Scientia Iranica
Volume:16 Issue: 6, 2009

In the present study, the in uence of the coecient of friction on the seismic response of sliding base isolated structures is investigated. The building is modeled as a simpli ed single-story structure resting on a group of sliding supports. The frictional forces mobilized at the sliding supports are assumed to have a hysteretic plasticity behavior. Bilateral interaction between the sti ness of the two horizontal orthogonal directions of the isolators has been taken into consideration. The results show that the variations of the coecient of friction in uence the response of a sliding base isolated building. E ects of vertical excitation on the normal and frictional forces are considered too. The in uence of the bi-directional interaction of frictional forces and vertical excitation on the response is investigated by comparing the response of the system to mono-directional (excluding vertical component and no interaction between the two horizontal orthogonal directions), bi-directional (excluding vertical component) and tridirectional earthquake excitations. It is demonstrated that the response of the sliding isolated structures is in uenced signi cantly by the bi-directional interaction of frictional forces and by incorporation of the vertical component. Further, the base shear response may be underestimated if the e ects of the vertical component are neglected and the sliding structures are designed merely on the basis of single-component or two-component excitation.

Performances of tuned mass dampers and friction dampers to mitigate the wave induced vibrations in jacket type o shore platforms have been compared in this study. Due to the random nature of ocean waves, a full stochastic analysis method has been used to evaluate the response of the structures equipped with these devices. A stochastic linearization technique has been used to take the nonlinear behavior of friction dampers into account. The developed mathematical formulation has been applied to evaluate the response of realistic models, and to nd out the optimal values for the adjustable parameters of friction dampers. The results have been veri ed in comparison with time domain nonlinear analyses results. Also, a computer utility has been provided in FORTRAN to perform the spectral fatigue analysis of platforms and together with a Genetic Algorithm utility, it has been used to nd out the optimal parameters of a tuned mass damper to dissipate the wave induced vibrations of the platforms. Although the eciency of both dissipative systems increases for more exible platforms due to the dominancy of the dynamic response, the functionality of TMD devices is more dependent on the dynamic characteristics of the platform; friction dampers seem to be more ecient for xed steel jacket platforms.

Increasing population has dramatically increased the request for agricultural products. In addition, lands and water resources with suitable quality are limited. Therefore, optimal allocation of limited resources to di erent demands has become very important in recent years. In this study, a model is developed to optimize a water resources allocation scheme considering the conjunctive use of surface water and groundwater resources, as well as determining a suitable crop pattern. The economic objective function of the optimization model is to maximize the net bene t, considering the water pumping cost, crop cultivation cost and the bene t of total crop production during the study time horizon. The e ect of a marginal water de cit in the reduction of crop yield and water table uctuations are considered. A genetic algorithm is used to solve the optimization model. The proposed model is applied to the Varamin plain to determine the optimal crop mix and water allocation from surface and groundwater. In order to evaluate the results of the model, the performance indices are calculated. The e ect of imposed policies on the crop pattern is investigated through the de nition of two scenarios and the results are compared. Also, the potential of the study area, for applying de cit irrigation strategies using economic analysis, is investigated.

In this paper, the graph models of planar frame structures with di erent symmetries are decomposed and appropriate processes are designed for their healing in order to form the corresponding factors. The eigenvalues and eigenvectors of the entire structure are then obtained by evaluating those of its factors. The methods developed in this article simplify calculation of the natural frequencies and natural modes of the planar frames with di erent types of symmetry.

Many structural models such as grids, barrel vaults, trusses and frames with repetitive units, known as regular structures, have structural matrices in the form of M = F(B; A;BT). In this paper, a simple and ecient method is presented for calculating the eigenvalues of the adjacency and Laplacian matrices of regular structures. These eigenvalues can be used in studying the combinatorial properties of these structures. Examples are included to show the accuracy of the presented approach.

In this article, a numerical model based on the layer approach is introduced for nonlinear cyclic analysis of two-dimensional reinforced concrete frames. The advantage of the proposed analytical procedure is that it takes the bond-slip, shear-slip and pull-out e ects and, also, shears deformation in the joints into account. Bar and concrete stress-strain relations, the bond stress-slip relation and the shear stress-strain relation and, also, their cyclic behaviors are adopted as known speci cations. In the modeling, each frame is divided into two types of joint element and beam-column element. The e ect of bond-slip has been considered in the formulation of a beam-column element by replacing the perfect bond assumption from the ber analysis method. Joint elements are formulated upon major behaviors including the pull-out of embedded longitudinal bars, shear and exural deformation of joint panels and shear slip in interface sections between joints and neighboring elements. The reliability of the method has been assessed through a comparison of numerical and experimental results for a variety of specimens tested under cyclic loading. A good agreement between experimental and analytical results is obtained for both cases of strength and sti ness during the analysis. Keywords: Nonlinear analysis; RC frames; Bond-slip; Joint element; Pull-out e ect

To de ne failure under biaxial stress, a three-dimensional surface in terms of two principal stresses and their orientation to the bed joints is required. This article describes a series of biaxial compression tests on full-scale brick specimens. Tests were performed on square unreinforced grouted brick masonry specimens with the principal compressive stresses oriented at 0 and 90 degree angles to the bed joints, and a failure surface was obtained in terms of these parameters. Test results indicated that the masonry strength under equal biaxial compression is higher by about 36% on average than that under uniaxial compression; the in uence of joint orientation is very insigni cant and negligible for these models.

This paper presents a nonlinear analysis of reinforced concrete structures. Various yield surfaces of concrete are reviewed in the beginning and then a recently proposed yield surface for concrete is introduced. The yield surface considers the behavior of concrete in a three-dimensional stress state. Based on the yield surface, a nonlinear nite element formulation is provided to facilitate a three-dimensional analysis of reinforced concrete structures. An eight-node brick element is used in the analysis. Several numerical examples are given to show the ability of the yield surface in solving nonlinear reinforced concrete problems.