جستجوی مقالات مرتبط با کلیدواژه "moving load" در نشریات گروه "فنی و مهندسی"

Civil structures are always considered one of the most valuable properties of each country. Many factors can lead to local damages in different parts of structures during their operational life. These damages are reflected in the vibration responses of structures. This research aims to detect the existence and determine the location of damage in a truss bridge under a moving load using an artificial neural network and experimental wavelet transform. For this purpose, a two-dimensional truss bridge was built in the laboratory to investigate this research's objectives. Earlier experimental studies in damage detection were subjected to excitations such as impact loads and electrodynamic shakers. Since the appearance of damage effects in the vibration responses of the structure mainly depends on the applied location of the impact load, a moving load that crosses the entire length of the bridge can be used as input excitation to detect the presence and location of damages for which there is no available data. After measuring the vibration responses of the bridge, 17 time-domain features were extracted from the raw signals, which were used to detect the presence of damage. Although feature extraction is applied to raw signals, the signal processing stage was not eliminated for damage localization. By processing the response signals of the healthy and damaged state of the bridge using experimental wavelet transform, these signals were decomposed into different modes and 5 non-parametric damage-sensitive features such as Shannon and Tsallis entropies, Root Mean Square (RMS), Shape Factor and kurtosis which are all based on statistical parameters in addition to energy, were extracted. Finally, these damage-sensitive features were presented as input to the neural network whereas the state of the bridge (healthy or damaged) was considered as its target. The obtained results showed that the proposed method can effectively detect the presence and the location of the damage in the truss bridge.

The cost of construction is one of the influencing factors in the design of various structures, and the structure can be designed in such a way that with the same construction cost, it has a better performance against the incoming loads. Various methods for optimal design of structures such as genetic algorithm, biological growth and evolutionary algorithms have appeared. The ability of these methods to find the optimal design of structures is much more than mathematical methods.In this research, using the genetic method, the geometric characteristics and height of a two-dimensional Pratt type truss have been improved to make the design more economical. This truss is designed under the concentrated force in the critical position in such a way that with the minimum amount of steel consumption, the maximum stresses in the members do not exceed the limit value. Then this force is considered in a mobile form along the entire length of the truss and its optimal design has been done with the objective function expressed.Finally, the results of two problems are compared. The results show that with the number of about  17000 calls of the objective function during 50 generations, the results lead to a suitable accuracy and the optimal height of the truss in the fixed load mode is equal to 2.53 meters and the moving load mode is 2.56 meters, which for each Two cases are about 50% of the length of each opening. Also, in the case of moving load, the weight of the optimized truss was about 36% more than that of fixed load.

In this paper, formulation of a recently proposed time-weighted residual method has been developed for the vibration analysis of Timoshenko beams under moving loads. Employing pre-integration relations as well as equilibrium equations, is the main idea of this method. In the first step of the proposed method, the time interval is subdivided into a number of sub-intervals and then the acceleration field in each sub-interval is defined as the combination of an unknown function and an exponential series with constant coefficients. Finally, the solution of the problem is estimated by the time-weighted residual method along with exact satisfaction of the initial and the boundary conditions at the two ends of the beam. Storing the information of solution at each time step on the exponential coefficients, is the most important advantage of this method, so that the solution is progressed in time without the need to discretize beams and only by using an appropriate recursive relation to update the exponential coefficients. In order to investigate the accuracy and efficiency of the proposed method, the results of solving three sample problems of constant and accelerated moving load on the beams with different boundary conditions, are compared with the results of finite element method. This comparison illustrates the speed and accuracy of the proposed method in estimating the internal shear forces and bending moments rather than those obtained by the finite element method.

The vibration of axially graded Rayleigh and Euler-Bernoulli micro-beams under a moving load on Pasternak foundation is studied numerically and analytically. Accurate mathematical modeling is acquired to analyze the effect of various parameters such as longitudinal gradient parameter of material, whirling inertia factor, the stiffness of Pasternak foundation, and strain gradient parameter on the critical velocity, and cancellation mechanism and the maximum amplitude of vibrations. Natural frequencies are obtained and compared with available results in the technical literature. Closed-form expressions are extracted for dynamic magnification coefficient and maximum amplitude of free vibration. The changes in material characteristics of the system have inverse influences on the amplitude of free and forced vibrations for lower and higher values of the critical gradient parameters. It is concluded that in comparison with homogenous Euler-Bernoulli beams, in the axially graded Rayleigh micro-beams surrounded by shear Pasternak foundation. It can be controlled the cancellation and maximum free vibration phenomenon, by choosing the accurate values of gradient parameter, whirling inertia coefficient, the stiffness of foundation, and strain gradient parameter. Also, the results of the present study can be used as a criterion for the optimal design of heterogeneous structures under the moving loads.

In this paper, the effects of supersonic flutter and moving load are studied simultaneously on a honeycomb sandwich beam with a cermet covered layer. The core layer ratio is considered as a regular nomex honeycomb which has the high stiffness to weight ratio. Also cermet layer which has a high thermal strength is considered as aluminum oxide in mild steel matrix, for optimized fractional ceramic concentration. The structural formulation is based on the classical Euler-Bernoulli beam theory and the quasi-steady first order supersonic piston theory is employed to describe the aerodynamic loading. Hamilton’s principle in conjunction with the generalized Fourier expansions and Galerkin method are used to develop the dynamical model of the structural systems in the state-space domain. The critical dynamic pressures are obtained by p method for a honeycomb sandwich beam. Simulation results shows that using cermet layer as a constrained layer has an important role in postponding the flutter to higher dynamic pressures compared to the same sandwich layer with aluminum constrained layer. The thickness effect of cermet layer on flutter phenomena is also considered. Finally, in order to obtain efficeient operational results, the aeroelastic responses of honeycomb sandwich beam in supersonic regime under moving loads with different velocities are calculated.