Petros Komodromos

Dynamic simulation of multiple deformable bodies using combined discrete and finite element methods

The discrete element methods (DEM) are numerical techniques that have been specifically developed to enable simulations of systems of multiple distinct, typically infinitely rigid, bodies that interact with each other through contact forces. However, there are multibody systems for which it is useful to consider the deformability of the simulated bodies and enable the evaluation of their stress and strain distributions. This paper focuses on the simulation of deformable multibody systems using a combination of DEM and finite element methods (FEM). In particular, an updated Lagrangian (UL) finite element (FE) formulation and an explicit time integration scheme are used together with some simplifying assumptions to linearize this highly nonlinear contact problem and obtain solutions with realistic computational cost and sufficiently good accuracy. In addition, this paper describes a software implementation of this formulation, which utilizes the Java programming language and the Java3D graphics application programming interface (API), as well as database technology.

Investigating the seismic response of ancient multi-drum colonnades with two rows of columns using an object-oriented designed software

A software application, based on the Discrete Element Method (DEM), has been developed, using a modern object-oriented design and programming approach, in order to enable the effective simulation of multi-drum columns and colonnades under harmonic and earthquake excitations. This paper examines specifically colonnade systems with two rows of columns, one over the other, under various ground motions. The computed results show that the frequency content of the ground motion affects mostly the response. The displacements of the upper level columns in respect to the displacements of the lower level columns are affected more by the frequency content of the excitation.

Utilization of Java and Database Technology in the Development of a Combined Discrete and Finite Element Multibody Dynamics Simulator

The Java programming language and the Java3D graphics application programming interface (API) provide certain benefits over traditional programming languages and computer graphics libraries in the development of engineering applications, such as the implementation of discrete element methods (DEM, [1 ]). Java, as a pure objectoriented programming (OOP) language, enables the implementation of more robust and extendable programs through its information hiding, encapsulation, inheritance, and polymorphism characteristics. In addition, it provides portability, architectureneutrality, and multithreading capabilities. This paper addresses performance concerns about the use of Java that arise due to the intensive computational demands of the DEM. Furthermore, the utilization of database technology to efficiently manage the huge amounts of data that are typically generated by DEM programs is discussed. An implementation of combined DEM and finite element methods (FEM), as discussed in [3], is presented as it utilizes the Java and database technology.

On the Simulation of Deformable Bodies Using Combined Discrete and Finite Element Methods

In discrete element methods (DEM, [2] and [4]) the simulated bodies are typically assumed to be infinitely rigid in order to reduce the computational cost. However, there are multibody systems where it is useful to take into account the deformability of the simulated bodies in order to enable the evaluation of their stress and strain distributions. This paper focuses on the simulation of systems of multiple deformable bodies using a combination of discrete and finite element methods (FEM), with some simplifying assumptions that are necessary to make the solution of the problem feasible. In traditional mixed FE formulations the contact effects can be taken into account using Lagrange multipliers methods and keeping the contact surfaces and forces as unknowns together with the unknown displacements. This approach results in huge systems of highly nonlinear coupled equations due to geometric as well as boundary nonlinearities. Furthermore, the parts of the bodies that may come in contact, typically, have to be identified before performing the simulation. However, no prior knowledge of the upcoming contacts is available in the multibody systems under consideration. Considering the excessive computational requirements, due to the huge number of degrees-of-freedom (DOF) and the high nonlinearities of the coupled systems of equations, it is unrealistic to solve problems involving many interacting bodies using such classical contact FE approaches. Simulations of deformable bodies with reasonable computational cost are enabled by incorporating FEM in DE analyses using certain assumptions that uncouple the contact interactions from the equations of dynamic equilibrium. In particular, the DEM are employed to identify, at each simulation step, the bodies in contact and determine the contact forces. Then, either a FE or a DE formulation is used at the individual body level to describe the equations of motion, depending

Optimized retrofit of multi-storey buildings using seismic isolation at various elevations: assessment for several earthquake excitations

This work is concerned with the seismic retrofit of multi-storey buildings by installing isolation devices at various levels along their height. The design of an effective retrofit solution of this type requires the selection of the appropriate number of isolation levels to introduce in a building, the elevations at which to place these isolation levels and the properties of each of the isolators to install. The task of identifying configurations of isolators vertically distributed over the height of a building that yield favourable structural behaviour is handled in the present paper with a specially developed optimization procedure, which can automatically and effectively explore the huge set of potential retrofit solutions formed by all possible combinations of isolator numbers, locations and properties. Specifically, a genetic algorithm is implemented to detect the isolation configuration that minimizes the maximum floor acceleration of the building under retrofit, subject to constraints ensuring that maximum allowable values for interstorey drifts, base displacements and overall isolation cost are not exceeded. A 6-storey building is used to test the presented optimization procedure, while several recorded strong earthquake motions are considered, which are applied either individually or in sets to the building in the framework of time-history analyses. The numerical results obtained demonstrate the validity and effectiveness of the optimization procedure, which manages to identify feasible isolation configurations for all test cases examined. Of particular importance is the ability of the optimization procedure to provide valid retrofit solutions for buildings with narrow seismic gaps subjected to very strong earthquakes, in which configurations employing only base isolation usually prove to be ineffective.

Effect of Planar Impact Modeling on the Pounding Response of Base-Isolated Buildings

During strong earthquake excitations, base-isolated buildings may collide, either with the surrounding moat wall or with adjacent buildings if the available clearance is exceeded. This undesirable possibility has been recently investigated by several researchers, adopting various types of force-based impact models. Evidently, an important issue that arises regarding such numerical studies is the way of taking into account potential impacts. This paper parametrically investigates the effects of impact modeling characteristics on the computed overall peak response of a base-isolated building that experiences structural pounding. Specifically, the Kelvin-Voigt impact model and various other modifications of this linear viscoelastic impact model are considered in the conducted analyses. In order to efficiently conduct this investigation a specially developed software is utilized. The results indicate that the excitation’s and isolator’s characteristics do not significantly influence the variation of the normalized peak response of the superstructure. In contrast, the impact parameters can have a significant effect on the superstructures’ peak accelerations with overestimations up to 70%. In general, the normalized peak response ratios of the inter-story drifts tend to increase as the available seismic gap clearance and the coefficient of restitution decrease, although the magnitude of the deviations is within 5%, which can be considered insignificant.

THE EFFECT OF MODIFIED LINEAR VISCOELASTIC IMPACT MODELS ON THE POUNDING RESPONSE OF A BASE-ISOLATED BUILDING WITH ADJACENT STRUCTURES

Abstract. During strong earthquake excitations, base-isolated buildings may collide, either with the surrounding moat wall or with adjacent buildings. This unfavorable possibility has been recently investigated through numerical simulations and parametric studies. A very important issue regarding these numerical studies is the modeling of impacts, which are typically simulated using various types of force-based impact models. This paper parametrically investigates the effects of impact modeling characteristics on the overall structural response of a base-isolated building that is subjected to seismic pounding. Specifically, the KelvinVoigt impact model and various other modifications of this linear viscoelastic impact model are considered in the performed analyses. In order to effectively and efficiently conduct this investigation, a specialized software application, which has been specifically developed to simulate buildings subjected to pounding, is employed. A smooth bilinear (Bouc-Wen) model is used for the simulation of the seismic isolation system. The influence of particular impact parameters, as well as the width of the seismic gap, on the dynamic response of the structure under strong excitations is quantified. Furthermore, the effect of using different impact models for the calculation of the impact forces on the overall seismic response during pounding is simulated and discussed, since a reasonable question arises regarding the accuracy of an impact model, which is a simplification of the actually very complicated impact phenomenon.

Numerical Investigation of the Effectiveness of Rubber Shock-Absorbers as a Mitigation Measure for Earthquake-Induced Structural Poundings

Very often, especially in densely-resided areas and city centers, neighboring buildings are constructed very close to each other, without sufficient clearance between them. Thus, during strong earthquakes, structural poundings may occur between adjacent buildings due to deformations of their stories. Furthermore, in the case of seismically isolated buildings, pounding may occur with the surrounding moat wall due to insufficient seismic gap at the base of the building. The current study presents a simple but efficient methodology that can be used to numerically simulate the incorporation of rubber layers between neighboring structures with relatively narrow seismic gaps in order to act as collision bumpers and mitigate the detrimental effects of earthquake-induced poundings. The efficiency of this potential impact mitigation measure is parametrically investigated considering both cases of conventionally fixed-supported and seismically isolated buildings subjected to various earthquake excitations. The results indicate that under certain circumstances the incorporation of rubber bumpers in an excising seismic gap can reduce the amplifications of the peak responses of the structures due to pounding.

Utilization Of Object-oriented Programming, Design Patterns And Java For Simulating Earthquake-induced Poundings Of Base-isolated Buildings

Base-isolated buildings experience large horizontal relative displacements during strong earthquakes due to the excessive fl exibility that is purposely incorporated, through seismic bearings, at their bases. When the available clearance around a base-isolated building is limited, there is a possibility of the building pounding against the surrounding moat wall or adjacent structures. Considering the nonlinearities involved in this structural impact problem, it is evident that the effects of potential pounding on the overall seismic response of base-isolated buildings during earthquake excitations should be investigated numerically through appropriate simulations. Object-oriented programming (OOP), design patterns (DPs), and the Java programming language have been utilized in order to design and implement a fl exible and extendable software application that can be effectively used to perform the necessary numerical simulations and parametric studies of base-isolated buildings that may experience structural poundings during earthquake excitations. The aim of this paper is twofold: (i) to explain the signifi cant advantages of utilizing OOP, DPs, and Java in structural analysis software and (ii) to use the developed software to study earthquake-induced poundings of base-isolated buildings.

Seismic Behaviour of Ancient Multidrum Structures

Strong earthquakes are common causes of destruction of ancient monuments, such as classical columns and colonnades. Ancient columns of great archaeological significance can be found in high seismicity areas in the Eastern Mediterranean. Understanding the behaviour and response of these historic structures during strong earthquakes is useful for the assessment of conservation and rehabilitation proposals for such structures. The seismic behaviour of ancient columns and colonnades involves complicated rocking and sliding phenomena that very rarely appear in modern structures. Analytical study of such multi-block structures under strong earthquake excitations is extremely complicated if not impossible. Computational methods can be used to simulate the dynamic behaviour and seismic response of these structures. The discrete element method (DEM) is utilized to investigate the response of ancient multi-drum columns and colonnades during harmonic and earthquake excitations by simulating the individual rock blocks as distinct rigid bodies.