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Showing 89 results for Finite Element

H.r. Ghafouri,
Volume 1, Issue 1 (9-2003)

A two-dimensional mathematical model for the prediction of time-variations of river-bend displacements was developed which is particularly applicable to meandering rivers. The computational procedure consists of two stages, that is , in the first stage by utilizing depth-averaged continuity and momentum equations, velocity field as well as water surface profile in a river is determined. The well-known Finite-Element technique was applied to the governing equations. In the second stage the rate of river bank erosion is computed in terms of determined depths and velocities. The model utilizes Odgaards (1989) bank-erosion model in this stage. The procedure is then performed repeatedly over the entire time span in a staggered manner. The developed model was applied to simulate the migration of Qezel Ozan river. The fairly good match obtained indicates the applicability of the model.
Asghari K.,
Volume 1, Issue 2 (12-2003)

The solution of shallow-water equation for a two-dimensional .simulation of overland /low for an actual watershed, is presented. The Petorv-Galerkin weighted residual method is used to overcome spurious oscillations inflow depth. For modeling overland flow over complex topography and variable surface, ct pre and postprocessor was developed to utilize the Triangulated Irregular Network (TIN) model and to generate finite element mesh. Close agreement of the model with measured data is obtained. This model can be used to better analyze the influence of varying surface roughness and topography on overland flow characteristics, including distribution of flow depth and velocity (is well as resulting hydrographs. Detailed spatial and temporal output parameters provide a basis fur further study of the patterns of soil erosion and evaluation of runoff natural slopes.
Asghari K.,
Volume 1, Issue 2 (12-2003)

The solution of shallow-water equation for a two-dimensional .simulation of overland /low for an actual watershed, is presented. The Petorv-Galerkin weighted residual method is used to overcome spurious oscillations inflow depth. For modeling overland flow over complex topography and variable surface, ct pre and postprocessor was developed to utilize the Triangulated Irregular Network (TIN) model and to generate finite element mesh. Close agreement of the model with measured data is obtained. This model can be used to better analyze the influence of varying surface roughness and topography on overland flow characteristics, including distribution of flow depth and velocity (is well as resulting hydrographs. Detailed spatial and temporal output parameters provide a basis fur further study of the patterns of soil erosion and evaluation of runoff natural slopes.
Afshar M.h.,
Volume 1, Issue 2 (12-2003)

A least squares finite element method for the .solution of steady incompressible Navier Stokes equations is presented. The Navier-.Stocks equation is first recast into a system of first order partial differential equations with the velocitv. pressure and the vorticity as the main variables. Finite element discretization of the domain introduces a residual in the governing equation which is subsequently minimized in a least squares sense. The method so developed clearly. falls into the minimization category card hence circumventing the L.B.B. condition. Furthermore. the method produces symmetric positive definite matrices which makes the way for using more efficient iterative sobers. A Conjugate Gradient algorithm is, therefore, used for the solution of the resulting .system of linear algebraic equations. To improve the efficiency , of this iterative solver an incomplete Cholesky factorization of the stiffness matrix is used as ct pre-conditioner. Since the storage requirement of the Cholesky factor depends on the bandwidth of matrix. an effective algorithm for the reduction of this bandwidth has also been employed. The application of the method to solve cavity problem and .step flow with different Remolds number is presented to show the applicability of the method to solve practical flows of incompressible fluid The use of both linear and quadratic elements with selective reduced integration is also investigated and the results are presented.
Kheyr Aldin A., Mortezaei A.r.,
Volume 2, Issue 1 (3-2004)

Structural walls are used extensively in moderate- and high-rise buildings to resist lateral loads induced by earthquakes. The seismic performance of many buildings is, therefore, closely linked to the behavior of the reinforced concrete walls. The analytical models used in this paper are developed to study the push-over response of T-shaped reinforced concrete walls andinvestigate the influence of the flange walls on laterally loaded walls and nonlinear behavior of shear walls, namely strength, ductility and failure mechanisms. A layered nonlinear finite element method is used to study the behavior of T-shaped and rectangular (barbell) shear walls. This paper introduces a computer program to practically study three-dimensional characteristics of reinforced concrete wall response by utilizing layered modeling. The program is first verified bysimulated and reported experimental response of 3-D reinforced concrete shear walls. Subsequently, a study considering eighteen analytical test specimens of T-shaped and barbell shear walls is carried out. Finally, based on analytical results, a new equation for minimum ratio of shear wall area to floor-plan area is proposed.
Kimiaei M., Shayanfar M.a., Hesham Ei Naggar M., Agha Kouchak A.a.,
Volume 2, Issue 2 (6-2004)

The seismic response of pile-supported offshore structures is strongly affected by the nonlinear behavior of the supporting piles. Nonlinear response of piles is the most important source of potentially nonlinear dynamic response of offshore platforms due to earthquake excitations. It is often necessary to perform dynamic analysis of offshore platforms that accountsfor soil nonlinearity, discontinuity condition at pile soil interfaces, energy dissipation through soil radiation damping and structural non linear behaviors of piles.In this paper, an attempt is made to develop an inexpensive and practical procedure compatible with readily available structural analysis software for estimating the lateral response of flexible piles embedded in layered soil deposits subjected to seismic loading. In the proposed model a BNWF (Beam on Nonlinear Winkler Foundation) approach is used consisting of simple nonlinear springs, dash pots and contact elements. Gapping and caving-in conditions at the pile-soil interfaces are also considered using special interface elements. This model was incorporated into a Finite Element program (ANSYS), which was used to compute the response of laterally excited piles. A linear approach was used for seismic free field ground motion analysis. The computed responses compared well with the Centrifuge test results.This paper deals with the effects of free field ground motion analysis on seismic non linear behavior of embedded piles. Different parts of a BNWF (Beam on Nonlinear Winkler Foundation) model, together with quantitative and qualitative findings and conclusions for dynamic nonlinear response of offshore piles, are discussed and addressed in detail. The proposed BNWF model (only using the existing features of the available general finite element software) could easily be implemented in a more comprehensive model of nonlinear seismic response analysis of pile supported offshore platforms.
Khalou A.r., Ghara Chour Lou A.,
Volume 3, Issue 1 (3-2005)

This paper presents the results of analytical studies concerning the flexuralstrengthening of reinforced concrete beams by external bonding of high-strength lightweightcarbon fiber reinforced plastic (CFRP) plates to tension face of the beam. Three groups of beamswere tested analytically and compared with existing experimental results. Results of the numericalanalyses showed that, although addition of CFRP plates to the tension face of the beam increasesthe strength, it decreases the beam ductility. Finite element modeling of fifteen different beams in aparametric study indicates that steel area ratio, CFRP thickness, CFRP ultimate strength andelastic modulus considerably influence the level of strengthening and ductility.
Sabouri Ghomi S., Kharazi M.h.k., Asghari A., Javidan P.,
Volume 3, Issue 1 (3-2005)

Design and construction of efficient and economic Reinforced Concrete (R.C.) Hyperbolic Cooling Towers have driven the engineers toward the design of tall and thin-shell towers which have considerable high slenderness aspect ratio. Consequently, the shell of R.C. Cooling Towers with relative high slenderness aspect ratio is extremely prone to buckling instability due to wind loading. To increase the structural stability or buckling safety factor, one economic approach is to design and construct stiffening rings for the R.C. Hyperbolic Cooling Towers. Despite the research previously performed to determine the effect of stiffening rings on the buckling behavior of the R.C. Hyperbolic Cooling Towers, information resulting in maximum buckling stability is absent considering the optimized utilization of the quantity and dimension as well as the location of this type of stiffeners. In this paper, not only the effect of the stiffening rings on the buckling stability of the R.C. Cooling Tower is studied but also the optimized location,quantity and dimension of the stiffening rings are carried out for a sample RC Cooling Tower. The dimensions of the selected sample cooling tower are in average typical dimensions which are used in the current practice. In this study, finite element (F. E.) analyses has been carried out to define the buckling modes and resistance of this tower due to wind loading for different number of stiffening ring configurations. Based on the conducted buckling analysis, the optimized number, location and dimension of the stiffening rings that maximizes the tower.s buckling stability are defined and the methodology to achieve this information is discussed in this paper.
M.h. Bagheripour, S.m. Marandi,
Volume 3, Issue 2 (6-2005)

Arational approach is introduced for numerical modeling of unbounded soil foundations based on coupled dynamic periodic infinite and conventional finite elements (IFE-FE). The model can be applied for analysis of various dynamic problems in geomechanics, especially in Soil Structure Interaction (SSI), where determination of stiffness properties and response of unbounded soil domains are of prime importance. In numerical SSI analysis, there exists important problems a) the discretization of natural soil foundation, especially defining the boundaries to prevent reflecting body waves and avoiding spurious results, b) the definition of the matrices related to the soils impedance functions which are essentially dependant on the excitation frequency, c) the evaluation of free field motion of the natural foundation, especially those of irregular geometry and material diversity. An efficient way and integrated solution to these important problems is found to be the use of periodic infinite elements. The wave equation of motion is derived numerically for discretization of the soil domain. Shape functions and mapping coordinates for dynamic periodic infinite elements are presented in this paper. The accuracy of the IFE is examined for the evaluation of free field motion of a visco-elastic soil foundation. Derivation of impedance function is shown and leads to the determination of dynamic stiffness characteristics of the unbounded soil medium including spring and dashpot coefficients. Application of the approach introduced here is shown by analysis of SSI for a semi-tall building subjected to earthquake loading. Other advantages of the approach are the substantial reduction in degrees of freedom involved in numerical SSI analysis, the computational time and costs without sacrificing the accuracy of the results.
S. Mohammadi, A. Bebamzadeh,
Volume 4, Issue 4 (12-2006)

Explosion has always been regarded as one of the most complicated engineering problems. As a result, many engineers have preferred rather simplified empirical approaches in comparison to extremely complex deterministic analyses. In this paper, however, a numerical simulation based on the combined finite/discrete element methodology is presented for analyzing the dynamic behavior of fracturing rock masses in blasting. A finite element discretization of discrete elements allows for complex shapes of fully deformable discrete elements with geometric and material nonlinearities to be considered. Only a Rankine strain softening plasticity model is employed, which is suitable for rock and other brittle materials. Creation of new lines/edges/bodies from fracturing and fragmentation of original objects is systematically considered in the proposed gas-solid interaction flow model. An equation of state is adopted to inexpensively calculate the pressure of the detonation gas in closed form. The model employed for the flow of detonation gas has resulted in a logical algorithmic procedure for the evaluation of spatial distribution of the pressure of detonation gas, work done by the expanding gas and the total mass of the detonation gas as functions of time indicating the ability of model to respond to changes in both the mass of explosive charge and the size of the solid block undergoing fracture. Rock blasting and demolition problems are amongst the engineering applications that are expected to benefit directly from the present development. The results of this study may also be used to provide some numerical based reliable solutions for the complex analysis of structures subjected to explosive loadings.
Ali Kheyroddin, Hosein Naderpour,
Volume 5, Issue 1 (3-2007)

A parametric study is performed to assess the influence of the tension reinforcement index, ( ω = ρ fy /f Bc), and the bending moment distribution (loading type) on the ultimate deformation characteristics of reinforced concrete (RC) beams. The analytical results for 15 simply supported beams with different amounts of tension reinforcement ratio under three different loading conditions are presented and compared with the predictions of the various formulations and the experimental data, where available. The plastic hinge rotation capacity increases as the loading is changed from the concentrated load at the middle to the third-point loading, and it is a maximum for the case of the uniformly distributed load. The effect of the loading type on the plastic rotation capacity of the heavily reinforced beams is not as significant as that for the lightly reinforced beams. Based on the analytical results obtained using the nonlinear finite element method, new simple equations as a function of the tension reinforcement index, ω, and the loading type are proposed. The analytical results indicate that the proposed equations can be used for analysis of ultimate capacity and the associated deformations of RC beams with sufficient accuracy.
H.r. Ghafouri, B.s. Darabi,
Volume 5, Issue 2 (6-2007)

A new mathematical model for identifying pollution sources in aquifers is presented. The model utilizes Lagrange Constrained Optimization Method (LCOM) and is capable to inversely solve unsteady fluid flow in saturated, heterogeneous, anisotropic confined and/or unconfined aquifers. Throughout the presented model, complete advection-dispersion equation, including the adsorption as well as retardation of contaminant, is considered. The well-known finite element method is used to discretize and solve the governing equations. The model verification is implemented using a hypothetical example. Also, the applicability of the developed code is illustrated by the real field problem of Ramhormoz aquifer in southwestern Iran.
F. Amini, R. Vahdani,
Volume 5, Issue 3 (9-2007)

In this research, an innovative numerical simulating approach for time domain analysis of multi degrees of freedom structures with uncertainty in dynamic properties is presented. A full scale finite element model of multi-story and multi bays of three sample structures has been constructed. The reduced order model of structure with holding the dominant and effective Gramians in the balanced state-space realization has been achieved for easy and safe design of the optimal control forces applied to the structure. Some easy selective control algorithms based on the Optimal-Stochastic control theories such as LQG, DLQRY and modified sliding mode control has been programmed with the simulation control sequences. Some real features of accurate control system such as time delay and noise signals in earthquake time histories and also measurement sensors are considered in illustrative simulation models. These models can be analyzed under either various intensity of corresponding earthquakes or desired random excitations passed through the suitable filters providing stochastic parameters of earthquake disturbances. This control procedure will be shown to be very efficient suppressing all the severities and difficulties may arise in design of a multi-objective optimal control system. The obtained results illustrate the feasibility and applicability of the proposed stochastic optimal control design of active control force providing a stable and energy-saving control strategy for tall building structures.
H. Behbahani, S.a. Sahaf,
Volume 5, Issue 3 (9-2007)

The available methods for predicting mechanical characteristics of pavement layers are categorized into two general groups, Destructive and Non-destructive. In destructive method, using coring and pavement subgrade and performing necessary experiments on them, the quantities of layers properties will be identified. In Non-destructive method, the attained deflection is measured by applying the loading on pavement surface using equipments such as FWD which charges the impact dynamic load, and the mechanical characteristics of pavement layers are determined using back calculations. The procedure of conducting these calculations is that by knowing the thickness of the pavement layers and assuming the initial amounts for mechanical characteristics of the layer, the attained deflection at the desired points on the pavement surface will be calculated. Then, new figures are assumed for the characteristics of layers in a reattempt and calculations are repeated again. This trial and error is continued until the produced basin deformations from the calculations with true value, differs in an acceptable range. Using this method may have no accurate and single answer, since the various compositions of layers characteristics can produce similar deformations in different points of pavement surface. In this article, using an innovative method, a measurement is taken in constructing and introducing a mathematical model for determining the elastic module of surface layer using deflections attained from FWD loading equipment. The procedure is such that by using dynamic analysis software of finite elements like ABAQUS and ANSYS, the deformation of corresponding points on the surface of the pavement will be attained by FWD loading equipment. This analysis will be performed on a number of pavements with different thicknesses and different layers properties. The susceptibility analysis of different points deformations show, which will be performed as a result of the change of properties and layers thicknesses. Using this artificial data base as well as deflection basin parameters (DBP), a measurement will be taken toward constructing a regression model for determination of asphalt layer model, i.e. Eac =f(DBP) function shall be attained. To achieve the maximum correlation coefficient, an attempt is made to use the parameters of deformations basin which has the most susceptibility in changing asphalt layer module.
Habib Shahnazari, Hosein Salehzadeh, Amin Askarinejad,
Volume 6, Issue 1 (3-2008)

Classical soil mechanics involves the study of fully saturated soils. However, many problems encountered in geotechnical engineering practice involve unsaturated soil, in which behavior is significantly different from classical saturated soil. Negative pore pressure and capillary forces develop a virtual cohesion between the grains of semi saturated soils. This kind of cohesion is dependent on different factors such as grain size, saturation degree, soil-water characteristic curve and relative density of the soil. In this research the virtual cohesion of fine silty sand with 5% water content and a saturation degree of 17% is estimated. A vertical slope is constructed and is accelerated in the geotechnical centrifuge until failure. During the test, the model was monitored by a wireless video camera, attached to the strong box. The cohesionless tested sand was unsaturated. Based on the scaling laws and considering parameters such as sample unit weight, failure acceleration and the sample dimensions, a slope stability analysis was performed, and the virtual cohesion generated in the sample was calculated. The factor of safety of the prototype modeled in the centrifuge is calculated either by Finite Element Method and Finite Difference Method by using the resulted virtual cohesion from physical modeling. Results of this research show the validity of physical modeling for calculating the virtual cohesion in unsaturated silty sand.
Hasan Ghasemzadeh,
Volume 6, Issue 2 (6-2008)

Solute transport in unsaturated porous media can be viewed as a coupled phenomenon with water

and heat transport, together with mechanical behaviour of media. In this paper, solute transport is formulated

mathematically considering heat and water flow in deformable porous media. Advection, dispersion and

diffusion of chemical species in the liquid phase are considered. Convection and conduction for heat flow is

taken into account. Water flow is considered in both vapour and liquid phases. Equilibrium equation, energy

conservation, mass conservation and linear momentum for water, gas and solute are written and solved

simultaneously using finite element method. The developed model is validated by solving some examples and

comparing results with the results of experimental observation.

Kourosh Shahverdiani, Ali Reza Rahai, Faramarz Khoshnoudian,
Volume 6, Issue 2 (6-2008)

Large capacity cylindrical tanks are used to store a variety of liquids. Their Satisfactory

performance during earthquake is crucial for modern facilities. In present paper, the behavior of cylindrical

concrete tanks under harmonic excitation is studied using the finite element method. Liquid sloshing, liquid

viscosity and wall flexibility are considered and additionally excitation frequency, liquid level and tank

geometry is investigated. The results show a value for wall thickness to tank diameter ratio which may be used

as a guide in the consideration of wall flexibility effects.

S.h. Ebrahimi, S. Mohammadi, A. Asadpoure,
Volume 6, Issue 3 (9-2008)

A new approach is proposed to model a crack in orthotropic composite media using the extended finite element method (XFEM). The XFEM uses the concept of partition of unity in addition to meshless basic idea of approximating a field variable by its values at a set of surrounding nodes. As a result, higher order approximations can be designed with the same total number of degrees of freedom. In this procedure, by using meshless based ideas, elements containing a crack are not required to conform to crack edges. Therefore mesh generating is performed without any consideration of crack conformations for elements and the method has the ability of extending the crack without any remeshing. Furthermore, the type of elements around the cracktip is the same as other parts of the finite element model and the number of nodes and consequently degrees of freedom are reduced considerably in comparison to the classical finite element method. Developed orthotropic enrichment functions are further modified to enable modeling isotropic problems.
S. N. Moghaddas Tafreshi,
Volume 6, Issue 4 (12-2008)

This paper presents the numerical analysis of seismic soil-pile-superstructure interaction in soft clay using free-field soil analysis and beam on Winkler foundation approach. This model is developed to compute the nonlinear response of single piles under seismic loads, based on one-dimensional finite element formulation. The parameters of the proposed model are calibrated by fitting the experimental data of largescale seismic soil-pile-structure tests which were conducted on shaking table in UC Berkeley. A comparative evaluation of single piles shows that the results obtained from the proposed procedure are in good agreement with the experimental results.
Sassan Eshghi, Khashaiar Pourazin,
Volume 7, Issue 1 (3-2009)

Confined masonry buildings are used in rural and urban areas of Iran. They performed almost satisfactory

during past moderate earthquakes of Iran. There is not a methodology in Iranian Seismic Code (Standard 2800-3rd

edition) to estimate their capacities quantitatively. In line with removing this constraint, an attempt is made to study

in-plane behavior of two squared confined masonry walls with and without opening by using a numerical approach.

These walls are considered based on Iranian Seismic Code requirements. Finite element 2D models of the walls are

developed and a pushover analysis is carried out. To model the non-linear behavior of the confined masonry walls, the

following criteria are used: (1) The Rankine-Hill yield criterion with low orthotropic factor to model the masonry

panel (2) The Rankine yield criterion to model reinforced concrete bond-beams and tie-columns (3) The Coulomb

friction criterion with tension cutoff mode to model the interface zone between the masonry panel and reinforced

concrete members. For this purpose, the unknown parameters are determined by testing of masonry and concrete

samples and by finite element analysis. Comparing the results show that the initial stiffness, the maximum lateral

strength and the ductility factor of walls with and without opening are different. Also, the severe compressed zones of

the masonry panels within the confining elements are found different from what are reported for the masonry panels

of infilled frames by other researchers. This study shows that a further investigation is needed for estimating capacity

of confined masonry walls with and without opening analytically and experimentally. Also where openings, with

medium size are existed, the confining elements should be added around them. These issues can be considered in the

next revisions of Iranian Seismic Code.

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