فهرست مطالب

  • Volume:2 Issue:2, 2012
  • تاریخ انتشار: 1391/06/18
  • تعداد عناوین: 7
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  • A.M. Kaynia Page 127
    A frequency-wavenumber-domain formulation is presented in this paper for calculation of the Green's functions and wave propagation modes in a stratified fluid body underlain by a layered viscoelastic soil medium. The Green's functions define the solid and fluid displacements and fluid pressures due to uniform disk loads acting in either the soil or fluid media. The solution is in the frequency domain and is based on a Fourier expansion of the displacements in the azimuthal direction together with Hankel transform in the radial direction and analytical solutions in the vertical direction. The presented formulation uses the notion of layer stiffness matrices for the solid and fluid layers. Derivation of the expressions for the fluid layer matrices is presented, and a simple procedure for numerical integration of the Green’s functions is proposed. Two examples of the use of the presented model are presented. The first example deals with computation of the dynamic impedances of seabed foundations, and the second example concerns computation of fundamental modes of surface waves at the solid-fluid interface. This mode is often used in characterization of the seabed material from the direct seismo-acoustic measurements underwater.
    Keywords: Wave propagation, Offshore, Viscoelastic media, Dynamic impedances, Green's functions
  • A. Shahjouei, G. Ghodrati Amiri Page 145
    Through the last three decades different seismological and engineering approaches for the generation of artificial earthquakes have been proposed. Selection of an appropriate method for the generation of applicable artificial earthquake accelerograms (AEAs) has been a challenging subject in the time history analysis of the structures in the case of the absence of sufficient recorded accelerograms. In this paper we have spotlighted the application of the evolutionary algorithms in the AEAs generation approaches. In this regard, we have statistically apprised the two novel methods; the genetic algorithm based and the hybrid evolutionary neural network-based methods. The main feature of this paper is to provide some statistical information of the two proposed methods to make some quantitative criteria for assessing the future models and algorithms. The assessment is performed based on three major functions of the spectrumcompatibility, the stochastic diversity of generated seismographs and the computational efforts. The results demonstrate the practical advantages of the evolutionary algorithms in this context.
    Keywords: Artificial earthquake accelerograms, Genetic algorithm, Response spectrum, Artificial neural network
  • A. Eslami, S. Salehi Malekshah Page 153
    For the optimum design of Piled Raft Foundations (PRF), the concept of settlement reducer piles has been applied. However, when these piles are connected to the raft, a high stress may develop in the relatively small number of piles. Therefore, an alternative approach is to disconnect the piles from the raft forming a new system called a non-connected or disconnected piled-raft foundation (NCPRF or DCPRF). To facilitate load transfer mechanism from the raft to the soft to medium subsoil, a cushion, which is a compacted coarse grained soil layer beneath the raft, has been used. In this study, three dimensional finite element analyses via ABAQUS software is performed and applied to a few PRF cases including those reported in the literature and some cases of monitored and instrumented high rise buildings. The analysis consists of an investigation on factors such as: pile disconnection, cushion, raft thickness, and pile arrangement on the performance of NCPRF. Results indicate that when the non connected piles are used, the maximum axial stress decreases in the piles and the neutral plane lowers down its location. Cushion height and stiffness have a major influence on the stress ratio of piles to the total external load applied to the mat. Also, increasing the raft thickness and concentrating longer piles in the central raft area affect significantly on the reduction of settlements particularly the differential settlement.
    Keywords: Non, connected piled raft foundations, Cushion, Finite element method
  • F. Khoshnoudian, S. Mestri, F. Abedinik Page 169
    The proposal lateral load pattern for pushover analysis is given in two forms for symmetric concrete buildings: 1-(X/H)0.5 for low-rise and mid-rise buildings, 2- Sin(ΠX/H) for high-rise buildings. These two forms give more realistic results as compared to conventional load patterns such as triangular and uniform load patterns. The assumed buildings of 4, 8, 12, 16, 20 and 30 story concrete buildings are special moment frame which have been designed according to 2800 standard. Then using conventional load patterns and proposal load patterns, the pushover analysis has been done and results have been compared with the outcomes of nonlinear time history analysis. Results show the accuracy of proposed load pattern in comparing to the load patterns proposed by standards such as FEMA356.
    Keywords: Pushover analysis, Performance, based seismic design, Lateral load pattern, Nonlinear time history analysis
  • M.M. Eslami, A. Aminikhah, M.M. Ahmadi Page 185
    Study on the seismic behavior of piled rafts and pile groups while the same amount of construction material and excavation is used in their construction, are the main objective of this research. The process where the raft interaction with soil can affect the seismic response and stress distribution is also discussed in the current study. By means, ABAQUS software was applied for the finite element modeling. Firstly, for calibration and verification of the procedure of modeling with two other experimental studies, results were compared by the analysis of models under single frequency sinusoidal dynamic loads. Moreover, analyses of the same models were applied under the acceleration time history of the El Centro earthquake. Results indicate that the participation of the raft interaction with soil under seismic loading, well improves the seismic response and behavior of a piled foundation systems i.e. internal moments and shear forces as well as deflection. Reductions of maximum acceleration response, horizontal displacement and bending moment in the piled raft system, are some important gained results by this study. In addition, the more uniform stress distribution in soil deposit and prevention of stress concentration leads to the reduction of the piled rafts settlement during earthquakes.
    Keywords: Piled raft foundations, Finite element modeling, Earthquake loading, Seismic response
  • S.A. Sadrnejad Page 201
    A few of the presented soil behavior models are capable of predicting the triggering and post liquefaction and also shear band mechanism through the soil media. The assessment of earth-dam body behavior as a soil structure made of cohesive soil in core and non-cohesive soil as the core supports, including water interaction at upstream through earthquake upon a multi-line constitutive equations is the aim of this paper. A multi-plane mechanism-based approach is successfully employed for assigning postliquefaction displacement of earth-dam structures. This approach is derived from total stress procedures with two major advantages: 1) the triggering and post liquefaction response have been multi-lined into one analysis, and 2) the modeling of post liquefaction behavior is greatly improved. Analyses are performed in the time domain, allowing the imposed earthquake motion to affect both the triggering and postliquefaction deformations. This Multi-plane based framework is employed to sum up the strength effects on integrated sampling planes and the resultant of this simulated multi-lined behavior is implemented at each finite element Gauss point. This multi-plane based model is also capable of predicting the effects of both induced and inherent anisotropy plus the rotation of principal stress/strain axes through the plastic behavior of both cohesive and non-cohesive soils. The approach is presented through the simulated of the case history as the response of the lower San Fernando dam to the 1971 San Fernando earthquake. The magnitude and pattern of the predicted displacements are shown to be in good agreement with the measured values.
    Keywords: Multi, line model, Liquefaction, Earth, dam
  • Arjangpay, M. Darvizeh, R. Ansari, Gh. Zarepour Page 219
    In this paper the meshless local Petrov-Galerkin (MLPG) method is implemented to study the buckling of isotropic cylindrical shells under axial load. Displacement field equations, based on Donnell and first order shear deformation theory, are taken into consideration. The set of governing equations of motion are numerically solved by the MLPG method in which according to a semi-inverse method, a new variational trial-functional is constructed to derive the stiffness matrices and critical buckling loads are obtained in various boundary conditions. The moving least squares interpolation is employed to construct both trial and test functions. The present method is a truly meshless method based on a number of randomly located nodes upon which no global background integration mesh is needed and no element matrix assembly is required. In the present MLPG formulation, a local variational form is constructed over a local sub-domain instead of using the conventional weighted-residual procedure. The influences of some commonly used boundary conditions and effects of shell geometrical parameters are studied. The results show the convergence characteristics and accuracy of the mentioned method.
    Keywords: Axial buckling, Isotropic cylindrical shells, MLPG method