Christos Dimou | National Technical University of Athens (original) (raw)
Papers by Christos Dimou
Proceedings of the Second International Conference on Soft Computing Technology in Civil, Structural and Environmental Engineering, 2011
In this paper, several variants of Differential Evolution, Particle Swarm Optimization and Geneti... more 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 problem. Interesting results are produced that reveal the strengths and weaknesses of each algorithm.
Journal of Computing in Civil Engineering, 2013
Journal of Computing in Civil Engineering, 2003
Competition is introduced among the populations of a number of genetic algorithms ͑GAs͒ in solvin... more Competition is introduced among the populations of a number of genetic algorithms ͑GAs͒ in solving optimization problems. The aim is to adapt the parameters of the GAs, by altering the resources of the system, so as to achieve better solutions. The evolution of the different populations, having different sets of parameters, is controlled at the level of metapopulation, i.e., the union of populations, on the basis of statistics and trends of the evolution of every population. An overall fitness measure is introduced that incorporates a diversity measure and the required resources to rank the populations. The fuzzy outcome of the conflict among the populations guides the evolution of the different GAs toward better solutions in the statistical sense. The proposed scheme is applied to two different problems-a multimodal function with six global and several near-global optima, and a reliability based optimal design of a simple truss. Numerical results are presented, and the robustness and computational efficiency of the proposed scheme are discussed.
Journal of Computing in Civil Engineering, 2009
The Binary Particle Swarm Optimization (BPSO) algorithm has been implemented with success in the ... more 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 against the simple BPSO.
This paper summarizes the results of a verification study of the effects of an eventual fault rup... more This paper summarizes the results of a verification study of the effects of an eventual fault rupture on the Thessaloniki to Skopje Crude Oil Pipeline. Two cases are analyzed, one corresponding to a normal fault and the other to a strike-slip fault. The study is carried out using a nonlinear displacement-based finite element formulation. The analysis is based on two models; one employing pure beam elements and one employing shell and beam elements that can also consider internal pressure effects. The analysis results, including displacement and stress/strain distributions along the pipeline, are used to carry out strength and serviceability verifications and to formulate construction recommendations.
In this paper, the design of a seismically isolated prestressed concrete curved railway bridge, c... more In this paper, the design of a seismically isolated prestressed concrete curved railway bridge, constructed by the cantilever method under the provisions of EC8, is presented. A combination of Lead Rubber Bearings and Flat Sliders together with Pot Bearing devices and Linear Springs are used to effectively isolate the structure from earthquake loads and satisfy the minimum requirements of DS804 regarding the longitudinal stiffness of the bridge. The analysis of the bridge is performed using three different methods, the equivalent static analysis of Single Degree of Freedom systems, in the longitudinal and lateral direction, the spectral analysis where the isolators participate with their effective stiffness and damping and the non-linear time history analysis using a collection of earthquake records properly scaled with respect to the horizontal and vertical spectrum. In this study, the LRBs are used also to carry the breaking and centrifugal forces resulting in relatively small displacements, using stiffness properties appropriate for static loads. Numerical results are presented for the specific bridge that justify all the selected parameters and establish an effective seismic isolation of a prestressed concrete bridge erected by the cantilever method.
Although an increase of population size usually improves the average performance of the Particle ... more 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 schemes is examined against the PSO algorithm with inertial parameter. The variation of the population is based on the Saw -Tooth oscillation scheme, implemented within various Evolutionary Algorithms (EAs) for demanding optimization problems. The performance is examined for a wide selection of unimodal and multimodal functions.
A method for the reliability analysis of structural systems of medium complexity is proposed. The... more A method for the reliability analysis of structural systems of medium complexity is proposed. The aim is to reduce the necessary computing effort for an accurate evaluation of the failure probabilities of the structure and its elements. This is achieved through domain decomposition (partitioning) of the probabilistic space and subsequent biased sampling in the areas close to the fail-safe surface. The results from the proposed algorithm are compared with those obtained from Monte Carlo simulation and the performance of the method is examined. In particular, the reliability of "optimal" deterministic designs based on the provisions of Eurocodes is investigated. Moreover, the robustness and computing efficiency of the method is examined for a non-linear limit state function and a 31-bar truss. Two parametric studies are performed with respect to the parameters of the method and the effects of indeterminacy on the reliability index of the structure are outlined.
Here, the Particle Swarm Optimization is implemented for the reliability based optimal design of ... more Here, the Particle Swarm Optimization is implemented for the reliability based optimal design of truss structures. This population-based, stochastic optimization technique is based on the movement of swarms and is inspired by the social behavior of flocks of birds and insects. The velocity vector which defines the trajectory followed by each individual is calculated based on information from, the individual's current velocity vector, the individual's memory with regard to its best personal solution and information obtained with regard to the best solution found in the entire population. As the optimization process evolves, the individuals search the design space and eventually are attracted towards the areas where the optimal solutions reside. This optimization technique is used for the reliability based optimal design of two determinate truss structures. The results of the analysis are presented and the robustness of the optimization scheme for the selected problems is discussed.
This paper summarizes the results of a verification study of the effects of an eventual fault rup... more This paper summarizes the results of a verification study of the effects of an eventual fault rupture on the Thessaloniki to Skopje Crude Oil Pipeline. Two cases are analyzed, one corresponding to a normal fault and the other to a strike-slip fault. The study is carried out using a nonlinear displacement-based finite element formulation. The analysis is based on two models; one employing pure beam elements and one employing shell and beam elements that can also consider internal pressure effects. The analysis results, including displacement and stress/strain distributions along the pipeline, are used to carry out strength and serviceability verifications and to formulate construction recommendations.
In this work a method for the Reliability Based Optimal Design (RBOD) of structures is presented.... more In this work a method for the Reliability Based Optimal Design (RBOD) of structures is presented. The aim is to develop a robust reliability analysis method that extracts information from a list of systematically generated Incomplete Failure Modes (IFMs), that represent sequences of failure events that drastically reduce the safety margin of the structure. An optimization algorithm that couples the advantages of Genetic Algorithms (GA) with a scheme of Competing Populations (CP) is developed in an effort to improve the robustness of GAs in handling multimodal optimization problems with reduced computational cost. These methods are combined for the RBOD of structures of medium complexity. Emphasis is given on the development of methods for reliability analysis of complex systems and RBOD. Moarefzadeha and Melchers [1], address the problem of directional importance sampling for ill-proportioned spaces to increase robustness for problems with highly correlated Random Variables (RV). Zheng and Das [2], employ Response Surface Methods (RSM) for the evaluation of failure probabilities. Mahadevan and Raghothamachar [3], present a method particularly suitable for the analysis of complex structures using the Branch & Bound (BB) method [4] to identify the important failure sequences. The method traces the Maximum Likelihood Failure Points (MLFP) of these sequences and confines sampling in regions close to these points. Olsson et al. [5] present variations of the Latin Hypercube Method (LHM) to minimize the relative error by adapting the sampling process on the shape of failure region. Thampan and Krishnamoorthy [6] couple the GA with a reliability analysis method based on a modified BB method [4] to minimize the structural expected cost.
, the velocity vector uses information obtained from an optimal solution found by the neighbors o... more , 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 . 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 . 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.
Proceedings of the Second International Conference on Soft Computing Technology in Civil, Structural and Environmental Engineering, 2011
In this paper, several variants of Differential Evolution, Particle Swarm Optimization and Geneti... more 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 problem. Interesting results are produced that reveal the strengths and weaknesses of each algorithm.
Journal of Computing in Civil Engineering, 2013
Journal of Computing in Civil Engineering, 2003
Competition is introduced among the populations of a number of genetic algorithms ͑GAs͒ in solvin... more Competition is introduced among the populations of a number of genetic algorithms ͑GAs͒ in solving optimization problems. The aim is to adapt the parameters of the GAs, by altering the resources of the system, so as to achieve better solutions. The evolution of the different populations, having different sets of parameters, is controlled at the level of metapopulation, i.e., the union of populations, on the basis of statistics and trends of the evolution of every population. An overall fitness measure is introduced that incorporates a diversity measure and the required resources to rank the populations. The fuzzy outcome of the conflict among the populations guides the evolution of the different GAs toward better solutions in the statistical sense. The proposed scheme is applied to two different problems-a multimodal function with six global and several near-global optima, and a reliability based optimal design of a simple truss. Numerical results are presented, and the robustness and computational efficiency of the proposed scheme are discussed.
Journal of Computing in Civil Engineering, 2009
The Binary Particle Swarm Optimization (BPSO) algorithm has been implemented with success in the ... more 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 against the simple BPSO.
This paper summarizes the results of a verification study of the effects of an eventual fault rup... more This paper summarizes the results of a verification study of the effects of an eventual fault rupture on the Thessaloniki to Skopje Crude Oil Pipeline. Two cases are analyzed, one corresponding to a normal fault and the other to a strike-slip fault. The study is carried out using a nonlinear displacement-based finite element formulation. The analysis is based on two models; one employing pure beam elements and one employing shell and beam elements that can also consider internal pressure effects. The analysis results, including displacement and stress/strain distributions along the pipeline, are used to carry out strength and serviceability verifications and to formulate construction recommendations.
In this paper, the design of a seismically isolated prestressed concrete curved railway bridge, c... more In this paper, the design of a seismically isolated prestressed concrete curved railway bridge, constructed by the cantilever method under the provisions of EC8, is presented. A combination of Lead Rubber Bearings and Flat Sliders together with Pot Bearing devices and Linear Springs are used to effectively isolate the structure from earthquake loads and satisfy the minimum requirements of DS804 regarding the longitudinal stiffness of the bridge. The analysis of the bridge is performed using three different methods, the equivalent static analysis of Single Degree of Freedom systems, in the longitudinal and lateral direction, the spectral analysis where the isolators participate with their effective stiffness and damping and the non-linear time history analysis using a collection of earthquake records properly scaled with respect to the horizontal and vertical spectrum. In this study, the LRBs are used also to carry the breaking and centrifugal forces resulting in relatively small displacements, using stiffness properties appropriate for static loads. Numerical results are presented for the specific bridge that justify all the selected parameters and establish an effective seismic isolation of a prestressed concrete bridge erected by the cantilever method.
Although an increase of population size usually improves the average performance of the Particle ... more 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 schemes is examined against the PSO algorithm with inertial parameter. The variation of the population is based on the Saw -Tooth oscillation scheme, implemented within various Evolutionary Algorithms (EAs) for demanding optimization problems. The performance is examined for a wide selection of unimodal and multimodal functions.
A method for the reliability analysis of structural systems of medium complexity is proposed. The... more A method for the reliability analysis of structural systems of medium complexity is proposed. The aim is to reduce the necessary computing effort for an accurate evaluation of the failure probabilities of the structure and its elements. This is achieved through domain decomposition (partitioning) of the probabilistic space and subsequent biased sampling in the areas close to the fail-safe surface. The results from the proposed algorithm are compared with those obtained from Monte Carlo simulation and the performance of the method is examined. In particular, the reliability of "optimal" deterministic designs based on the provisions of Eurocodes is investigated. Moreover, the robustness and computing efficiency of the method is examined for a non-linear limit state function and a 31-bar truss. Two parametric studies are performed with respect to the parameters of the method and the effects of indeterminacy on the reliability index of the structure are outlined.
Here, the Particle Swarm Optimization is implemented for the reliability based optimal design of ... more Here, the Particle Swarm Optimization is implemented for the reliability based optimal design of truss structures. This population-based, stochastic optimization technique is based on the movement of swarms and is inspired by the social behavior of flocks of birds and insects. The velocity vector which defines the trajectory followed by each individual is calculated based on information from, the individual's current velocity vector, the individual's memory with regard to its best personal solution and information obtained with regard to the best solution found in the entire population. As the optimization process evolves, the individuals search the design space and eventually are attracted towards the areas where the optimal solutions reside. This optimization technique is used for the reliability based optimal design of two determinate truss structures. The results of the analysis are presented and the robustness of the optimization scheme for the selected problems is discussed.
This paper summarizes the results of a verification study of the effects of an eventual fault rup... more This paper summarizes the results of a verification study of the effects of an eventual fault rupture on the Thessaloniki to Skopje Crude Oil Pipeline. Two cases are analyzed, one corresponding to a normal fault and the other to a strike-slip fault. The study is carried out using a nonlinear displacement-based finite element formulation. The analysis is based on two models; one employing pure beam elements and one employing shell and beam elements that can also consider internal pressure effects. The analysis results, including displacement and stress/strain distributions along the pipeline, are used to carry out strength and serviceability verifications and to formulate construction recommendations.
In this work a method for the Reliability Based Optimal Design (RBOD) of structures is presented.... more In this work a method for the Reliability Based Optimal Design (RBOD) of structures is presented. The aim is to develop a robust reliability analysis method that extracts information from a list of systematically generated Incomplete Failure Modes (IFMs), that represent sequences of failure events that drastically reduce the safety margin of the structure. An optimization algorithm that couples the advantages of Genetic Algorithms (GA) with a scheme of Competing Populations (CP) is developed in an effort to improve the robustness of GAs in handling multimodal optimization problems with reduced computational cost. These methods are combined for the RBOD of structures of medium complexity. Emphasis is given on the development of methods for reliability analysis of complex systems and RBOD. Moarefzadeha and Melchers [1], address the problem of directional importance sampling for ill-proportioned spaces to increase robustness for problems with highly correlated Random Variables (RV). Zheng and Das [2], employ Response Surface Methods (RSM) for the evaluation of failure probabilities. Mahadevan and Raghothamachar [3], present a method particularly suitable for the analysis of complex structures using the Branch & Bound (BB) method [4] to identify the important failure sequences. The method traces the Maximum Likelihood Failure Points (MLFP) of these sequences and confines sampling in regions close to these points. Olsson et al. [5] present variations of the Latin Hypercube Method (LHM) to minimize the relative error by adapting the sampling process on the shape of failure region. Thampan and Krishnamoorthy [6] couple the GA with a reliability analysis method based on a modified BB method [4] to minimize the structural expected cost.
, the velocity vector uses information obtained from an optimal solution found by the neighbors o... more , 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 . 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 . 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.