a new semi analytical method with high efficiency is developed for soil-structure interaction (SSI) analysis, which is called decoupled scaled boundary finite element method (DSBFEM). This method has analytical solution in radial direction and uses a specific shape functions as the interpolation function in the circumferential direction. In addition, the boundaries of the problem are discretized by specific new non-isoparametric elements. In these elements, new special shape functions as well as higher-order Chebyshev mapping functions are implemented. For the shape functions, Kronecker Delta property is satisfied for displacement function, simultaneously. Moreover, the first derivatives of shape functions are assigned to zero at any given control point. In fact, to model the geometry of the problems, we consider a local coordinate origin (LCO) for transportation of the geometric characteristics of global coordinate and local coordinate. Consequently, using a form of weighted residual method and implementing Clenshaw-Curtis numerical integration, coefficient matrices of the system of equations are converted into diagonal ones, which leads to a set of decoupled partial differential equations for solving the whole system. This means that the governing partial differential equation for each degree of freedom (DOF) becomes independent from other DOFs of the domain. Due to the soil flexibility effect on structural responses, in this paper, SSI problem has been investigated considering different values of modulus of elasticity for soil domain. To achieve this, two different LCOs have been used to discretize the soil domain and the structure domain. Thus, a three-step algorithm is proposed, which consists of: (1) considering an initial stress on the interaction boundary, (2) analysis of soil domain, and (3) analysis of structure domain. Therefore, after the initial assumption of stress on the interaction boundary, the soil domain will be completely analyzed by two-stage traction redistribution and the results on interaction boundary will be used as boundary conditions of structure domain. It should be noted that in the proposed algorithm, only one-stage traction redistribution will be used to analyze the structure domain. Finally, validity and accuracy of DSBFEM are fully demonstrated through some benchmark examples with different values of modulus of elasticity for the soil domain, and the results are compared with Finite Element Method (FEM). The results indicate that the proposed method has high accuracy and flexibility to consider the SSI effect, determine the resonant frequency and the maximum displacement amplitude of the structure. In addition, the number of elements used in the DSBFEM is much less than the FEM, which will lead to a reduction in computational costs.

Maximum interstory drift ratio is a useful engineering demand parameter for predicting damage and structural collapse. In recent decades, some research studies have focused on Maximum Residual Interstory Drift Ratio (MRIDR) of structures as another engineering demand parameter. MRIDR plays a key role in assessing the seismic performance of structure after seismic events, because it indicates that if structure is safe or not, and if the repair of structure is economical or not. Nowadays, passive control systems are employed for designing new structures and improving the seismic performance of existing structures. Among them, the use of Fluid Viscous Dampers (FVDs) has become very common because of their remarkable energy dissipation capacity, negligible maintenance cost and the possibility of being used in multiple earthquakes. Linear FVDs have a velocity exponent of α=1.0 and nonlinear FVDs have velocity exponents of α≠1.0. This study evaluates the effects of employing linear and nonlinear FVDs and different vertical distributions of damping coefficients on the MRIDR response of steel Special Moment Resisting Frames (SMRFs) with FVDs. For this purpose, low- and mid-rise steel SMRFs including the 3- and 9-story SMRFs designed for Los Angeles as part of SAC steel project are considered. Moreover, the height of the first story in the 3-story SMRF is increased by a factor of 1.4 to generate a 3-story SMRF with a soft story. Each of these three structures is equipped with FVDs to limit maximum interstory drift ratio under the design earthquake to 0.015. Two types of vertical distributions of damping coefficients that include uniform distribution and Interstorey Drift Proportional Distribution determined on the basis of the first mode deformations (IDPD) are assumed for each of the structures. Moreover, four values of α=0.25, 0.5, 0.75 and 1.0 are considered for FVDs. OpenSees software is applied to model the structures. Concentrated plasticity approach is used for modeling beams. In this approach, each beam is modeled by an elastic beam-column element and two zero-length elements simulating inelastic response. However, columns are modeled using nonlinear beam-column elements, which are based on the concept of distributed plasticity. The P-Delta effects of gravity columns are accounted for by a leaning column. Four MRIDR values of 0.002, 0.005, 0.01 and 0.02 are assumed as limit states, and Incremental Dynamic analyses (IDAs) are performed on the structures using a set of far-field ground motion records considering each of these limit states. For performing the IDAs, 5% damped pseudo spectral acceleration at the fundamental period of structure, Sa(T1), is selected as ground motion intensity measure. Using the results of the IDAs median MRIDR capacity, i.e., median SaRD, and its corresponding logarithmic standard deviation are calculated for each of the structures. Then, assuming lognormal distribution for SaRD, residual drift fragility curves are obtained for the structures given each of the MRIDR limit states. The results indicate that given each of these limit states, the structure equipped with linear FVDs has higher median SaRD compared with its corresponding structure equipped with nonlinear FVDs. Furthermore, reducing α causes reduction in median SaRD. Residual drift fragility curves corresponding to all the limit states for each of the structures are combined with the seismic hazard curve for the site assumed to calculate the mean annual frequencies of exceeding these MRIDR limit states (λRD). According to the results, the values of λRD for the structures with linear FVDs are between 6.87% to 80.24% lower than those for the structures with nonlinear FVDs. Comparing the results obtained using the two height-wise distributions of damping coefficients shows that when first story height is greater than typical story height, using IDPD leads to higher median SaRD and lower λRD.

In recent years, the use of seismic isolators and dampers in protecting important structures against earthquake or explosion loads have increased dramatically. Increase in terrorist attacks have motivated many researchers in studying the effects of explosive load on structures. This paper presents the dynamic response of three structures (i.e., five, ten, and fifteen-story buildings) with fixed base, isolated using lead core rubber bearing and viscous dampers subjected to earthquake and explosion loads. For this purpose, the intended explosion load is considered to be due to the surface explosion of 264 kg of TNT at distances of fifteen and twenty meters from the structure. The pressure load that should be applied to the structures is computed at different points of the considered structures using the AUTODYN software. Finally, the structures are analyzed using SAP 2000 software, and the values of relative displacement, drift ratio, base shear, and plastic joints created in structure's members are computed for all controlled and uncontrolled structures. The results show that relative displacement values in a structure with viscous dampers are significantly reduced (82.7%) for blast loading. Base shear reduces by 40.85% in structures using rubber separator system with lead core rubber bearing system, and it is reduced by 36.19% for structures using the combination of lead core rubber bearing system and viscous damper. The results show that the use of a combined system in the low-rise structures has reduced the structure's drift ratio by 95.17%. For the structures controlled with the viscous dampers, the base shear is increased. This increase was 89.35% for the controlled structure subjected to the blast load at a distance of 20 m from the structure. In contrast, the use of a lead core rubber bearing system resulted in a decrease of 33.19% to 40.84% of the base shear in the controlled structures. The results indicate that the simultaneous use of lead core rubber bearing system and viscous damper improves structure's performance level, and no plastic joints were formed in structures studied. Therefore, according to the above results, it is clear that use of a combined system consisting of lead core rubber bearing system and viscous damper can lead to a safer design of steel structures subjected to the blast load.

The recorded recent earthquake events show that near-field earthquakes have different characteristics than far-field earthquakes. The most important distinguishing feature of near-field movements is the production of pulses due to the effect of orientation and the effect of permanent displacement. Therefore, it is necessary to study such effects on structures. In this research, first, the proposed mathematical model is validated according to the results of experiments. In order to study the nonlinear seismic behavior of post-tensioned precast concrete walls (PT-PCW), nonlinear dynamic analysis has been performed on six-story structures under near-field accelerations. Also, the effect of wall pier confinement on the self-centered performance and energy dissipation has been evaluated by performing dynamic analysis. Each acceleration is scaled to two levels of design-based earthquake (DBE) and the maximum considered earthquake (MCE) and then is used in the analysis. The performed analyzes show the optimal performance of the post-tensioned precast concrete wall system in response to earthquakes on DBE, so that at the end of the seismic load, the system does not suffer any structural damage, and minor lateral displacement remains in the system. The results also show that increasing the height of the wall pier confinement to a value slightly higher than the minimum specified in the seismic design codes promotes seismic behavior for MCE and increases the reliability of the system against overturning. The study also found that the use of post-tensioned cables in the boundary areas of the wall as an energy dissipation factor does not have a significant effect on improving the energy absorption performance of the system and the small deformations created in these members cause energy dissipation through crushing of concrete.

In the recent decades designing buildings against progressive collapse has been subjected to growing attention. In progressive collapse, the failure of one single structural member is transmitted to other members resulting in collapse of the entire load-resisting system of the building. Progressive collapse in buildings can be triggered by diverse factors such as accidental gas blast, impact between vehicles and one of the columns, etc. It is, thus, of great importance to design a building to withstand such catastrophic collapse. In this manner, in the design process, the building is subjected to different failure scenarios of single elements, e.g. a column, and investigate whether the failure spread to the remaining parts of the structure. While a substantial effort has been devoted to design earthquake-resilient structures based on the current seismic codes and state-of-the-art researches in earthquake engineering, still further studies are required to investigate the resistance of structures against progressive collapse.In this research the progressive collapse of dual special steel moment-resisting frames and buckling-restrained braces, as an earthquake-resilient structural system for high-rise buildings, has been numerically investigated considering several failure scenarios. Buckling-restrained braces have been considered to provide appropriate seismic performance due to fair tensile and compressive behavior as well as almost symmetrical hysteresis response. In the performed numerical analyses, progressive collapse in four high-rise residential and commercial 20-story buildings with dual special steel moment-resisting frames and buckling-restrained braces with different configuration, such as (combined V-Inverted V) and (V) located in the corner and edge bays is studied, regarding different failure scenarios. The above-mentioned scenarios include the removal of a corner or edge column in the first or fifteenth floor, as well as the removal of two edge columns of the braced bays in the first or fifteenth floor. The numerical models were verified against existing published experimental and numerical results before being applied to the analytical cases. This is achieved by comparing the results reproduced by the numerical tool to the previously published results regarding both cyclic analysis, at the sub-assembly level, and time-history analysis, at the structural level.The adopted numerical models were based on Finite Element (FE) simulation of the structures taking into account both material and geometric nonlinearities. The Inelastic force-based plastic hinge frame element type in SeismoStruct software was used to model beam and column elements, while Inelastic force-based frame element type elements was implemented to model the nonlinear behavior along the buckling-restrained braces. Each of the bracing elements consisted of three parts, including: a middle part to simulate the core as well as transition and elastic segments of bracings, and two end parts with large stiffness to simulate the panel zone and gusset plates. Rayleigh damping was also selected to model the damping in time-history analyses of the progressive collapse process. The performance criteria for all the members and connections were selected according to ASCE/SEI 41-17.Based on the obtained results, the peak vertical deflection of the top node of the removed column was approximated to be as large as 6.5 cm, and collapse was observed in one of the numerical models (commercial building contains X-bracing in the corner bays).The dual structure with X-bracing in the corner bays represents the weakest performance, while the best performance was related to the case of V-bracing in the edge bays, and the V-bracing in the corner bays and X-bracing in the edge bays have shown similar behaviors. In addition, in all the single column removal scenarios, the peak value of deflection in the cases in which the column removal was not located at the bracing bays was significantly greater, approximately 1.5 to 3 times larger, compared to those of column removals in the bracing bays.