سال:1387 | دوره: | شماره: |تعداد مقالات:15

The effect of Alkali-Aggregate Reactions (AAR) in concrete dams was investigated using the finite element method. Two models have been presented to assess the effects of the AAR in concrete dams. The stress dependency of AAR strains has been taken into account in the models and the degradation of concrete properties during the reaction has been implemented in the finite element program. The results of the isothermal and nonisothermal analyses of a gravity section of a Beauharnois power plant have been presented and compared with the measured data and the results are in close agreement. The results of the analysis demonstrate the importance of the AAR in concrete dams. High stresses and large deflections occur in the dam during the reaction. Because of the stress dependency of the reaction, vertical displacements are smaller than horizontal displacements.

Concrete is a heterogeneous material with a wide variety of usage in structural design. Concrete under tension exhibits strain softening, i.e., a negative slope in the stress-deformation diagrams. Different softening curves have been proposed in the literature to interpret this phenomenon. In current research, a new softening curve for concrete has been proposed by using the newly introduced concept of fractal geometry. This new softening curve is denominated a `Quasi-fractal' softening curve and consists of two parts, a linear portion at the beginning and an exponential portion in the rest of the curve. A comparison of a \Quasi-fractal" softening curve with a set of proposed experimental softening curves has been performed, which reveals good agreement.

Theoretical predictions regarding the ultimate resistance of slender plate girders to applied shear loading, based on existing theories and formulas, show close correlation with the test data presented. When a plate girder is subjected to a bending moment in addition to shear, the determination of the ultimate resistance becomes more complex. Herein, an interaction formula for the ultimate resistance of slender plate girders to combined bending and shear loading isproposed, which shows satisfactory correlation with the available theories and which is acceptable for practical purposes. The proposed interaction equation covers web panel aspect ratios, bw/dw, from 1 to 2 and slender ratios, dw/tw, from 150 to 300.

In this paper, the effectiveness of Tuned Mass Dampers (TMDs) in controlling building structures under earthquake excitations is studied through investigating the practical considerations and vibration control efficiency of tuned mass dampers for 5-, 8-, 10- and 15-story buildings utilizing a structural system with special Moment-Resisting Frames (MRFs) in both directions. Assuming its frequency to be near the 1st natural frequency of buildings, it is designed to control the largest response of the buildings. The effect of detuning, on some TMD parameters, on the seismic performance is studied through time-history analysis using the El Centro and Tabas earthquake records. In addition, the results of time-history analysis are compared with those of a response spectrum analysis for the structures with and without TMD, in order to judge its effectiveness. Under earthquake excitation, the performance of structures having TMDs greatly depends on the characteristics of the ground motion. When the first mode of a MDOF structure dominates the structural response, a seismic response reduction can be easily achieved. While the first mode response of a structure with TMD is proved to be substantially reduced, the higher mode response, in fact, increases as the number of stories increases. It is observed that TMD is effective in reducing maximum displacement in MRF buildings by as much as 32.2% in the Tabas earthquake and 45.3% in the El Centro earthquake. The maximum displacement results of a response spectrum analysis for the uncontrolled and controlled case in the El Centro earthquake, in an 8-story structure, are 25.70 cm and 14.59 cm, respectively, whereas themaximum displacement using time-history analysis, in the uncontrolled and controlled cases, are 27.54 cm and 15.14 cm, respectively.

Available water resources are often not sufficient or too polluted to satisfy the needs of all water users. Therefore, conflict over water, as a result of limitations on quantity and quality, is a major challenge in water allocation. In this paper, a methodology for conflict resolution over water allocation in river-reservoir systems is presented. The proposed model includes the genetic algorithm (GA)-based optimization and a water quantity/quality simulation model. The objective function of the optimization model is based on the Nash bargaining theory. Nash theory can incorporate the utility functions of the decision makers and the stakeholders, as well as their relative authorities over the water allocation process. The WQRRS (Water Quality for River- Reservoir Systems) model of the U.S. Hydrologic Engineering Center (HEC) and Qual2e model of the U.S. Environmental Protection Agency (EPA) are used for simulating the Karkheh reservoir and river water quality. In these models, the reservoir thermal stratification cycle, the reservoir discharge quality and the water quality downstream of the reservoir are simulated. The model is applied to the Karkheh river-reservoir system in the southern part of Iran. The utility functions are based on the reliability of the allocated water to different sectors, the environmental water demands (quality of the allocated water and in-stream flow), water storage in the reservoir and the quantity and quality of the return flows. The results show that this model can be effectively used in optimal water allocation of river-reservoir systems with conflicting objectives. In this paper, in order to generate the policies of the Karkheh reservoir operation and the river water quality management, the results of the optimization model are used to train the ANN model.

The purpose of this paper is to evaluate performance-based procedures in the Iranian Guidelines for the Seismic Rehabilitation of Existing Buildings, which is currently being used for the vulnerability assessment of existing buildings in Iran. For this evaluation, two construction programs are studied for buildings: 1) The displacement coefficient method and 2) The Iranian seismic code (Standard 2800). In this study, several special steel moment-resisting frames are designed, according to Standard 2800  requirements, and their vulnerability is assessed. Analytical results show that some columns do not satisfy life safety requirements at the design hazard level. Moreover, the target displacement estimated by the displacement coefficient method is larger than the maximum displacement calculated by nonlinear dynamic analysis

Turbulent heat transfer, in a three dimensional channel ow, in the presence of a square cylinder, was investigated numerically. The existence of a square cylinder in a channel, compared to a plain one, changes the heat transfer rate from the walls of the channel. A Large Eddy Simulation (LES) of a turbulent flow was performed to simulate flow behavior in a channel for Reynolds numbers in the range of 1000 to 15000. The results obtained for the Nusselt number distribution along the wall of the channel, at Re = 3000, followed those of experimental data with good accuracy. It was observed that the existence of a square cylinder makes the attached wall boundary layer separate, with a subsequent recirculation zone downstream of the cylinder. The Nusselt number distribution along the wall of the channel shows an increase, with a relative maximum, slightly downstream of the reattachment point. Heat transfer from the wall of the channel increases with increasing Reynolds number. A correlation was obtained for the variation of the mean total Nusselt number with the Reynolds number.

In this paper, a Multi-Input Multi-Output (MIMO) Model Reference Adaptive Control (MRAC) scheme for a flexible rotating arm is developed. In order to construct a reference model to be followed by this distributed parameter system, a finite element method is used to approximate the behavior of the arm. An input error direct adaptive control algorithm is utilized as the control approach to account for parameter uncertainty. Assuming the same approximation and structure as the model for the actual system, the stability analysis of the proposed controller will be straightforward. Simulation results are provided to illustrate the performance of the proposed algorithm in the presence of disturbance and uncertainties. Also, the proposed algorithm results are compared with those of a conventional PD controller.

Welding is an important manufacturing process that can be automated and optimized. This paper focuses on the development of a robotic arc welding system, wherein a three-degree- of-freedom Selective Compliance Assembly Robot Arm (SCARA) is interfaced to a Gas Metal Arc Welding (GMAW) process. The entire system is composed of a robot linked to a GMAW system. Set points are derived using the desired mass and heat input, along with the weld speed. The stick-out and the current of the welding process are controlled using an Adaptive Neural Network Controller (ANNC). The trajectory of the robot or the weld profile is also controlled by implementing a Mixed Fuzzy-GA Controller (MFGAC) on a three-axis SCARA robot. The system is, then, analyzed and the results show adequate improvement in the efficiency and performance of the proposed controller in welding a curvilinear weld profile.

To manipulate an object with several cooperating manipulators, the Multiple Impedance Control (MIC) is a model-based algorithm that enforces designated impedance on all cooperating manipulators and the manipulated object. For tuning the inner object forces, it is needed to model the inner forces/torques and include them in the MIC law. In this paper, a virtual linkage model is introduced to determine the inner forces in the MIC law. Also, open loop and a closed loop controllers are designed for inner forces tuning. The MIC law will be compared to the relevant algorithms, i.e., Object Impedance Control (OIC) and Augmented Object Control (AOC). Next, the MIC is used to manipulate an object on a planned path with desired inner forces. The grasp condition is considered either solidly (with all cooperating end-effectors), or, as flexible. Finally, the effects of gain tuning on the variations of inner forces will be discussed. The obtained results reveal the merits of the proposed scheme, in terms of system flexibility and good tracking errors, as well as inner forces tuning, even in the presence of impacts caused by contact with the environment.

A computational effort is undertaken to investigate the effect of corona discharge in air and nitrogen gas on the enhancement of heat transfer. A corona is a visible luminous emission that occurs in the vicinity of sharp edges where the electric field is intense. This phenomenon is associated with gas ionization and acceleration of ions in strong electric fields, dragging the gas particles and generating a secondary flow, known as corona wind. Corona wind may be employed as an active enhancement technique for heat transfer. Here, corona wind is used to enhance natural convection inside circular tubes and triangular ducts. The paper describes, not only the enhancement of heat transfer, but also, the shortcoming and challenges encountered in the computational modeling of corona discharge. It is shown that the solution of the electric charge density suffers from some degree of irregularity and asymmetry, while an azimuthally symmetric solution is expected. If this deficiency is not rectified, the resulting electric body force and the predicted flow field are not accurate. Two remedies are recommended to improve the solution, namely, artificial viscosity and structured grids. Moreover, the present computations indicate that, for the Rayleigh number of 3737 and the applied voltage of 8.25-9.5 kV, the corona discharge enhances heat transfer by 17.9-106.9.

Single-product oligopolies, without product differentiation, are examined under the assumption that any increase in production levels has additional cost to the firms. Therefore, the best response of each firm depends on the current output of the rest of the industry and on the previous output of the firm. Two dynamic models are introduced. In the first case, the firms form adaptive expectations on the output of the rest of the industry and select the best response output levels and, in the second case, it is assumed that they adjust their output levels adaptively. Conditions are derived in both cases for the asymptotic stability of the equilibrium.

In a production process, when the quality of a product depends on more than one characteristic, multivariate quality control techniques are used. Although multivariate statistical process control is receiving increased attention in the literature, little work has been done to deal with multi-attribute processes. In this paper, a new methodology has been developed to monitor multi- attribute processes, in which the defect counts are important and different types of defect are dependent random variables. In order to do this, based on the symmetric square root transformation concept, first, multi-attribute data is transformed, such that the correlation between variables either vanishes or becomes very small. Then, by a simulation and bisection method, the symmetric control limits are found and a symmetric rectangular region is formed for control. In simulation studies, some numerical examples are presented to illustrate the proposed method and to evaluate and compare its performance to the ones of the existing method.

In this paper, a computational method for obtaining the maximum Dynamic Load Carrying Capacity (DLCC) for the 6-UPS Stewart platform manipulator is developed. In this paper, the manipulator is assumed to be non-rigid and the joint actuator torque capacity and accuracy of motion are considered major limiting factors in determining the maximum payload. The maximum dynamic payload carrying capacity of the manipulator is established, while the dynamic model of a typical hydraulic actuator system is used in the joint actuator force capacity for a given trajectory. The flexibility of the manipulator is assumed to be eventuated from the manipulator's joints flexibility. According to the high complexity of the dynamic equations system of the flexible joints parallel manipulators, the effects of the flexibility of the prismatic joints are considered in a static situation to show the considerable effects of the joint's flexibility on the motion accuracy of the 6UPS-Stewart platform. This method can be used for determining the maximum dynamic payload, which acts as an end-effector for the mechanical design of the manipulator and the optimized selection of the actuator, such as machine tools, based on the hexapod mechanism.

Studies of the solar magnetic field are one of the key method of explaining important phenomena found in the Sun. In order to determine the contribution of the magnetic field on the solar outer shape, here, the dynamo model of the Babcock-Leighton type [1] is considered. This model explains that the surface eruptions of the toroidal magnetic field, such as the eruptions of the ux tubes, are the source of the poloidal field, whereas, generation of the toroidal field takes place in a thin, deep seated layer, called the Generating Layer (GL), at the bottom of the Solar Convection Zone (SCZ). To calculate the indicating quantity of the solar shape, i.e. the gravitational moments, Jn, several methods can be used: Stellar equations combined with a differential rotation model, inversion techniques applied to helioseismology and based on the Von Zeipel theorem, the theory of figures of the Sun [2]. In this paper, this last theory was used, but adding the magnetic field contribution. Different estimates were obtained for the successive Jn (n = 2, 4, 6, 8), in terms of different values of Bcr (the critical field), where the maximum value of the toroidal magnetic field in the GL is 1.5 × Bcr.