Year:2008 | Volume:41 | Issue:6 (108)|Papers Count:16

This paper describes a fast approach for solving elasticity problems using boundary element method. The key idea is based on compressing matrices, to sparsify them, using fast wavelet transforms. Wavelet compression technique is successfully used to overcome the main drawback of BEM, dense and fully populated matrices. Compactly supported orthogonal wavelets have been used to compress matrices arising from applying BEM to practical engineering problems, which could be added as a black box to existing BEM codes. Dense and fully populated matrices arising from BEM have been changed to semi banded sparse matrices by applying wavelets and permutation matrices at the same time. Applying permuted wavelets converts dense matrices arising from BEM to semi-banded and sparse ones like what are encountered in FEM. Transforming matrices from the physical variables to wavelet basis needs much less computational effort than that saved by solving sparse linear system. Transformed linear system is solved by iterative GMRES solver. Numerical results including a precise study on threshold parameter, solution accuracy for displacements and stresses and savings in computer time and memory are presented. High speed-ups are achieved for large problems without significant loss of accuracy. Compression ratio is tested by different thresholding parameter to optimize the method. The proposed method works on the final linear algebraic system then, it could be used in any other numerical method, which leads to linear equations. The results demonstrate the competence of the method in terms of both computation time and memory requirements and since wavelet technique is used as a black box in the existing BEM code, the results encourage application of the combined wavelet-BEM codes to analyze practical engineering problems.

An axisymmetric transversely isotropic material is considered. A complete solution in terms of two retarded potential functions for the wave equations of general elastodynamics problems in transversely isotropic media has been presented in literature, which reduced to one for torsionless axisymmetric case. In the cylindrical coordinate system, the body force in the torsionless axisymmetric case given there is at most in the form of. In this paper, we show that by introducing another potential function, which is directly obtained from the body force in radial direction, a new complete solution is introduced for more general torsionless axisymmetric case, where the body force may be as general as. The new solution is degenerated for the elastostatic case, and in addition it is degenerated for the torsionless axisymmetric elastodynamics and elastostatics problem of isotropic materials. The solution for elastostatics problem of isotropic material is the same as extended Love's potential function.

Modal approach is one of the usual methods utilized in dynamic analysis of structures due its computational time efficiency. This paper presents the general application of this approach for analyzing dams. Dam body and water domain are discretized by finite elements. Material of the dam body is assumed to be isotropic and homogeneous with linear viscoelastic behavior. The foundation is assumed to be rigid. However, the effects of wave absorption in sediments are included. Water in the reservoir is considered as an irrotational, inviscid and compressible fluid. Furthermore, Sommerfeld boundary condition is imposed for the upstream boundary of the reservoir. Pressure and nodal displacements degrees of freedom are used for the water in the reservoir and dam body, respectively. It is noteworthy that these selection of degrees of freedom leads to a coupled equation of the dam - reservoir system with unsymmetric mass and stiffness matrices. Therefore, an unsymmetric eigenproblem should be solved to obtain the mode shapes of the system, which requires a complicated computer programming. Decoupled modal approach is used as an alternative to avoid this problem. In this method, a symmetric eigenproblem is solved which is obtained by eliminating the unsymmetric terms of the initial eigenproblem. Thus, a series of mode shapes are calculated that can be used for the analysis of the coupled system. By increasing the number of mode shapes utilized, results of this approach converges to the exact response of the system. Moreover, unsymmetric eigenproblem is also studied for comparison purposes. As mentioned above, this unsymmetric eigenproblem is difficult to solve and a special technique is utilized for calculating mode shapes of system. Results of the coupled modal approach are compared with the decoupled method and it is observed that the former technique gives good results with relatively low number of modes. Moreover, a direct method referred to as the pseudo-symmetric method is also utilized for comparison purposes. Based on the above techniques, a special computer program is prepared in FORTRAN. This program can analyze 2D and 3D models of dam – reservoir systems with first and second order finite elements.

Overland flow (OF) systems were evaluated and compared for advanced treatment of municipal and industrial effluents, including nutrients and nondegradable chemical oxygen demand (COD) removal. Three pilot plants were constructed at the Shahin Shahr Wastewater Treatment Plant (WWTP), Isfahan, Iran. Each pilot was assigned a specific wastewater and all were simultaneously operated for eight months. Treatment of primary effluent, activated sludge secondary effluent, and lagoon effluent of textile wastewater was investigated at application rates (ARs) of 0.15, 0.25, and 0.35 m3 m1 h1. During five months of stable operation after a three month acclimation period, mean removals of total 5-day biochemical oxygen demand (TBOD5), total COD (TCOD), total suspended solids (TSS), total nitrogen (TN), total phosphorus (TP) and turbidity were 74.5, 54.8, 66.2, 39.4, 35.8, and 67.7% for primary effluent; 52.9, 52.9, 66.5, 44.4, 39.8, and 50.1% for activated sludge effluent; 65.7, 58.7, 70.3, 41.7, 41.3, and 54.9% for textile wastewater lagoon effluent, respectively. The model of Smith and Schroeder (15) was satisfactory for TBOD5. For all treatment parameters a standard first-order removal model was inadequate to represent the data but a modified first-order model provided a satisfactory fit to the data. Overall, it can be concluded that an OF system as advanced treatment has the ability to meet effluent discharge permit limits and is an economical replacement for stabilization ponds and mechanical treatment options.

Because of different behavior of reservoir water and dam material, the determination of hydrodynamic pressure during earthquake is very complicated. Thus, different formulations have been presented for modeling of the dam reservoir system under dynamic loading such as earthquake. These formulations can be categorized into two general groups, which are Lagrangian and Eulerian, each having advantages and disadvantages. In this paper, a proper formulation is presented for modeling of reservoir of concrete dams and determination of the hydrodynamic pressure based on finite element, and the Lagrangian and Eulerian methods. The Lagrangian and Eulerian methods are then compared with together for the determination of hydrodynamic pressure on the dam upstream and the displacement of the crest of dam for a variety of dimensions of dam under different types of earthquake. The results brought up from these two formulations are then compared with each other in different aspects such as accuracy, the required amount of calculations, the frequency effect, the boundary conditions, the dam upstream slope, the sediment in energy absorption, the reservoir height and its bed slope and eventually, the type of elements used in finite element method. Furthermore, the formulation of reservoir using Lagrangian method for applying boundary conditions as especial element has been used and the results show that this method is an improvement of the existing finite element method in comparison with usual Lagrangian method in other to decrease the degree of freedom.

Flanged shear 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. They must be carefully designed to provide not only adequate strength, but also sufficient ductility to avoid brittle failure under strong lateral loads, especially during an earthquake. When concrete in the compression zone of a shear wall is confined by transverse reinforcement, both the strength and ductility of the wall would be increased. However, none of the existing analysis methods in the codes allows for such effect. Herein, a finite element model that takes into account the effect of concrete confinement is developed for nonlinear analysis of reinforced concrete structures. In this model, the confinement effect of the transverse reinforcement is incorporated by adjusting the compressive stress-strain relation of the concrete according to the confinement index proposed by Kappos. Shear wall models tested by others under constant vertical load and monotonically increasing horizontal load are analyzed and the analytical results for the failure modes and load-deflection curves of the walls are found to be in good agreement with the experimental results. Using the finite element model, a parametric study on the effect of concrete confinement on the behavior of shear walls has been carried out. It is revealed that the confinement of the concrete in the compression zone can significantly increase the lateral strength and ductility of the wall. The increases in lateral strength and ductility due to concrete confinement are greater in walls with greater height to width ratio and/or subjected to larger vertical loads. There is an obvious diminishing return of the further gain in lateral strength or ductility as the quantity of confinement reinforcement increases to beyond certain limit. The contribution of the concrete confinement to ductility is generally more significant than the corresponding contribution to lateral strength especially when the amount of confinement reinforcement is relatively large.

In this paper, the performance of multi-layer electrometric seismic isolators, which have been reinforced by layers of woven carbon fibers, is studied. Specimens of this kind of isolators as well as a specimen of similar size and shape reinforced by steel plates have been designed and manufactured. They have then been tested under cyclic vertical and horizontal loads to determine their mechanical properties and performance. Design of the steel reinforced specimen has been carried out according to current method of design of isolators ; for the fiber reinforced isolators the same method has been applied and the tensile and bending flexibility of the fiber cords have also been considered. The studies show that the fiber reinforced isolators which include steel top and bottom end plates, in addition to having the desired shear characteristics such as ability of sustaining of very large shear strains, can demonstrate vertical stiffness and ratio of vertical to horizontal stiffness equal or even greater than the similar steel reinforced isolators. Therefore they can be used in seismic isolation of structures more economically compared to steel reinforced isolators in view of advantages such as being lighter and simpler to manufacture.

Acceleration time histories, recorded during the destructive 26 December 2003 (Mw 6.5) Bam earthquake, have been simulated using the stochastic method for finite faults. In this method, the ground motion amplitudes are simulated as a summation of stochastic point sources. An inhomogeneous slip-distribution model, derived from previous studies is used to asses the source effect. It was found that the overall agreement between simulated and observed waveforms and spectra is well satisfied. The calibrated model is then used to simulate ground motion for the most damaged areas where strong-motion data are not available. This result clearly shows that source effect is capable of explaining the main features of the damage distribution pattern of the 2003 Bam earthquake, especially why eastern part of Bam city suffered the heaviest damage

In this paper, stability of slopes with a shallow foundation located on top including the shape of the failure mechanism is investigated in static and seismic states. This is a three dimensional problem which is analyzed two dimensionally because 3D analyses are complex and appropriate softwares are not available. The conventional method of bearing capacity evaluation of foundations on top of slopes is reviewed and results are compared with those obtained from using a specific three dimensional slope stability algorithm based on upper bound limit analysis theorem. It is concluded that increasing the foundation load to a threshold level, the effect of the load on the stability of the slopes is negligible, a phenomenon which can not be observed in two dimensional analyses. Determination of this threshold by 3D analyses is a useful criterion for selection of the adequate safety factor in static and seismic slope stability analyses.

The use of steel flush end-plate moment connection is practiced in the construction of the light steel frames around the world particularly in parts of Europe and US. In the past most research was concentrated on studying the behavior of the flush end-plate connection subjected to only monotonic type loadings. The majority of that research carried out such investigation through experimental programs. Although experimental results reveal the behavior of such system to some extent, but the cost of using the numerical techniques such as finite element for the analysis appears to be much less and takes not long time and effort in comparison with the test program. In this study, the numerical behavior of flush end-plate connection using finite element method is investigated. The moment-rotation characteristic of the connection when subjected to cyclic loading is obtained. The numerical results obtained in this study are compared with the experimental results of other researchers. Some of the pertinent parameters effecting on the overall behavior of the connection as well as the design equations obtained from the method of yield line analysis are discussed.

One of the common questions in all projects, is "How much would be the final cost of the project?" or "How much would be the completion duration of the project?". Earned Value Management presents a method for estimation of these results. In general methods and formula of Earned Value Management, future trend of project, are anticipated completely based on its previous trend, and changes in environmental conditions or other effective elements in future performance of the project are neglected. But risk management looks a future horizon in project's future and explores unknown future to identify potential threats and opportunities and control their occurrence results on project performance. In this paper, using risk management process and challenging effective aspects & elements on project future performance in different visions, a risk coefficient in accordance with current coefficient of "EVMS", is presented for better estimate of final results of projects. The goal of this method is concentration of management's attention on reaching project goals, by using interaction of these two methods in determining exact performance coefficient of project future. Anticipating exact results of project is attainable by this coefficient.

Using the statistical characteristics is one of the methods to justify the random nature of the ocean waves. Probability function is used to facilitate the studies of the random wave’s parameters, such as the surface and height and period of the waves. Since, the force of the ocean waves are the prevalent principal forces on the offshore structures, the assignment of the significant structural responses such as applied inline and transverse forces and also total moment on the piles is the main step for design and construction of the structures. In this paper, the random waves and their effects on the cylindrical pile will be investigated. These waves have been recorded during some tests which will be illustrated separately. The oscillation of some linear or nonlinear responses of the pile has been discussed statistically and then spectral analysis performed and their extremity amplitude statistical analysis interpreted and prevalent probability functions determined.

Construction of dams by using roller compacted concrete (RCC) is a relatively recent technology which has increasingly developed in the past few years. With regard to various horizontal lift joint in the body of RCC dams and in order to increase the safety factor in the design, the experimental investigation was conducted on the basis of improving bond strength in RCC by using interface cement grout and imparting changes in normal pressure. RCC samples were constructed in two layers with maximum aggregate size of 25 mm, type I cement of Isfahan plant and Khash pozzulan. On the whole, in different conditions of using grout with w/c ratio of 0.5, 0.75, and 1 as well as no grout condition, the first stage of bond strength tests in normal pressure (0, 5, 10, and 15 kg/cm2) by using the designed machine in this study and the second stage tests were conducted in normal pressure (20, 25, 30, 35, and 40 kg/cm2) by using stone direct shear machine. From the sum of 250 made samples, the results of bond strength and compressive strength in 7, 28, and 90 days in the first stage, and the results of maximum bond strength and residual bond strength in 28 days in second stage have been obtained. On the basis of the results obtained, bond strength parameters have been obtained and analyzed. The results of this study indicated that the Mohr coulomb criterion shows the upward tend of bond strength with an increase in normal pressure very well, and by increasing the normal pressure from the first stage to the second stage, interface cohesion decreases and the interface angle increases. Also, by using grout between layers paves, the way for an increase in bond strength compared to the no grout condition, and there will be a decrease in bond strength with an increase in w/c ratio of grout. In this case, changes in cohesion parameter are in phase with changes in bond strength while to sensible changes in interface conditions are evident on the interface friction angle.

In some natural events such as soil failure the deformations are localized in narrow restrictions, which are called shear bands. This event which is a fundamental phenomenon in granular material has been widely investigated during recent decades within expensive experimental tests and also some numerical simulations. Most of previously used numerical methods are based on continuum theories describing shear bands as interfaces along which solid masses move like rigid blocks. In this interpretation many physical events such as changes of the soil structure around failure line are neglected. In this paper the discrete element method is used to simulate the shear bands. Since in this method some of the problems of experiments and simulations are solved, It would be an ideal method to obtain the stresses and strains, and also to investigate the behavior of shear bands in a granular media, while exposing to external forces. In this research by the use of DEM and conducting series of biaxial tests on assemblies of two-dimensional ellipse shaped particles, the effect of different factors such as average grain size, particle shape and confining pressure on the shear bands are studied and the results show that some investigated factors like average grain size and confining pressure have considerable effect on the shear bands characteristics. The main results can be summarized as follows: The amount of rotation is a very sensitive characteristic and it changes considerably by all of the factors measured in this research. The most affecting factors on the displacement of the particles across the shear bands are the loading rate and confining pressure. Moreover by increasing the particle size the displacements increase with a great amount but if these values are divided by the particle radius, no significant changes in the particle displacement will be observed. Other factors do not seem to have any effects on this issue. 6-13d50 seems to be the best estimation for the shear band thickness, and other controlled factors affect this value within this restriction. The width of the shear bands seems to increase by the loading rate and confining pressure increase. The inclination of this localization is mostly affected by confining pressure (which increase leads to angle decrease), porosity (which increase leads to angle decrease), grading (its uniformity causes smaller shear band angles), and size (greater sizes of grains would result in failure with lower angles of sear band). Among these factors grain size has the least effect.