فصلنامه مدل سازی در مهندسی
پیاپی 18 (پاییز 1388)

In this paper an effective, applicable method and a simple model are presented for accurate modeling of buckling restrained braces. In the beginning, all components of buckling restrained brace are defined. A test specimen of this brace type is completely modeled in ANSYS finite element program (complete model). The modeling is validated by comparing the nonlinear dynamic results with experimental results.A simple model consisted of core steel and spring is recommended, modeled, analyzed and evaluated. Based on the results a method is presented for calculating the spring stiffness. It elucidates that the spring prevents the buckling of steel core and fining the mesh elements size resulted in smaller spring constant. Comparison of the hysteresis curves indicates a good agreement between the experiment and models (simple and complete models). In general, the difference between the experimental and analytical results is less than five percent.

In this paper with the use of theory of variable mass systems and dynamic analysis, rotary motion of variable mass cylinder has been evaluated. Effects of angular rates, the methods of mass reduction and propellant burn, the shape and measurement of geometrical elongation in direction of symmetry axis of cylinder have been known in stability of the system. The results of this paper lead to understand unexpected and sudden changes in angular velocities of spin-stabilized rockets which in some of them is happened near the end of motor burn and has made problems in control and stability of the system. In this paper the equations of motion have been derived from the methods of analytical dynamics–Kane's formalism.

Crack changes the static and the dynamic characteristics, and as a result, the structural stiffness decreases and flexibility increase. The location and size estimation of cracks are inverse of analysis process. Most of the damage assessments are based on minimization of error function, that is difference between experimental and analytical responses. In this paper, a triangular element with through crack is introduced and non-destructive method is applied for crack detection in plane structures that uses changes in the natural frequencies of a structure. The genetic algorithm is used for optimization of the error function and the size of crack is obtained. Unlike the traditional mathematical methods, which guide the direction of hill climbing by the derivatives of objective functions, GA searches the problem domain by the objective function itself at multiple points. This method is able to detect the location of the cracks. Finally, crack detection procedure is evaluated by numerical examples. The results show that Several parameters affect on the crack detection accuracy and some of them are: size of mesh, number of generations and population, genes length, crossover and mutation operations.

Pulse-width modulation (PWM) is a commonly used technique in control of electric DC motors. PWM control systems have on/off control inputs with fixed amplitude and variable duty cycle. Here, a PWM control law is determined from a given discrete-time (DT) control law by applying the principle of equivalent area. An approach to the DT control system design is Plant Input Mapping (PIM) digital redesign, which gives a DT controller from a predesigned continuous-time (CT) controller. The PIM is a well-known digital redesign method which can guarantee the closed-loop stability for any non-pathological fast and slow sampling frequencies, has a superior performance even for low sampling frequencies, and is applicable to a variety of CT control systems. The proposed design method is applied to an experimental DC motor control setup to illustrate its effectiveness.

In recent years an increase in simulation of metallurgical processes can be seen. This increase is mainly due to a decrease in the number of required experiments in order to control the process and therefore, decrease the spent time, energy and cost. One of these processes is the gas carburizing process. The percent of surface carbon content in parts manufactured by this process is the most important specification to facilitate manufacturing of standard parts. In order to determine the carbon content on the surface of steels in this surface heat treatment process, a lot of time and resources must be spent on conducting experiments.In the current study, the independent reactions in a carburizing furnace were first studied and then simultaneously modeled with the aid of MATLAB software. Afterwards, the equilibrium constant equations were solved with a computer and the equilibrium surface carbon content of the steel was calculated. As a final point, by inputting the amount of generated gases in the endothermic gas generator (air to fuel ratio), furnace temperature, furnace internal pressure, ambient temperature and humidity in the programmed software, the percent of surface carbon on the steel part is calculated and the effect of the mentioned parameters on the surface carbon content can also be studied. In addition, the results obtained from this simulation were confirmed by empirical results acquired from gas carburizing furnaces and a good relation between these two was found.

Civil engineering structures as well as office or apartment building are affected by earthquakes. A common cause of failure seems to be shear stress. The earthquake forces developed at different floor levels in a building need to be brought down along the height to the ground by the shortest path. Short column phenomena is one of the effective causes of buildings failure in past earthquakes.This destructive phenomenon is due to column height difference in a story level that is predominantly because of locating building on sloppy ground. These buildings have unequal height columns along the slope, which causes ill effects like twisting and damage in shorter columns. Short columns comparing with slender columns attract high level of earthquake force due to high stiffness. This paper present the results of linear and nonlinear static analysis (Push over) of reinforced concrete structures on flat and slope surface. The results show that in slope structure due to short column phenomenon, earthquake force increase and column need to further reinforcement. Moreover, this phenomenon causes increasing of stiffness and decreasing in drift.

Due to nonlinear and discontinuous functions, discrete and continuous variables and discontinuous constraints, optimal dispatch of reactive power is a complex problem. Moreover, because of multiple local minima, solving this problem by various types of optimization techniques such as analytic methods and stochastic search methods is difficult. PSO algorithm is one of the newest search algorithms that in this paper is used for solving optimal dispatch of reactive power problem. Comparing results from applying PSO algorithm and genetic algorithm on modified IEEE 14 bus system shows the effectiveness and superiority of PSO in rate of convergence and quality of response.