Scientia Iranica
Volume:15 Issue: 5, 2008

Roo ng systems have always been vulnerable to strong wind uplift pressures. Wind forces have dynamic e ects on structures, as they change in time and space. Therefore, a dynamic means of evaluating roo ng systems is necessary in order to identify the component of the system having the least resistance to wind uplift forces. Although researchers worldwide have conducted tests on roo ng structures, they have all used various table sizes, as there is still no standard chamber size for experimental purposes. This paper aims to study the impact of table size on roo ng system performance. To achieve this objective, extensive analytical work has been conducted to investigate the performance of roo ng systems subjected to wind pressure. Analytical results compared well with those obtained from experimental work, validating the numerical modeling. This paper presents some of these result comparisons. It was found that an increase in table width beyond a certain level, about 3 m for cases considered here, did not signi cantly change the results, while the rate of fastener load change might be high for a smaller table width. This speci c limit depends on the roo ng system con guration. Furthermore, a larger membrane width (fastener row spacing) would increase the width of the ideal table. Ideal table sizes were also suggested for various con gurations having a TPO (Thermo-Plastic Ole ns) membrane and correction factors were eventually developed for di erent table sizes.

Hydraulic fracturing is a widely used and ecient technique for enhancing oil extraction from heavy oil sands deposits. Application of this technique has been extended from cemented rocks to uncemented materials, such as oil sands. Models, which have originally been developed for analyzing hydraulic fracturing in rocks, are in general not satisfactory for oil sands. This is due to a high leak-o in oil sands, which causes the mechanism of hydraulic fracturing to be di erent from that for rocks. A thermal hydro-mechanical fracture nite element model is developed, which is able to simulate hydraulic fracturing under isothermal and non-isothermal conditions. Plane strain or axisymmetric hydraulic fracture problems can be simulated by this model and various boundary conditions, such as speci ed pore pressure/ uid ux, speci ed temperature/heat ux, and speci ed loads/traction, can be modeled. The developed model has been veri ed by comparing its results to existing analytical and numerical solutions for thermoelastic consolidation problems. The model has been used to simulate a laboratory experiment of hydraulic fracture propagation in oil sands. The results from the numerical model are in agreement with experimental observations. The numerical model and laboratory experiments both indicate that, for uncemented porous materials, such as sands (as opposed to rocks), a single planar fracture is unlikely to occur and a system of multiple fractures or a fracture zone consisting of interconnected tiny cracks should be expected.

The occurrence of the western Iran earthquake of 31 March 2006 provided an important opportunity to study the source properties of earthquakes in this region. Although moderate in size (ML = 6:1, IIEES), this earthquake was the largest to have occurred in the region since the deployment of the Global Digital Seismograph Network. The far- eld data determination of body wave (P) spectra, interpreted in terms of the circular seismic source model, are used to estimate the parameters seismic moment (MO), corner frequency (f0), source radius (r) and stress drop (
). P waves recorded at teleseismic distances can be obtained from stations of this network that are at to displacement, in a frequency range of 0.19 to 0.32 Hz. The average seismic moment (MO = 14:92  1019 N-M) and source radius (r = 9281 m) were calculated from the long period spectral levels, which were corrected for the radiation pattern of a double couple point source. In addition, the stress drops (
 = 87106 N/m2) of this event have been calculated by using an average seismic moment and source radius. Additional errors in the stress drop determination are produced by uncertainty in the seismic moment. Scatter in the seismic moment values is caused by such factors as site condition and errors in the radiation pattern

A numerical modeling procedure was used to quantify calibration chamber size and boundary e ects for cone penetration testing in sand. In the numerical analyses, chamber diameter and boundary conditions were varied to investigate the e ects of chamber size and boundary conditions on cone tip resistance. These analyses show that, for loose sand, a chamber-tocone diameter ratio of 33 is sucient for the boundaries to have no in uence on the cone tip measurements. However, for very dense sand, the numerical analyses show that the chamberto- cone diameter ratio should be more than 100 to ensure that boundaries have no in uence on cone tip measurements. Numerical analysis indicates that, not only the sand relative density but its stress state is also a signi cant factor in in uencing the chamber size e ects. The results of the numerical analyses were compared to existing empirically based relationships. Suggestions are provided to reduce the e ects of chamber size and boundaries on cone tip resistance measurements in sand.

In this paper, the data transfer operators are developed in three-dimensional elasto-plasticity using the Superconvergent Patch Recovery (SPR) method. The transfer operators are de ned for mapping of the state and internal variables between di erent meshes. The internal variables are transferred from Gauss points of old mesh to the nodal points. The variables are then transferred from the nodal points of old mesh to the nodal points of new mesh. Finally, the values are computed at the Gauss points of new mesh using their values at the nodal points. Aspects of the transfer operators are presented in a three-dimensional superconvergent path recovery technique, based on C0, C1 and C2 continuity. Finally, the eciency of the computational algorithms is demonstrated using a circular tube subjected to internal pressure.

Generating an orthoimage from high-resolution satellite images is an important undertaking for various remote sensing and photogrammetric applications. In this paper, a method is proposed that uses Arti cial Neural Networks (ANN) to generate orthoimage Ikonos Geo images. For orthoimage generation, a Digital Elevation Model (DEM) with a cell size of 4 m and RMS error of 0.24 m is constructed with neural networks, based on a Quad Tree (QT) structure. In order to determine object-to-image relationships, rational function models, polynomials and neural networks with back propagation learning algorithms were used. Ground Control Points (GCPs) and check points were taken from topographic maps of 1:2000, with a contour interval of 2.5 m, to evaluate the accuracy of DEM and object-to-image transformations. The method described in this paper is tested with an Ikonos Geo image from a region of Bilesavar, Iran.

A CFD code has been developed to describe the salt solution density current, which propagates three-dimensionally in deep ambient water. The height and width of the dense layer are two dominated length scales in a 3-D structure of the density current. In experimental e orts, it is common to measure the height and width of this current via its brightness. Although there are analytical relations to calculate the current height in a two-dimensional ow, these relations cannot be used to identify the width and height of a 3-D density current, due to the existence of two unknown parameters. In the present model, the height and width of the dense layer are obtained by using the boundary layer concept. Also, a comparison is made between depth averaged and characteristic variables. Then, the computed velocity and concentration pro les are compared with the experimental data and the results show good agreement between them. In this work, the entrainment coecient was also calculated using depth-averaged parameters and compared with the experimental data. The result has the same trend as the Ellison and Turner experiments. Present results show that the boundary layer concept can be useful in identifying the height and width of a 3-D density current.

The objective of this article is to study gas temperature estimation in a partially lled rotating cylinder. From the measured temperatures on the shell, an inverse analysis is presented for estimating the gas temperature in an arbitrary cross-section of the aforementioned system. A nite-volume method is employed to solve the direct problem. By minimizing the objective function, a hybrid e ective algorithm, which contains a local optimization algorithm, is adopted to estimate the unknown parameter. The measured data are simulated by adding random errors to the exact solution. The e ects of measurement errors on the accuracy of the inverse analysis are investigated. Two optimization algorithms are used in determination of the gas temperature. The conjugate gradient method is found to be better than the Levenberg-Marquardt method, since the former produces more accurate results for the same measurement errors. A good agreement between the exact value and the estimated result has been observed for both algorithms.

E ective prevention and treatment management of spinal disorders can only be based on accurate estimation of muscle forces and spinal loads during various activities such as lifting. The infeasibility of experimental methods to measure muscle and spinal loads has prompted the use of biomechanical modeling techniques. A major shortcoming in many previous and current models is the consideration of equilibrium conditions only at a single cross section, rather than along the entire length of the spine, when attempting to compute muscle forces and spinal loads. The assumption of extensor global muscles with straight rather than curved paths and of the spinal segments as joints with no translational degrees-of-freedom, are additional issues that need to be critically evaluated when simulating lifting tasks. The kinematics-driven approach, which satis es equilibrium conditions in all spinal directions and levels and yields spinal postures compatible with external loads, muscle forces and nonlinear passive properties, while also taking into account the wrapping of trunk muscles, is employed. Results demonstrate that, regardless of the method used (optimization or EMG-assisted), single-level free body diagram models yield estimations that grossly violate equilibrium at other levels. The computed results are also markedly leveldependent. The crucial e ects of the proper consideration of global muscles with curved paths and of spinal segments with translational degrees-of-freedom when attempting to estimate muscle forces and spinal loads in isometric lifting tasks are also demonstrated.

A known theorem in probability is adopted and through a probabilistic approach, it is generalized to develop a method for generating random deviates from the distribution of any continuous random variable. This method, which may be considered as an approximate version of the Inverse Transform algorithm, takes two random numbers to generate a random deviate, while maintaining all the other advantages of the Inverse Transform method, such as the possibility of generating ordered as well as correlated deviates and being applicable to all density functions, regardless of their parameter values.

Through the emergence of information and communication technologies and customer-oriented approaches in business and industry, for achieving competitive advantages and in order to remain at the top in every business, more exible and responsive supply chain systems are required. The next generation of supply chain systems must be agile, adaptive, cooperative, integrated and exible. Agent-based supply chain management is an approach that addresses the next generation of supply chain system features. This paper focuses on the role of an information agent in agent-based supply chain management within an uncertain environment. For this purpose, a proper modular architecture for the information agent, based on fuzzy theory, is proposed. Here, the knowledge-based module in the architecture is fuzzy rules. The system is used for updating forecasted values and implementing customer commitment in a proper manner. Finally, the proposed architecture is tested and veri ed and the results of the developed approach are discussed

Damping e ects on THE vibration behavior of a sandwich cylindrical shell with a passive constrained viscoelastic layer are investigated in the time-domain. Equations of motion in terms of transverse modal coordinates in the frequency-domain are obtained by means of an energy method and the Lagrange equation, and they are solved by the assumed-mode method. The viscoelastic behavior is represented by the frequency complex modulus model. The equations of motion are transferred from the frequency-domain to the time-domain by the Inverse Fast Fourier Transform, (IFFT). Thickness e ects of constrained and viscoelastic layers are investigated by a transient external load response and evaluation of the damping factor and settling time.

The goal of this paper is to introduce some new robust gaits of the snakeboard. This is achieved by de ning two parameters; the ratios of the frequencies and amplitudes of the snakeboard''s sinusoidal shape variable dynamics, and properly adjusting their variations. The gaits are produced via stable and/or moving limit cycles. The highly symmetric patterns generated by these gaits, besides their inherent beauty and coherency, exemplify the rich information content of the underlying nonlinear system.

IN this paper, a new approach to formulate a class of scheduling problems is introduced, which can be applied to many other discrete problems with complicated structures. The concept of graph circular coloring is applied to develop a model for the special case of an open shop scheduling problem. In this problem, there are some independent jobs to be processed in a shop with dedicated renewable resources. Each job consists of several tasks with no precedence restriction. Each task is processed without preemption. The processing time of the tasks is given. Processing each task requires using some multiple speci ed types of resource, while no more than one task can use each resource, simultaneously. Some tasks can be shared by more than one job and the process may be repeated more than once. The objective is to develop a schedule which yields the minimal makespan length of all jobs, as well as the number of cycles. The model is rst developed for cases when the processing time of each task is one unit and, then, it is generalized by relaxing this restriction. In both cases, a circular coloring formulation is shown in comparison with traditional formulation (single process execution) results in an improved makespan and also the required information regarding the optimum number of cycles to repeat the process.