Aristotelis E. Charalampakis

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• Ph.D.
• Professor (Assistant) at University of West Attica

We present a method for the analytical evaluation of globally optimal solutions for the minimum weight design of trusses. The basis of the methodology is the Cylindrical Algebraic Decomposition (CAD) algorithm, in tandem with powerful symbolic computation for the discovery of stationary points. Certain final answers to well-known problems are produ...

A simple planar truss solver using Microsoft Excel (R) and VBA (Visual Basic for Applications). It contains code for in-situ matrix inversion with Gauss-Jordan elimination and full pivoting, based on the Numerical Recipes code which was translated to VB.

In this paper a new design methodology is presented to optimize the natural frequencies of functionally graded beams and arches by tailoring appropriately their material distribution. The Functionally Graded Materials (FGMs) are deemed to have an advantageous behavior over laminated composites due to the continuous variation of their material prope...

Metaheuristic algorithms have emerged as the best way of solving complex optimization problems. Consequently, the literature includes a large and growing number of applications of metaheuristics for the size optimization of trusses. Generally speaking, these studies do not focus in the comparison between algorithms. Motivated by this, we present a...

This program analyzes arbitrary composite cross sections in biaxial bending and axial load using a precise new algorithm. The only restriction / assumption is Bernoulli - Euler’s assumption that plane sections remain plane after bending. The cross section is defined by curvilinear polygons i.e. polygons with straight and / or curved edges and can b...

This study investigates a Subbuteo‐inspired rolling isolation system (RIS) as an effective means for protecting rigid blocks. The block is assumed to be rigidly connected to a curved base sector, allowing it to roll back and forth freely under dynamic excitations. The intrinsic trait of the proposed isolation concept is that the rigid block is part...

This paper examines the static and dynamic responses of strain gradient planar trusses. Classical elasticity (CE) theory lacks a material microstructural length parameter in its governing equations, making it insufficient to capture size-dependent effects. To address this limitation, higher-order continuum theories—such as micropolar, couple-stress...

This paper examines the static and dynamic responses of strain gradient planar trusses. Classical elasticity (CE) theory lacks a material microstructural length parameter in its governing equations, making it insufficient to capture size-dependent effects. To address this limitation, higher-order continuum theories—such as micropolar, couple-stress...

This paper revisits our recent work on rapid and accurate design of reinforced concrete (R/C) columns and bridge piers using Artificial Neural Networks (ANNs). Both rectangular and circular, solid and hollow sections are treated. The new functions for rectangular sections now accommodate a much greater aspect ratio, making them suitable for all sec...

Mathematical optimization refers to the process of finding the values of variables that maximize or minimize a function. Structural optimization is the process of designing a structure in such a way as to minimize its weight or cost, while meeting a set of performance requirements, ensuring that it is robust, lightweight, and efficient. Two large c...

MS 22 "Hysteresis Phenomena in Structural Dynamics and Earthquake Engineering: Experiments, Modeling, and Design"

In this work, a new analysis and design methodology of laminated composite beams is presented. The problem is formulated for both symmetric and antisymmetric ply-stacking configurations, based on a refined higher-order shear deformation theory. The solution is obtained using the Analog Equation Method (AEM) while the design process is based on the...

In this paper, the underlying energy flow in rocking dynamics of rigid blocks is utilized to elucidate their response details and overturning conditions. Based merely on the law of the conservation of energy, the exact criteria for a rigid block to overturn in the free vibration regime are established for the first time in the context of both nonli...

A comparative study of machine learning regression algorithms for predicting the deflection of laminated composite beams is presented herein. The problem of the scarcity of experimental data is solved by ample numerically prepared data, which are necessary for the training, validation, and testing of the algorithms. To this end, the pertinent geome...

In this study, the seismic performance of the mass-reduction design of multistory buildings is assessed utilizing sliding isolation systems. The proposed design concept can provide seismic protection by effectively reducing the seismic mass of the structure. This is achieved using floating slabs, i.e., slabs which are detached from the skeleton of...

The development of Machine Learning, which is deemed to be the path to Artificial Intelligence , has changed tremendously the way many computationally intensive tasks are treated nowadays. Regarding the design of R/C columns and bridge piers, the results of a recent project which proposes a number of design functions are examined and discussed in t...

In this paper, the underlying energy flow in rocking dynamics of rigid blocks is utilized to elucidate their response details and overturning conditions. Based merely on the law of the conservation of energy, the exact criteria for a rigid block to overturn in the free vibration regime are established for the first time in the context of both nonli...

In this work, various functions developed with Machine Learning techniques are proposed for the rapid and accurate design of R/C columns and bridge piers. Both rectangular and circular as well as solid and hollow sections are examined. Using powerful modern-day hardware and software, it is found that large Artificial Neural Networks (ANNs), in tand...

A comparative study of the seismic response of structures equipped with linear and nonlinear mass damping systems is presented herein. Three optimized mass damping systems mounted at the top of a linearly elastic two-degree-of-freedom building are tested under an artificial seismic motion to assess their response. The artificial seismic motion is g...

In this work, the estimation of the fundamental period of vibration of masonry infilled RC frame structures is achieved using both Machine Learning techniques and concise nonlinear formulas. The data used are extracted from a recently published extensive database that associates the period with relevant information, such as the height of the struct...

A mass reduction concept for seismic hazard mitigation is investigated herein. The proposed method is implemented through floating slabs, i.e. slabs that have been seismically isolated from the skeleton of the structure. The investigation is based on time history analyses of MDOF models under scaled strong-motion seismic records complying with an E...

This study critically compares variants of Genetic Algorithms (GAs), Particle Swarm Optimization (PSO), Artificial Bee Colony (ABC), Differential Evolution (DE), and Simulated Annealing (SA) used in truss sizing optimization problems including displacement and stress constraints. The comparison is based on several benchmark problems of varying comp...

The use of floating slabs, i.e. slabs that have been seismically isolated from the skeleton of the structure, as structural control system is investigated herein. Depending on their period, the purpose of these slabs is twofold; on one hand they act as mass-damping system for the overall response of the structure and on the other hand they provide...

In this paper, three specific uniaxial phenomenological models commonly used for the description of a Shape Memory Alloy (SMA) behavior are examined in detail. In particular, the models examined are the Graesser-Cozzarelli model, the Wilde-Gardoni-Fujino model, and the Zhang-Zhu model. The pertinent model parameters are examined with respect to the...

The aim of this paper is to investigate the effects of hysteresis and negative stiffness on seismic response reduction. For this reason, the novel Hysteretic Nonlinear Energy Sink (HNES) is used as a passive vibration control device for seismic response mitigation. So far, HNES performance has been tested in shock mitigation and has proved to exhib...

In this study, a method for the analytical evaluation of globally optimal solutions for the minimum weight design of trusses is presented. The basis of the methodology is the cylindrical algebraic decomposition algorithm, in tandem with powerful symbolic computation for the discovery of stationary points. Certain final answers to well-known benchma...

The behavior of a new Hysteretic Nonlinear Energy Sink (HNES) coupled to a linear primary oscillator is investigated in shock mitigation. Apart from a small mass and a nonlinear elastic spring of the Duffing oscillator, the HNES is also comprised of a purely hysteretic and a linear elastic spring of potentially negative stiffness, connected in para...

The behavior of a new Hysteretic Nonlinear Energy Sink (HNES) coupled to a linear primary oscillator is investigated in shock mitigation. Apart from a small mass and a nonlinear elastic spring of the Duffing oscillator, the HNES is also comprised of a purely hysteretic and a linear elastic spring of potentially negative stiffness, connected in para...

In this paper a new design methodology is presented to optimize the natural frequencies of axially functionally graded beams and arches by tailoring appropriately their material distribution. Functionally Graded Materials (FGMs) are deemed to have an advantageous behavior over laminated composites due to the continuous variation of their material p...

A simple space truss solver using Microsoft Excel (R) and VBA (Visual Basic for Applications). It contains code for in-situ matrix inversion with Gauss-Jordan elimination and full pivoting, based on the Numerical Recipes code which was translated to VB.

mySpec calculates the response of a SDoF or 2DoF system subjected to seismic or user defined excitation. The analysis may be linear elastic or non-linear. The response is calculated for several models, including linear elastic, bilinear inelastic, and Bouc-Wen. The typical excitation corresponds to a ground motion specified by an earthquake. Howeve...

myBWDE calculates the dissipated energy of Bouc-Wen hysteretic systems analytically. This program uses analytic procedures to calculate the dissipated energy of Bouc-Wen hysteretic systems. The calculation refers to a complete cycle during steady-state response under symmetric cyclic excitation. For more information, please refer to the following p...

myBWID employs evolutionary algorithms for the identification of Bouc-Wen hysteretic systems. Viscous damping can also be identified based on suitable experiments. The program implements a new hybrid evolutionary algorithm which is based on gradual decrease of the allowable domain of model parameters. For more information, please refer to the follo...

A simple single-degree-of-freedom implementation of the Bouc-Wen model in Mathematica. The excitation is sinusoidal. The differential equation is solved parametrically so that the user can experiment with the effect of each parameter. This was made possible after version 6 of Mathematica, with the addition of Manipulate[ ] function. This function c...

A simple single-degree-of-freedom implementation of the Bouc-Wen model in Mathematica. The excitation is arbitrary (The El Centro excitation is included in the file). The differential equation is solved parametrically so that the user can experiment with the effect of each parameter. This was made possible after version 6 of Mathematica, with the a...

myBWMod implements a modified Bouc-Wen model. The modified Bouc-Wen model is compatible with Drucker’s and Il’iushin’s plasticity postulate. For more information, please refer to the following papers: Charalampakis AE, Koumousis VK. A Bouc–Wen model compatible with plasticity postulates. Journal of Sound and Vibration, 322 (2009): 954–968, doi:10.1...

mySpec calculates the response of a SDoF or 2DoF system subjected to seismic or user defined excitation. The analysis may be linear elastic or non-linear. The response is calculated for several models, including linear elastic, bilinear inelastic, and Bouc-Wen. The typical excitation corresponds to a ground motion specified by an earthquake. Howeve...

In this work, the response and dissipated energy of the well-known Bouc–Wen model are examined in detail. New analytical and numerical solutions are derived using a generic model formulation without any parameter constraints. Theoretical issues, such as the inexistence of an elastic domain and the evaluation of the residual deformation after a load...

In this work, a consistent smooth Bouc–Wen-type degrading hysteretic model is presented which incorporates stiffness degradation, strength deterioration, pinching, asymmetric hysteresis and strain-hardening characteristics. The smooth rate-independent model originally proposed by Bouc [1] and later extended by Wen [2] was further developed by Sivas...

Transit systems should provide attractive yet low cost services to passengers. However, reduction of operating costs can significantly affect performance of bus systems. Cutbacks in deadhead mileage and depot costs becomes attractive since it leads to reducing operating expenses without interfering with passenger services. We revisit previous work...

In this paper, several variants of Differential Evolution, Particle Swarm Optimization and Genetic Algorithms are employed for the identification of a Bouc-Wen hysteretic system that represents a full-scale bolted-welded steel connection. The purpose of this paper is to assess their comparative performance in a highly non-linear identification prob...

We examine the concept of storing all evaluated chromosomes and directly reuse them in genetic algorithms (GAs). This is achieved by a fully encapsulated operator, called Registrar, which is effortlessly placed between the GA and the objective function. The Registrar does not approximate the objective function. Instead, it replaces the chromosomes...

Steel angle members are usually designed by elastic methods, which may lead to uneconomical design. In order to make use of the plastic reserves of the section, the knowledge of its exact failure surface is important. In this study, it is shown that idealizing the section with lines of no thickness, which is common in the literature, entails a sign...

Particle swarm optimization (PSO) is a population-based stochastic optimization technique suitable for global optimization with no need for direct evaluation of gradients. The method mimics the social behaviour of flocks (swarms) of birds and insects [1] and satisfies the five axioms of swarm intelligence, namely; proximity, quality, diverse respon...

In this paper, several variants of Differential Evolution, Particle Swarm Optimization and Genetic Algorithms are employed for the identification of a Bouc-Wen hysteretic system that represents a full-scale bolted-welded steel connection. The purpose of this work is to assess their comparative performance in a highly nonlinear identification proble...

The Binary Particle Swarm Optimization (BPSO) algorithm has been implemented with success in the Reliability Based Optimal Design (RBOD) of truss structures. In this study, a BPSO implementing time-varying schemes for the inertia factor and maximum velocity is used for the same purpose. Various schemes are examined and the performance is compared a...

The concept of using full memory in Genetic Algorithms (GAs) is examined. This is achieved by an encapsulated operator that is placed between the GA and the subroutine evaluating the objective function. The operator records all evaluated solutions and selectively replaces the solutions requested by the GA with similar ones taken from the memory. Si...

In this paper, two variants of the particle swarm optimization (PSO) algorithm are employed for the identification of Bouc–Wen hysteretic systems. The first variant is simple while the other is enhanced, as it implements additional operators. The algorithms are utilized for the identification of a Bouc–Wen hysteretic system that represents a full s...

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, an...

Although an increase of population size usually improves the average performance of the Particle Swarm Optimization (PSO) algorithm, in some cases, however, it can become detrimental on robustness, or it can result to high computational cost. In this study, the relative performance of various PSO variants implementing time-varying population scheme...

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.

Steel trusses and other structures with angle section members are usually designed by elastic methods. This may lead to uneconomical designs when these sections are subjected – besides the axial force – to biaxial bending arising not only from eccentric connections, but also from direct transverse loading. This paper provides a simple inelastic int...

The versatile Bouc–Wen model has been used extensively to describe hysteretic phenomena in various fields of engineering. Nevertheless, it is known that it exhibits 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 post...

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 postulat...

A new generic fiber model algorithm is presented that allows for the efficient analysis of arbitrary composite sections under biaxial bending and axial load. The geometry of the cross section is described by multi-nested curvilinear polygons i.e. closed polygons with edges that are straight lines or circular arcs. The polygons may be convex or conc...

A new identification method that determines the parameters of Bouc–Wen hysteretic systems is presented. It is based on a hybrid evolutionary algorithm which utilizes selected stochastic operators, heuristics and problem-specific information. The proposed method exhibits efficiency, robustness and insensitivity to noise-corrupted data, while it iden...

An efficient formulation for the response and dissipated energy of Bouc–Wen hysteretic model is proposed. The displacement is associated with the hysteretic parameter in terms of Gauss’ hypergeometric function. The hysteretic equation is solved analytically for specific values of the exponential parameter that controls the transition between elasti...

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...

This paper presents the basic features of mySpec, a computer program that has been designed to enhance learning in subjects that cover structural dynamics, earthquake engineering and hysteretic systems in graduate and post-graduate level. The program features a fully graphical interface with many interactive capabilities and covers the following to...

This paper deals with the identification of a generic Bouc-Wen hysteretic system. It utilizes the so-called Sawtooth Genetic Algorithm that combines variable population size and periodic partial reinitialization of the population. It is shown that, although the complexity of the problem is significant, the Sawtooth GA can rapidly provide very accur...

A precise fiber model algorithm for the analysis of arbitrary cross sections under biaxial bending and axial load is presented. The method can be applied to complex cross sections of irregular shape and curved edges, with/without openings and consisting of various nested materials. The only assumption is that plane sections before bending remain pl...

A generic fiber model algorithm for the analysis of cross sections under biaxial bending and axial load is presented. The method is applied to any cross section (reinforced, composite, repaired etc.) of irregular shape with/without openings and consisting of various materials. The only assumption is that plane sections before bending remain plain a...