Vlasis K. Koumousis

Genetic Algorithm Optimization for the Active Control of a Beam by Means of PZT Actuators

The aim of this work is to optimize placement and sizing of a given number of piezoelectric (PZT) actuator patches on an Euler-Bernoulli beam, in order to control (or damp) desired modes in an optimal way. A modal approach for the dynamic influence functions is used to take directly into account the position, length and loading of a generic PZT. The optimization problem is solved from a stochastic point of view by means of Genetic Algorithms.

Hysteretic Finite Elements for the Nonlinear Static and Dynamic Analysis of Structures

A new numerical analysis procedure is presented for the nonlinear analysis of structures. The proposed methodology is developed within the framework of the direct stiffness method and the hysteretic formulation of finite elements. The derived numerical scheme relies on the natural evolution of localized inelastic quantities within the element, that is, the plastic deformation evaluated at properly defined collocation points rather than the evaluation of global and varying state matrices. This is accomplished by considering the additive decomposition of the total strain rate into elastic and plastic parts. Using the principle of virtual work, an equilibrium expression is derived in which the total applied load is equilibrated by an elastic internal force vector and an additional term acting as a nonlinear correction to the elastic component. The evolution of the plastic components is based on a smooth multiaxial hysteretic law that is derived within the framework of classical plasticity. Examples are presented that demonstrate the validity of the proposed method and its computational advantages with respect to existing methods of inelastic analysis.

Small and Large Displacement Dynamic Analysis of Frame Structures Based on Hysteretic Beam Elements

In this work, a beam element is proposed for the nonlinear dynamic analysis of frame structures. The classical Euler-Bernoulli formulation for the elastic beam is extended by implicitly defining new hysteretic degrees of freedom, subjected to evolution equations of the Bouc-Wen type with kinematic hardening. A linear interpolation field is employed for these new degrees of freedom, which are regarded as hysteretic curvatures and hysteretic axial deformations. By means of the principle of virtual work, an elastoplastic hysteretic stiffness relation is derived, which together with the hysteretic evolution equations fully describes the behavior of the element. The elemental stiffness equations are assembled to form a system of linear global equations of motion that also depend on the introduced hysteretic variables. The solution is obtained by simultaneously solving the entire set of governing equations, namely the linear global equations of motion with constant coefficient matrices, and the nonlinear local ...

Bouc-Wen Type Hysteretic Plane Stress Element

In this work, a plane-stress element is proposed for the elastoplastic dynamic analysis of two-dimensional structures exhibiting hysteretic behavior. The constant-strain triangular element formulation for the elastic case is modified by introducing the Bouc-Wen hysteretic model, properly defined in the 2D stress-strain space. Solutions are obtained by simultaneously solving two sets of governing equations, namely the global equilibrium equations and the local constitutive equations, using a predictor-corrector differential equation solver. In following the proposed method, the linearization of the constitutive relations usually performed in step-by-step solution approaches is avoided. The proposed element is capable of modeling cyclic induced phenomena such as stiffness degradation and strength deterioration. Furthermore, the solution method implemented improves the accuracy of the results without increasing the computational cost of the analysis. Examples are presented which demonstrate the efficiency of...

Lay-Out and Sizing Design of Civil Engineering Structures in Accordance with the Eurocodes

Construction of civil engineering structures is legalized by assuring that their design meets the requirements imposed by codes of practice. These codes are legislative documents that impose a minimum set of specifications, which correspond to the accumulated scientific knowledge and experience of the Engineering community, on the particular domain. The main phases of the design process and its computer implementation represents an important but yet incomplete problem. In this paper the basic methods that deal with the problem are reviewed and the logic based model is adopted as a general approach that easily simulates the course of actions taken by designers. The basic structure of this environment is discussed and the model is implemented for the design of steel roofs, that meet the requirements of Eurocode 3 for steel structures. The structural topology generator is presented, together with algorithms that modify the lay-out of the problem. The sizing problem is solved as a discrete optimization problem that selects from the data-base of the European sections, filtered by rules of practice. A sequential backtracking algorithm, that performs an exhaustive search in the design space, is presented written in Prolog. This yields the minimum cost or minimum weight design for the roof. As an alternative, the use of a genetic algorithm is presented which gives satisfactory results.

Collapse Fragility Curves of RC Frames with Varying Design Parameters

The inelastic behavior of reinforced concrete structures subjected to a number of strong motion excitations of escalated Intensity Measure (IM) and monitoring of characteristic Engineering Demand Parameters (EDPs) of the structure for all these different instances is presented. This provides the necessary data to estimate the overall performance of a structure at a particular site of specified seismic hazard within the framework of Incremental Dynamic Analysis (IDA). In this, generation of data regarding capacity and demand evolves following a lognormal distribution while the corresponding cumulative distribution function is used to define the corresponding fragility curves. This analysis facilitates further the deduction of statistically sound estimates of the measured parameters. The hysteretic inelastic response of reinforced concrete members, i.e. beams and columns designed on the basis of Eurocodes is of primal importance. The Bouc-Wen model, as implemented in “Plastique” code, with parameters established based on existing experimental data, is considered implementing the IDA procedure. Through this modeling, a series of plane frames of different number of spans and stories designed in a similar manner is investigated. Also, the effect of some general design code provisions on collapse capacity of these frames, such as stiffness distribution along height and strong column-weak beam design principle are examined. Numerical results are presented and their corresponding fragility curves are derived. Interesting features are revealed, regarding the effect of alternative designs on collapse capacity, which often deviate from collapse predictions made using the static pushover analysis.

”Plastique“ — A Computer Program For 3D Inelastic Analysis Of Multi-Storey Buildings

The code ”Plastique“, suitable for the inelastic dynamic analysis of R/C structures and its theoretical background are presented. Every structural entity is represented by a single nonlinear element through the implementation of a macro modeling approach. Three different types of 2D-macro elements are formulated namely; beams, columns and shear walls. The structural behavior of each element is evaluated using a flexibility formulation based on both element edge regions that follow a distributed plasticity law. A fiber model is used to define the monotonic strength envelope at each section. The hysteretic behavior of the structural elements is monitored by a smooth hysteretic model of Bouc-Wen type. This model is capable to express the stiffness degradation, strength deterioration and pinching phenomena which are observed in R/C elements under cyclic loading. Plane frames consisting of combinations of plane elements are linked at the levels of floors via diaphragms to assemble the 3D mathematical model of the structure. Solutions are obtained by direct integration of the equations of motion, while an iterative procedure is implemented to satisfy equilibrium at every time step. Finally, a damage analysis is performed using an appropriate damage model. The numerical examples presented herein the reveal the features of the proposed analysis scheme.

Floating Slab of the Athens Metro, Greece

The extension of the Athens Metro is the largest ongoing construction project in Greece. The design and construction of two new Metro lines with a total length of 18 km and 21 new stations, is expected to significantly alleviate the severe traffic problems of Athens by accommodating approximately 450,000 passengers daily. Part of the Metro went into service in October 2001, and the remaining lines should be completed for the 2004 Olympics.