Papers by Vlasis Koumousis
Solid Mechanics and its Applications, 2006
... By combining such elements one assembles multi-storey plane frames, which are linked ... stre... more ... By combining such elements one assembles multi-storey plane frames, which are linked ... stress redistribution which occurs due to the non linear behaviour of the ... Diploma Thesis, (2004), «Inelastic analysis of multistory buildings with a Bouc-Wen type hysteretic model», NTUA. ...
In this work a beam element based on the finite element method, suitable for the inelastic dynami... more In this work a beam element based on the finite element method, suitable for the inelastic dynamic analysis of structures is presented. The hysteretic beam element proposed by Triantafyllou and Koumousis [1] is extended to account for stiffness degradation, strength deterioration and pinching phenomena. The behavior of the element is governed by the BoucWen model of hysteresis while stiffness and strength degradation are based on Baber and Wen model [2] and pinching on Foliente’s model [3]. The case of non-symmetrical yielding, important for concrete members, is also taken into account. The proposed formulation is based on additional hysteretic degrees of freedom which herein are considered as hysteretic curvatures and hysteretic axial deformations of the crosssections. The elements are assembled using the direct stiffness method to determine the mass and viscous damping matrices, as well as the elastic stiffness and the hysteretic matrix of the structure. The entire set of governin...
Proceedings of the Ninth International Conference on Civil and Structural Engineering Computing, 2003
The fuzzy outcome of the conflict among the populations guides the evolution of the different GAs... more The fuzzy outcome of the conflict among the populations guides the evolution of the different GAs towards better solutions in the statistical sense. The optimization scheme utilizes the reliability analysis algorithm and the results from the analysis of a planar 10-bar truss and a 25-bar space truss are presented.
In this work, the robustness of two variants of the Binary Particle Swarm Optimization algorithm ... more In this work, the robustness of two variants of the Binary Particle Swarm Optimization algorithm (ΒPSO) are examined, in the Reliability Based Optimal Design (RBOD) of determinate truss structures. Particle swarm optimization (PSO) [1] is a population based stochastic optimization method inspired from the social behavior of birds or insects. In the first variant, the Binary Repulsive PSO (BRPSO), the velocity vector uses information obtained from an optimal solution found by the neighbors of the individual, the individual's memory, its current velocity vector and some "noise". Repulsive PSO is considered robust in very complex search space with numerous local optima [2]. The second variant is the Binary PSO incorporating the "craziness" operator. When craziness is implemented a subset of the particles is selected, and their position and/or velocity vectors are re-initialized [3]. The robustness of these variants is examined in the RBOD of a 25-bar truss and a 30-bar arch. The results of the investigated optimization schemes, for these two problems, are compared with the results obtained from the BPSO and the Standard Genetic Algorithm (SGA) and the relative performance of all variants is discussed.
Advances in Engineering Software, 2003
Competition is introduced among the populations of a number of genetic algorithms (GAs) having di... more Competition is introduced among the populations of a number of genetic algorithms (GAs) having different sets of parameters. The aim is to calibrate the population size of the GAs by altering the resources of the system, i.e. the allocated computing time. The co-evolution of the different populations is controlled at the level of the union of populations, i.e. the metapopulation, on the basis of statistics and trends of the evolution of every population. Evolution dynamics improve the capacity of the optimization algorithm to find optimum solutions and results in statistically better designs as compared to the standard GA with any of the fixed parameters considered. The method is applied to the reliability based optimal design of simple trusses. Numerical results are presented and the robustness of the proposed algorithm is discussed.
The effect of oscillating population size and partial initialization of the population on the rob... more The effect of oscillating population size and partial initialization of the population on the robustness and computational efficiency of a simple GA is examined. The results are compared with those of constant population size and equivalent computing time. The algorithm is applied to the reliability based optimal design of a 25 bar truss. The problem is solved for two crossover types and two different reproduction schemes. Parametric studies for various initial population sizes and crossover probabilities are performed. Oscillations of population size enhance the capacity of the algorithm in finding optimal solutions and improve the robustness.
Earthquake Engineering & Structural Dynamics
Computational Particle Mechanics
Soil Dynamics and Earthquake Engineering
Journal of Sound and Vibration, Apr 18, 1996
A method is presented for the determination of the dynamic characteristics of non circular cylind... more A method is presented for the determination of the dynamic characteristics of non circular cylindrical panels, based on Flu¨gge or Donnell type theories. The panels are simply supported along the curved edges, while at the straight edges they are supported by any type of admissible boundary conditions. The components of displacements are expanded in Fourier series in the axial direction and power series in the circumferential direction. The solution is determined by employing a recursive scheme in conjunction with the method of successive approximations. This approach leads to a characteristic equation in which the non-dimensional frequency appears in explicit form. The method is applied to catenary cylindrical panels. Numerical results are presented which describe the variation of the non-dimensional frequencies with respect to the geometric parameters. A comparison with circular panels is presented for different types of boundary conditions.
In this work, the parameter identification of non-linear dynamical systems is presented. More spe... more In this work, the parameter identification of non-linear dynamical systems is presented. More specifically the non-linear hysteretic behavior of seismic isolators modeled by the versatile Bouc-Wen model such as FPS is determined based on given data. The primal objective of this paper is to develop a parametric identification method for the modeling of FPS seismic isolator from periodic vibration experimental data. The estimation of the model parameters based on measured data from periodic experiments is implemented using a time domain method. The Bouc-Wen differential model is adopted to take into account the hysteretic frictional damping of the FPS bearings. Then, the parameter identification problem is solved using nonlinear optimization methods such as the Levenberg-Marquardt algorithm. The identification results, of the proposed method, are verified by numerical simulations and the accuracy of the identified parameters is verified by comparing the experimentally measured and the identified time histories.
Journal of Sound and Vibration
The Sivaselvan-Reinhorn model which is based on Bouc-Wen phenomenological model of hysteresis is ... more The Sivaselvan-Reinhorn model which is based on Bouc-Wen phenomenological model of hysteresis is explored from an engineering and mathematical perspective. Remarks and modifications of the original model are provided, concerning the hysteretic behavior of systems exhibiting stiffness degradation, strength deterioration and pinching. In addition, analytical solutions for the hysteretic response of the Sivaselvan- Reinhorn model are derived using Gauss’ hypergeometric function on the moment – curvature relations. Sivaselvan-Reinhorn model exhibits displacement drift, force relaxation and nonclosure of hysteretic loops when subjected to small amplitude reversals. This nonphysical behavior is eliminated with the introduction of a stiffening parameter in the hysteretic differential equations. Numerical results are provided that demonstrate the significance of the proposed modifications, particularly for seismic excitations.
Hysteresis is a term that describes macroscopically many phenomena observed in engineering. The c... more Hysteresis is a term that describes macroscopically many phenomena observed in engineering. The complexity of the actual mechanism behind hysteresis has given rise to the extended use of phenomenological models, such as the Bouc-Wen model. This paper presents a new stochastic identification scheme for Bouc-Wen systems that combines Sawtooth Genetic Algorithm and a Bounding technique that gradually focuses into smaller and better regions of the search space. Numerous studies show that the proposed scheme is very robust and insensitive to noisecorrupted data. Apart from frequency-independent hysteretic characteristics, the method is also able to identify viscous-type damping.
In this work, the parameters of Bouc-Wen hysteretic model are examined in detail. Their effect on... more In this work, the parameters of Bouc-Wen hysteretic model are examined in detail. Their effect on the overall response is clarified and discussed. The analysis is based on both mathematical and physical requirements, as well as on the analytical relations for the response and the dissipated energy of the model that were derived recently.
The versatile Bouc-Wen model has been used extensively to describe hysteretic phenomena in variou... more The versatile Bouc-Wen model has been used extensively to describe hysteretic phenomena in various fields of engineering. Nevertheless, it is known to exhibit displacement drift, force relaxation and nonclosure of hysteretic loops when subjected to short unloading – reloading paths. Consequently, it locally violates Drucker’s or Ilyushin’s postulate of plasticity. In this study, an effective modification of the model is implemented which eliminates these problems. A stiffening factor is introduced into the hysteretic differential equation which enables the distinction between virgin loading and reloading. Appropriate reversal points are utilized effectively to guide the entire process. It is shown that the proposed modification fully corrects the nonphysical behavior of the model. It is further demonstrated that the original and modified model may exhibit significantly different response under seismic excitation.
Mechanics of Advanced Materials and Structures, 2003
ABSTRACT Based on the cantilever beam solution and principles of superposition, analytical expres... more ABSTRACT Based on the cantilever beam solution and principles of superposition, analytical expressions for the displacements of statically indeterminate laminated beams with piezoelectric actuators are derived. Necessary optimality conditions determining the input voltages for shape matching are developed. The formulation allows for the effect of transverse shear deformation through the adoption of the first-order shear deformation beam theory. The effect of the thickness of the piezoelectric patch actuators is also taken into consideration for better accuracy. Optimal voltages for shape matching are obtained for beams with three different sets of boundary conditions, namely (1) beam with both ends clamped, (2) beam with one end clamped and the other end hinged, and (3) beam with one end clamped and the other end roller supported. The formulation allows for the handling of upper and lower bounds on the voltages, as in practice these voltages are constrained due to cost and material constraints. The solution procedure developed in this study can be extended to any beam with at least one clamped support, including multispan beams.
Uploads
Papers by Vlasis Koumousis