مجله علوم و مهندسی زلزله
سال دوم شماره 3 (پیاپی 4، پاییز 1394)

During an earthquake, the dynamic response of a structure located on a soil deposit could be very complex compared with the analysis of the same structure on bedrock due to the interactions between the soil and the structure. This phenomenon is technically termed as Soil-Structure Interaction (SSI) effects in literature. Most of what is currently known about soil-structure interaction (SSI) is based on theoretical and mathematical models. Therefore, it is necessary to investigate the structure treatments when they response to the ground strong motions transferred by SSI. In this regard, in an experimental field, the present study investigated the SSI effects on structure, evaluating the natural frequencies and damping of a pile-group-supported pier of Kermanshah’s LRT. The frequencies and damping evaluations were performed through ambient vibration test results and system identification procedures. The purpose of system identification is to evaluate unknown properties of a system, using known inputs and outputs. There are two principal system identification procedures to build mathematical models of dynamical systems from measured data: (a) non-parametric and (b) parametric procedures. Nonparametric procedures evaluate complex-valued transmissibility functions from the input and output recordings without fitting an underlying model. Accordingly, Fourier Transform (FT), response square of transfer function, peak picking and four spectra are considered as non-parametric procedures. On the other hand, parametric procedures develop numerical models of transfer functions. More precisely, in these procedures, a mathematical model with several parameters is defined first. The considered parameters are featured with specific values determined by experimental results. Then, the system’s input-output function is obtained using this described model. The studied pier was fully instrumented with two SARA and a CEM seismometers. The seismometers recorded signals of two horizontal and a vertical components that were digitally recorded at 200 Hz sampling rate. In general, measure of SSI effects was then obtained by comparing the flexible base and fixed-base parameters to calculate the two most important effects of SSI, period lengthening and foundation damping. SAP 2000 was used to create a finite element model of the whole structure and the accuracy of the model was tested using recorded data from ambient vibration at the structure site. In summary, the current study indicates that all the utilized system identification methods are appropriate in determining the dynamic characteristics of the structure in fixed condition. In addition, it was demonstrated that peak picking and four spectral methods did not have appropriate function in investigating the interaction. However, these two procedures have appropriate function in determining the dynamic characteristics of the structure in fixed condition. As Figure (1) shows, the diagram of the period lengthening obtained from parametric method with the ratio of the structure-to-soil stiffness for the pier is approximately consistent with system identification analyses performed for the 57 sites in Stewart et al. [1]. Accordingly, inertial interaction effects were generally observed to be small for 1/σ < 0.1 and for practical purposes could be neglected in such cases.

In the current study, a series of 1 g shaking table tests was performed to study the Tehran subway tunnel effect on the ground surface acceleration response. Two reduced-scale 1 g shaking table models, designated as FF and SF, were constructed in 1/32 scale. The FF was constructed to study the seismic response of the soil layer in free field condition, while the SF model includes a subway tunnel to study its effect on the acceleration response of nearby ground. In prototype scale, the subway tunnel with 8 m diameter and 0.35 m thickness was embedded in a soil layer with 32 m thickness. The soil was dense sand with 70% of relative density. The models were constructed in a rigid box made from Plexi-glass with dimensions of 178*80*120 cm (L.H.W.). Lateral boundaries of the models were covered with conventional foam in order to reduce the lateral boundary effect on the seismic response of the soil layer. The constructed SF model is depicted in Figure (1a). The accelerometers and LVDT transducers installed in the models to record the acceleration in the soil and settlement at model surface are illustrated in Figure (1b). The experimental study revealed that the tunnel does not affect the incident waves with dimensionless period (λ/D) larger than 10. Previous numerical studies [1-2] also demonstrated that an underground tunnel does not affect the free field response at λ/D greater than 10. Up to now, this finding has not been demonstrated by any experimental research. However, the physical modeling performed here is suffered from some limitations regarding the applied frequencies. Therefore, a numerical model was developed based on the results of the shaking table tests, and the effect of the tunnel on the excitations with higher frequency ranges was investigated. Besides, the effect of different parameters such as shear wave velocity of the soil, flexibility ratio and depth of the tunnel on the acceleration at the ground surface was numerically determined. Figure (2) shows the numerical model of the SF model in prototype scale. Six real earthquakemotions that were matched to the response spectrum of ground type I in Standard 2800 were used in the parametric analyses. PGA of the motions was scaled to 0.35 g. The push of the amplification during the analyses were considered as the maximum response and depicted as amplification pattern at the ground surface. Amplification pattern at the ground surface for a tunnel at h/a=1.5 in soils with different shear wave velocities (VS) is depicted in Figure (3). As presented, the maximum amplification occurred at X/a = 1.5 for all Vs. Moreover, as the shear wave velocity increases, the amplification ratio decreases. The study revealed that the amount of the amplification on the ground surface depends on the tunnel depth and shear wave velocity of the soil. The maximum amplifications at the ground surface was equal to 5, 8 and 10 percent for the tunnel depth rations of 1.5, 2 and 3, respectively, in a soil medium with 175 m/s of the shear wave velocity. The effect of tunnel depth on the amplification pattern was investigated in the parametric study. It was concluded that as the tunnel depth increases, the amplification ratio decreases. The tunnel depth affects the location of the maximumamplifications. As the tunnel depth increases, the location of the maximum amplification gets away from the tunnel center and occurs at longer distance from the tunnel center. The effect of the Tehran subway tunnel on the response spectrum at the ground surface in different soil for different ratios of the tunnel depth was investigated. It was concluded that the subway tunnel in soils with different shear wave velocity affects the different ranges of the periods. A subway tunnel with 8 m diameter influences the seismic response of the buildings with the period lower than 0.4 sec or the buildings smaller than 10 m. It means that the tunnel has an adverse effect on the short buildings.

Experimental and theoretical investigations indicate that the seismic bearing capacity of foundations is affected by earthquake excitation. In the present study, an analytical procedure is presented to obtain the seismic bearing capacity factor of shallow strip footing NγE for a foundation under inclined load on cohesionless soils. The limit equilibrium method with numerical iteration technique is utilized to calculate the seismic bearing capacity factor NγE. In the proposed analysis the Kötter’s equation and a failure surface consisting log-spiral and planar surface are employed. The results indicate that the seismic bearing capacity is reduced due to an increase in horizontal coefficient of earthquake acceleration. Besides, the results are in good agreement with solutions available in the literature. The failure pattern (Figure 1) is considered based on Budhu et al. work [1] with the difference that the pole of the log spiral is not fixed and varies with earthquake acceleration, friction angle, geometry etc. The failure surface has two passive parts, the log spiral of CD and DE. To obtain the distribution of soil reaction pressure for each of these two parts, the Kötter’s equation is employed. Asymmetrical elastic wedge with full mobilization of the passive resistance on one side (BC) of the footing and partial mobilization on the other side (AC) of the footing is assumed. In Figure (2), from the horizontal and vertical equilibrium. where QuE represents the ultimate seismic bearing capacity of the foundations, β is the inclination angle (tanβ= horizontal load on the foundation/QuE), Ws is the weight of the triangular soil wedge ABC, φ is friction angle, pmpγE and pmpγE represent the seismic passive thrust and mobilized seismic passive thrust, m and φm denotes the mobilization factor and mobilized friction angle: mtan j = tan jm (3) In the above equations, m and φm are unknown that can be determined by the trial and error. The trial and error continues until two calculated values for QUE from Eqs. (1) and (2) are approximately equal. Considering 2B, footing width: Figure (3) clearly indicates that the seismic bearing capacity factor NγE is reduced with an increase in the inclination angle. The comparison of the developed seismic bearing capacity coefficients, NγE with those obtained from the other methods for φ = 30º are presented in Figure (4). It is observed that there is a good agreement between the NγE values of the proposed method and those reported by other researchers.

During earthquakes, piles undergo stresses due both motion of the superstructure (i.e. inertial interaction) and that of the surrounding soil (i.e. kinematic interaction). In practice, structural engineers commonly take into account stresses induced by the inertial interaction, which is responsible for pile head failure, but they neglect the effects of the kinematic interaction that is responsible for failures along pile’s length in the case of layered soils with highly contrasting mechanical characteristics even in the absence of the superstructure. Thus, the evaluation of kinematic forces developing in piles during earthquakes has been receiving increased interest from the researchers. Numerical methods for the analysis of kinematic soil-pile interaction can be classified into two groups; continuum-based approaches and Winkler methods [1-3]. It has been customary in professional engineering and research practices to assume a linear behaviour for the soil and the pile foundation. However, under strong excitation, the nonlinear behaviour of soil media at the soil-pile interface has a strong influence on the response of the pile foundation. The aim of this study is to investigate the influence of soil nonlinearities on the kinematic interaction forces of pile groups embedded in layered soil deposits during seismic actions. Figure (1) shows the assumed soil-pile group case with 5 by 5 piles embedded in two layer subsoil profile. The pile has been considered as an elastic beam, while the soils have been modelled using the elastic-plastic solid element. The corresponding 3D finite element mesh has been shown in Figure (2). Based on the symmetry, only half of the model is meshed. Dynamic numerical analysis has been performed using the FE program ABAQUS [4]. Necessary parameters to simulate the examined cases are listed in Table (1). It is worth noticing that the comparison with availableexperimental and theoretical results in the literature has been made to validate the numerical model. The maximum displacements and the envelopes of kinematic bending moments and axial forces along the piles depth have been reported in Figure (3) due to the 1978 Tabas, Iran earthquake ground motion at the Dayhook station. As shown, it can be observed that the kinematic force distributions present relative maximum values very close to the layer interface. On the other hand, the diagram of the maximum displacements is characterized by a shape very similar to the first vibration mode of the soil deposit with maximum value at the piles head and almost zero value near the bedrock. In addition, the effects of kinematic group interaction lead to a decrease of bending moments at the pile head and also the layer interface as compared to the results from the single pile. Finally, the influence of main parameters governing the seismic response of piles like the space-diameter ratio, number of piles in the group, pile dimater, pile-to-cap fixity condition and the variation of soil layers properties are discussed.

In this study, the role of input motion characteristics on the amount of residual settlement of shallow foundations relied on liquefiable soil is investigated. First, two-dimensional numerical model is created using finite difference software FLAC-2D, and the UBCSAND constitutive model. After verification of the numerical model with the results of a centrifuge test, behavior of foundation is investigated under different earthquake loadings. To evaluate the effect of the input motion characteristics on foundation settlement, three records from Irpinia 1980, Northridge 1994, and Landers 1992 earthquakes are used. The frequency contents of these records are considerably different. These records are scaled to five levels of PGA (including 0.05g, 0.1g, 0.2g, 0.3g, 0.45g) to be used in the analyses. The results of the analyses demonstrate that the amounts of foundation settlement for the same PGA in three records are considerably different. Therefore, the use of PGA alone might be insufficient to predict the amount of foundation settlements. This fact is shown in Figure (1a). Figure (1b) illustrates the amounts of foundation settlement against the mean period of records at different PGA levels. It is observed that the mean period of records which indicates their frequency content, has a significant influence on the values of foundation settlements. It is concluded from the numerical analyses that the seismic parameters associated with ground velocity have better correlation with settlements of foundation. For example, the amounts of foundation settlements against the peak ground velocity (PGV) of the input motion are shown in Figure (2). This result reveals a high correlation between the ground velocity and the residual settlement of foundation, irrespective of frequency content and PGA of the earthquake records. It can be concluded from the results that both PGA and mean period of input motions have significant influence on the amounts of the foundation settlement, but they cannot be a good criteria for evaluating the amounts of the foundation settlements alone. In contrast, velocity-type parameters that are called intermediate-frequency intensity measures can be appropriate for prediction of foundations settlement in liquefaction condition. These results have been achieved for conditions of the calibrated foundation with its certain inherent parameters such as natural frequency content. For other conditions such as larger foundations’ width or sand density, more detailed studies are required.

Progressive collapse starts with local destruction of a few elements of structures, which extends into a significant part of the building. Regular buildings are designed for dead, live, wind, earthquake and other normal loads. Nevertheless, there are other possible risks and loads, including firing, vehicle collision, gas explosion, design or construction error, bomb blast, etc. These risks occur very rarely, but they may cause a catastrophic collapse; therefore, for very important structures, they should be considered in the designing phase [1]. Infills have a considerable improvement in the stiffness and the strength of the frame. Therefore, their influences on progressive collapse of buildings should be considered. Considering such elements in structuralmodelling is very complicated, that’s why many standards and codes ignore their local and global effects and just consider their effects in decreasing the building natural period of vibration [2]. However, they are considered in rehabilitation projects and should be considered in progressive collapse analyses [3-4]. Many methods have already been proposed to model solid infills in the structure; the most common approach is modelling by diagonal struts [4, 5]. For infills with openings, there is not a verified approach for the modelling. This study is to propose a method for modelling infills with and without opening and investigating their effects on progressive collapse of buildings. Perforated infills are modelled by the equivalent struts, considering the influence of the opening as a reduction factor for the width of the strut, verified in previous studies [6]. The proposed model is verified by the results of an experimental study on San Diego Hotel, obtained by Sasani [3]. The plan of the hotel is shown in Figure (1). The hotel had reinforced concrete structure and was destructed by explosion in two columns (A2 and A3 in Figure 1) of the first story. Opensees is applied to model the structures; “Nonlinear Beam Column Elements” with Fiber section is used for modelling the beams and columns. Concrete01 and Concrete02 are used for non confined and confined concrete of the sections, respectively. “Elastic Beam Column Element” is applied to model infill equivalent strut. Comparing the results of the modelling with the experimentally obtained values, for column forces (before and after columns explosion) as well as the displacement history of the top point in the removed columns shows robustness of the modelling, which is more accurate than Sasani models, as shown in Figure (2). To study the influence of infills on progressive collapse, the structure of the hotel is modelled and the proposed scenarios of GSA [1] are investigated. As shown in Table (1) for all scenarios, the presence of infill panels decreases the vertical displacement of the removed columns. This shows that infills improve the building against progressive collapse considerably.

During the last 20 years, part of the research work has focused on the seismic analysis of Liquefied Natural Gas (LNG) tanks, due mainly to (1) the increasing number of LNG tanks constructed in seismically active regions, resulting from the adoption of LNG as an environmentally friendly fossil fuel, and 2) the catastrophic environmental impact, associated with a potential local or total failure of such tanks, caused by the earthquake motion. Seismic isolation is a well-known method to mitigate the earthquake effects on the structures by increasing their fundamental natural periods at the expense of larger displacements in the structural system. In this study, the seismic response of isolated and fixed base vertical, cylindrical, liquid storage tanks is investigated using a numerical model, taking into account the fluid-structure interaction effects. The numerical model is validated by the comparison of its results with the experimental measurements of small-scale tank under harmonic and seismic excitations. The comparison reveals that the use of the considered model provides enough accuracy for evaluating the seismic behavior of nonlinear isolated and non-isolated tanks. Three vertical, cylindrical tanks with different ratios of height to radius (H/R = 2.6, 1.0 and 0.3 as the representatives of slender, medium and broad tanks) are analyzed and the results of response-history analysis, including base shear, overturning moment and free surface displacement are reported for isolated and non-isolated tanks. The isolated tanks are equipped with lead rubber bearings isolators, and the bearings are modeled by using a non-linear spring in numerical model. Long period ground motion is the main parameter that can significantly affect the seismic response of isolated tank. It is observed that the seismic isolation of liquid storage tanks is quite effective and the response of isolated tanks is significantly influenced by the system parameters such as their fundamental frequencies and the aspect ratio of the tanks. The average reductions of base shear forces of isolated tanks are 71%, 70% and 50% for broad, medium and slender isolated tanks. It seems that the effectiveness of base isolation system to mitigate the base shear force is not significantly affected by changing of tank aspect ratio. In terms of overturning moment, the average reductions of the order of 71%, 69% and 47% for broad, medium and slender tanks is obtained due to applying of isolation system. Therefore, overturning moment is considerably mitigated by the reduction of the tank aspect ratio. The effectiveness of base isolation considerably reduces for exerted earthquake records including long period motion. Especially for slender tanks, base isolation may even increase the overturning moment. However, the base isolation does not significantly affect the surface wave height, and even it can cause adverse effects on the free surface sloshing motion. The results of free surface displacement for both isolated and nonisolated tanks have quite similar trends for considered tanks. The errors between the maximum sloshing wave height of fixed base and isolated tank are less than 8% for most of the considered cases. Even, the sloshing height is slightly amplified in some cases. Therefore, the base isolation system can cause adverse effects on the free surface sloshing motion. It can be concluded that the effectiveness of the base isolation method is very sensitive to the physical and geometrical parameters of the considered tanks. This suggests that a careful selection of isolators with a certain limit on the mechanical properties of the isolators is required for the optimal seismic isolation design of liquid storage tanks.