مجله علوم و مهندسی زلزله
سال پنجم شماره 1 (پیاپی 14، بهار 1397)

The evaluation of potential human and economic losses arising from earthquakes, which may affect urban infrastructures that are spatially extended over an area, is important for national authorities, local municipalities, and the insurance and reinsurance industries. However, seismic-risk analysis of distributed systems and infrastructures need to apply a different approach with respect to the classical site-specific hazard and risk analysis. Ground motion intensity measures (IMs) and resulting structural responses are correlated in neighborhood sites. The correlation value depends on the distance between the adjacent sites and the natural vibration period of structures. In particular, when a lifeline system is of concern, classical site-specific hazard tools, which consider IMs at different locations independently, may not be accurate enough to assess the seismic risk. In fact, modeling of ground motion as a random field, which consists of assigning a spatial correlation to the IM of interest, is required. It is very common in the seismic design of spatially distributed structures and lifelines to include the correlation of the nearby earthquake records, through empirical semi-variogram functions. In this study, the semi-variogram of vertical components as a function of inter-site separation distance with respect to the ground motion prediction equations for the Iranian acceleration data (vertical peak ground acceleration (PGA) and vertical pseudo spectral acceleration (PSA)) are presented for the first time using acceleration data from 220 earthquakes. The calculations were carried out for five natural vibration periods in the range of 0 to 3 seconds and using ground motion prediction equations for vertical component. The selected ground motion prediction equation is the local model proposed by Soghrat & Ziaeifar (2017). For estimation of empirical semi-variogram, two classical and robust estimators, and to fit the data, the exponential and Goda models are used. For the ground motion prediction equation by Soghrat & Zyiaeifar (2017), the values of the range (b) in the exponential model and the values of α and β in the model of Goda (i.e. a continuous function fitted to experimental values in order to deduce semivariogram values for any possible site separation distance, Goda & Hong, 2008) are estimated. It is observed that the correlation trend range generally increases with period.

The Iranian Plateau, characterized by active faulting, active folding, recent volcanic activities, mountainous terrain, and variable crustal thickness, has been frequently struck by earthquakes resulting in the massive loss of life. Studying the seismic hazard as well as evaluating and predicting the strong ground motions require the knowledge of seismic wave attenuation. The complex structure of the Earth’s medium affects seismic wave propagation. Attenuation quantifies the behavior of the seismic energy propagation in the lithosphere and can be utilized for seismic hazard mitigation. Local seismicity makes a large body of data, which provides a unique opportunity to estimate the seismic attenuation. Data from the strong-motion network installed in Tehran region was used to study the seismicity and the frequency-dependent attenuation of the crust. 22 local accelerograms recorded at 14 stations were utilized for the present study. It is estimated that the quality factor of coda waves (Qc) and shear waves (Qs) in the frequency band of 1.5–24 Hz by applying the single backscattering method of S-coda envelopes and the extended coda normalization method, respectively. The values of Qc and Qs show a dependence on frequency in the range of 1.5–24 Hz for this region. Considering records from Shahr-e Rey earthquake (Ml 4, 1388), the estimated values of Qc and Qs vary from 151 ± 49 and 93 ± 14 at 1.5 Hz to1994 ± 124 and 1520 ± 123 at 24 Hz, respectively. The average frequency-dependent relationships estimated for the region are Qs=(92±16)f (0.98±0.15) and Qc=(114±5)f (1.12±0.04). These results evidenced a frequency dependence of the quality factors Qc and Qs, as commonly observed in tectonically active zones characterized by a high degree of heterogeneity, and the low value of Q indicated an attenuative crust beneath the entire region. The experimental results show that lower Q values can be observed for near main shock epicenter stations and higher Q values for distant stations. The quality factor Q is affected significantly by the presence of cracks, and that Q is sensitive to cracks. The environment of the epicenter is more affected by the released energy, and seismic waves recorded in the near field are propagated in the filled crack area. This paper makes a significant contribution to the understanding of crustal attenuation and provides data to fill an important gap in the knowledge of attenuation in this region.

Intergranular water in granular soils, such as sands, can produce apparent cohesion in the soil that results in changing the behavior of soil while shearing. Apparent cohesion in wet granular soil arises from surface tension and capillary effects of the small water bridges between adjacent soil grains. This intergranular water exerts an attractive force between the grains and an apparent cohesion is produced. Induced cohesion in wet soil depends on the amount of intergranular water within the pores of soil.
In this investigation, the effect of water content on the sand strength parameters (such as cohesion and internal friction angle) and deformational parameter (such as failure strain) were studied. For this purpose, some series of direct shear tests were conducted in which soil water content and its vertical stress were widely changed. Samples with 0% (dry condition), 5%, 10%, 15% and 24% (saturated soil condition) were examined in nine vertical stress (that categorized into two fields: low and high vertical stress condition). The low normal stress condition contained 0.015, 0.03 and 0.045 kgf/cm2. Vertical stresses of 0.1, 0.25, 0.4, 0.6, 0.85 and 1.25 kgf/cm2 were determined as the high stress condition. The importance of low stress condition is to obtain the exact behavior of soil at low stress condition that corresponds to vertical stresses in small scale physical models.
In this research, the investigation is based on two points of view: 1) from deformational point of view, the most important result is the relationship between soil’s water content and its failure strain. Adding water to sandy soil decreases its failure strain and it has its lowest value at about 5% water. It means that wet soil behaves more brittle than dry one. In this point of view, also a good similarity can be observed between the results of direct shear tests and physical models results (D0/h parameter); 2) From strength point of view, in low vertical stress tests, with an increase in water content to about 5%, internal friction angle increased and beyond this limit decreased. As in this situation, almost a constant cohesion obtained from the data, it can be concluded that by an increase in water content to about 5%, soils shear strength also increases and then decreases (in low stress condition that it corresponds to physical modeling stress level). Besides, discussions were made on the behavior of sand at high stress level and in residual condition.
Above-mentioned results have been employed to interpret the result of some fault rupture physical modeling tests on wet sand. The pure sand was used with various water content (0% (dry), 5%, 10% and 15%). One of the important results that can be achieved in physical modeling tests is the normalized required fault displacement (D0/h) at the bedrock, in which the fault rupture trace reaches the ground surface. Obtained results show that as the water content increased up to a certain value (around 5%), the D0/h parameter decreases at first and beyond this limit it increases a little. Generally, it seems that soil behaves more brittle when it is wet. Besides, it is in its most brittle behavior when the water content of soil is around 5%. The observed trend of D0/h parameter is almost similar to what observed on failure strain of the wet sand that was achieved by direct shear tests.

On the other side, with urbanization, city blocks contain clusters of closely spaced buildings. Under such circumstances, the dynamic interaction of adjacent structures should not be ignored. However, available evidences show that in the field of soil-structure interaction, little attention has been payed to adjacent structures. In addition, the vast majority of studies are subjected to consider two-dimensional models with plain strain behavior assumptions. Such simplifying assumptions lead to obtain not so accurate and reliable achievements.
In this paper, the effects of soil structure interaction and adjacent structures interaction on the seismic response of structures through considering three-dimensional models by using OpenSees software were studied. In this regard, structures are divided in to two major groups, fixed base structures (structures resting on the rigid base) and flexible base structures (superstructures resting on the flexible base), whereas flexible base structures contains Soil-Structures and Structure-Soil-Structure systems. As respects, the common dynamic analysis focus on the fixed base assumptions; therefore, in this study, the evaluations and comparisons between results of flexible base structures and fixed base structures analysis have been paid attention to. Due to the modeling of superstructures for analysis and design processes, three reinforced concrete moment resisting frame, 5, 10 and 15 stories, two spans, resting on shallow foundations with different structural neighborhoods are selected in conjunction with a soil type III (according to ground type classification of Iranian building code), based on the direct method, considering of appropriate lateral boundaries and interface elements to simulate frictional contact and probable slip due to seismic excitation. Besides, for the design and analysis of superstructures according to the Iranian concrete code, gravity and lateral loads with considering Iranian national building code part 6 and Iranian seismic code 2800, respectively, is conducted by using ETABS software, then structural sections are designed according to Iranian national building code part 9.
Nonlinear dynamic analysis using OpenSees software under influence of three different earthquake records is conducted. The study of the response of acceleration, drift and shear force in the stories indicates that effects of soil-structure and structure-soil-structure interaction depend on dynamic characteristics of buildings, frequency content of seismic data and the height of adjacent structures. The results show that considering adjacent structures with common distance lead to increase or decrease about tens of percent of dynamic responses.
The results indicate that when a short building (5-story) has two adjacent close tall buildings (15-story), maximum responses of acceleration, drift and shear force increase up to 10, 20 and 40 percent, respectively. Results show that a 10-story building with two adjacent 5-story buildings, up to 24% decreases in acceleration response; while with two adjacent 15-story buildings increase in responses of drift and shear force, 20 and 16 percent, respectively. In tall buildings (15-story in this study) with the same height adjacent structures, the acceleration up to 15% and drift up to 23% decrease with two 10-story adjacent structures and shear force increases up to 35% with two 5-story adjacent structures. On the other hand accepting the results of common structural dynamic analysis lead to unsafely design for tall buildings, and for short structures are not affordable.

According to the extensive development of urban texture and the vital necessity of lifelines, infrastructure and underground openings have found an important role in human societies. A full understanding of the behaviors of underground tunnels including tunnels for transportation, water, and facilities, can assist in presenting an optimum layout. The importance of this issue has increased because of the complex performance of the tunnels against seismic loads. The seismic analysis of underground tunnels has been used by the researchers for almost half a century. Technically speaking, there are some analytical as well as semi-analytical methods for analyzing the ground response subjected to subsurface structures such as lined/unlined tunnels. Although these methods have high accuracy and used in the benchmark purposes, arbitrary shaped models with different boundary conditions cannot be easily applied in establishing the real problems. On the other hand, by developing the computers and software knowledge, the numerical approaches are frequently used for analyzing the continuum media in recent two decades, especially in soil mechanics. Based on the formulation, numerical approaches used in dynamic problems analysis can be usually divided into domain and boundary methods. In domain methods, such as finite element method (FEM) and finite difference method (FDM), it is required to discretize the whole body including internal parts and its boundaries. Although the simplicity of domain methods makes them favorite for analyzing seismic finite mediums, the models are complicated by discretizing the whole body and closing the boundaries in a distance far away from the interested zone for dynamic analysis of soil medium. In the response to the mentioned issues, the boundary element method (BEM) can be practically used for analyzing infinite/semi-infinite soil mediums in a better manner, due to discretizing only the boundaries of the problem. The BEM approaches can be formulated in the classes of full-plane (FBEM) and half-plane (HBEM) boundary element method.
In this study, a direct half-plane time-domain boundary element method (HBEM) was developed and successfully applied to analyze the transient response of ground surface in the presence of circular lined tunnels, embedded in a linear elastic half-plane, subjected to propagating incident plane SH-waves. The Fundamental solutions for the case of propagating SH-waves in single-medium environments in the presence of unlined tunnels and ground surface topographies have been developed by Panji et al. [30]. For Multi-medium problems such as lined tunnels embedded in deep/shallow soil, the use of single-medium fundamental solutions was expanded by considering substructure procedure such that the problem was decomposed into a pitted half-plane and a closed ring-shaped domain. To solve the model, only the interface and inner boundary of the lining need to be discretized. After computing the matrices and satisfying the compatibility as well as boundary conditions, the coupled equations were solved to obtain the boundary values. There are two types of boundary conditions on interface boundaries, the first of which is the equality of displacement between mediums, and the second one is the equality of normal and shear stresses along the interface. These two conditions must be considered when the boundary equation is formed. In addition, to boundary conditions it is required to apply free-field displacement effects on the displacement of boundary and internal points. Free-field displacement provides the effects of wave incident and reflectance when the wave arrives to boundary or internal nodes. For Multi-medium problems such as lined tunnels embedded in deep/shallow soil, the free-field displacement accomplishes in the same way that it was done for single-medium problems.
The mentioned method was successfully implemented in a developed computer code called DASBEM [30]. To validate the responses, a practical example was analyzed and compared with those of the published works. Manoogian [13] has obtained ground surface frequency domain responses for the case of embedded lined circular tunnel in shallow soil medium. By implementing the problem into the DASBEM, the results were obtained and compared with those of Manoogian [13]. The results showed that the analytical and numerical responses are in good agreement. Finally, in a parametric study, a circular lined tunnel was evaluated and the effect of depth and incident wave angle on ground surface response was analyzed. The results mainly showed that the ground surface frequency response due to tunnels without lining is more than that of lined tunnels. The method used in this paper is recommended to obtain the transient response of underground structures in combination with other numerical methods.

In bridge design codes such as AASHTO, there is not a direct and explicit relation for considering the effects of the earthquake vertical component in the design of elastomeric supports. In lieu, AASHTO recommends a 20% reduction or increase of the dead load amount in its seismic isolators design guide for considering the earthquake vertical component without taking into account the acceleration vertical component, soil type, distance to fault, etc. While the earthquake vertical component in near-fault areas are strong and significantly increase responses in the bridge. In this study, Sadr bridge located in northeast of Tehran in which low damping rubber seismic isolators are used, is investigated under the effect of the vertical component of some earthquake accelerograms with near-fault properties. Besides, the non-linear time history analysis is performed for each of them. Results show that the earthquake vertical component in near-fault areas will significantly increase the response of bridge members such as the increase of the maximum axial force of the bridge piers, deck acceleration, shear and bending moment in the deck cross section and is much higher than the 20% increase or decrease of the dead load given in the code. In order to investigate the seismic isolators behavior due to the increase of the axial compressive force under the effect of the earthquake vertical component, a simple model of isolators with the capability of considering the stiffness variations in the vertical and horizontal directions and performing the buckling stability analysis was prepared, and the isolators responses for three different lateral loadings including the application of monotonic transverse displacement, cyclic loading and earthquake loading were obtained. For all three lateral loadings, the results show that the increase of the compressive axial force decrease the isolators’ lateral stiffness and increasing the lateral displacement will increase the isolators’ axial deformation. Besides, the hysteresis curve of the isolators will experience a reduction in the stiffness and lateral strength. In the following, the buckling capacity amount obtained from the isolation model with the capability of buckling analysis is compared with the buckling capacity amount obtained from the conventional recommended relations. The results show that the conventional relations slightly overestimate the buckling capacity and the use of buckling analysis models is preferable. In order to decrease the effects of the earthquake vertical component in the isolated structures, three-dimensional isolation is recommended through simultaneous application of vertical and horizontal flexibility in the isolators. The results show that the decrease in bridge response such as the midspan vertical acceleration, deck shear and bending moment has a descending trend with the isolators’ vertical stiffness reduction.

There are various design and construction deficiencies that exist in the contemporary moment resisting frames in Iran. This, of course, greatly increases the vulnerability of such structures. The most common defects observed in a real structure are as follows: cracks in welds due to the lack of complete penetration in groove welds and/or incomplete fusion, use of latticed built-up sections instead of standard wide-flange sections as columns, and misplacement of continuity plates at the column-girder joints. These defects have a negative impact on the behavior of connections and hence resulting in an unpredicted structural response. The rate of impact of such defects on structures can be estimated in a probabilistic analysis of the behavior of these frames. In this study, to investigate the probabilistic behavior of constructed structures in Iran, two existing frames with three and five stories having the above-mentioned deficiencies are considered. The mean annual loss and failure probability of these frames are obtained and compared with that of similar structures with no such defects. At first, the moment-curvature of different elements of the defected structure is estimated by an analytical method. The standard and defected connections are modeled in finite element software, and the hysteresis behavior of these connections is estimated using the standard SAC loading protocol. Based on these results, the backbone curve of connections is determined and employed for modeling of the frames. Then, the non-linear probabilistic behavior of the frames are evaluated by performing nonlinear dynamic analysis, when the frames are subjected to several earthquake records of soil type 2 in far- and near-field pulses. Thereby, the seismic fragility function of frames is estimated by applying the results of the probabilistic analysis. Using the seismic hazard curve of the location of the structure, the probability of failure of the studied frame in different damage states is estimated. In addition, the mean annual loss of the frames is calculated and compared. The results indicate that the presence of defects in connections cause a decrease in yielding and ultimate moment capacity of connections by 34 and 17.1 percent, respectively. Moreover, in three and five-story frames, the failure probabilities of the defected frames are 1.7 and 4.12 times of perfect frames, correspondingly. Similarly, the mean annual loss of defected three- and five-story frames are 1.76 and 2.36 times of perfect frames, respectively. It can be concluded that, first, the safety of constructed structures with defects in connections is significantly lower than that of code’s ideal frames. Second, the mean annual loss of defected frame is more than two times that of code’s complied frames; this demonstrates the significant vulnerability of existing structures. Third, the effect of deficiency on the reliability of mid-rise frames is higher than that of low-rise frames. This higher vulnerability of mid-rise frame indicates that particular attention should be paid to the construction of such structures. Besides, results have shown that the reliability of constructed steel frames with deficiencies is significantly lower than that of frames, which are constructed according to the requirements of the code, especially for high-rise buildings.

The pore water pressure increasement in saturated sands is a consequence of cyclic shear stresses induced by earthquake loads. Following this change, the shear strength of soil rapidly decreases and liquefaction of soil may be occurred. Most types of failure associated with the liquefaction phenomena are: sand boil, flow failure of slopes, ground oscillation, loss of bearing capacity, ground settlement, and lateral spreading.
Liquefaction-induced lateral spreading is one of the most important factors of major damage to Ground and underground structures during earthquakes. This type of permanent ground displacement, which has amplitudes ranging from a few centimeters to 10 meters and more, has caused substantial damages to lifelines and pile-foundations of buildings and bridge piers along the past earthquakes.
Although the mechanism of soil liquefaction is well recognized, the prediction of liquefaction-induced horizontal displacement is associated with the complexity and difficulty, due to the involvement of multiplex parameters. Several researches have been done to develop techniques for lateral ground displacement prediction. These techniques can be divided into four classes, including simplified analytical, numerical, empirical, and artificial neural network methods. Since the simplified analytical methods consider the shear strength of soil unchanged during an earthquake, these methods may not provide an accurate estimate of lateral displacements caused by liquefaction of soil. Besides, due to the complexities related to the accurate modeling and the difficulties in measuring the in-situ parameters of soil layers, it is obvious that the consideration of some simplifications in numerical methods is required, which may reduce their capabilities. Due to the limitations related to analytical and numerical methods, many researchers developed empirical models based on ground displacement records. Empirical methods detect the relationship between in-situ displacements and various effective parameters by regression method. It is believed that ANN models compared to the conventional regression methods can predict complex problems, such as liquefaction-induced lateral spreading more accurately.
In the present study, a new model with the ability to estimate the lateral displacement caused by liquefaction has been developed using the Group Method of Data Handling (GMDH) type neural networks. In this method, complicated relationships are developed according to their efficiency against a series of multi-input single-output data pairs. GMDH algorithms present a tool to find the appropriate relationship between data, recognize the optimal structure of the network, and improvement in accuracy of existing algorithms. In general, the GMDH type neural network includes certain advantages compared to other types of neural networks. In particular, it has the ability to find and select the most suitable input variables from a set of variables. By sorting different solutions, GMDH networks minimize the influence of the user on the structure and results of modeling. The computer automatically finds the optimal structure of the model and the laws acting on the system.
In this study, a comprehensive database containing 526 case histories and recorded over 18 major earthquakes was utilized to correlate the liquefaction-induced lateral spreading with the most effective parameters. Since the presented model has been developed based on numerous earthquakes and site conditions, it is more general and reliable than previous models. The obtained results indicate that the GMDH model has the ability to predict the
lateral spreading with a high degree of accuracy. In order to validate the new proposed model, the displacements obtained by 28 centrifuge tests were compared with the results of the GA-GMDH model. The comparison showed a high degree of accuracy of the new GA-GMDH model, indicating a good predictive capability of the model, even at small ground displacements. Moreover, comparing the performance of the model developed in the present study with experimental results in literature shows the accuracy of predicted values by the new model.