Olof Friberg

Updating Large Finite Element Models in Structural Dynamics

In model updating problems, one tries to find design variable values affecting the input to a numerical or analytical model, such that the model output is well correlated to some target response or experimental data. Because most proposed solution techniques are iterative or require large matrices to be inverted and multiplied, severe obstacles arise when large models, such as, for example, typical industrial finite element models, are at hand. Here we consider finite element model updating for structural dynamic applications and show how the problem with little effort may be projected onto a subspace spanned by, for instance, a few eigenvectors of the initial model. Because the subspace dimension is much smaller than the space spanned by the finite element basis, the subsequent correlation can be conducted speedily, although large changes from the initial design may require the subspace basis to be updated. The proposed method can conveniently be used for substructured models and, in addition, makes interactive design changes possible. A small model problem and an industrial application are used to illustrate the performance of the suggested approach.

Optimal placement of dampers in structures using genetic algorithm

Purpose – The objective is to study the optimal damper placement of a given number of passive viscoelastic dampers, when the structure (nuclear plant) is subjected to motions from earthquake, in order to reduce the acceleration responses at a particular location, for example, a vibration‐sensitive room (nuclear reactor).Design/methodology/approach – The IDESIGN software including genetic algorithms for the optimisation task has been interfaced with a finite element program, ABAQUS, to create a structural optimisation tool. The tool has been tested on a 3D building structure.Findings – Optimisation results for different constraints on number of dampers shows that eight optimally located dampers results in an overall maximum reduction of 59 per cent of the cost function for the uncontrolled structure. It is also found that six optimally placed dampers are more effective than the fully damped case with 12 dampers.Research limitations/implications – More powerful computers are needed in order to reduce the co...

Use of mode orthogonalization and modal damping in flexible multibody dynamics

Abstract In nonlinear multibody000namics simulations a model superposition for flexible bodies is often of advantage. For combined sets of modes such as constraint modes, fixed-interface eigenmodes, attachment modes, etc. it is of interest to use mode orthogonalization. This procedure gives information about the frequency content of a flexible body model discretization. Of special interest in this study is that some orthogonalized modes may be detected as rigid body modes which cannot be used in conjunction with the multibody technique of using a co-rotating and co-translating reference frame. The orthogonalization procedure thus can be used for both rigid body mode annd high frequency mode rejection. The orthogonalization procedure makes it possible to introduce modal damping in a convenient way.

A set of parameters for finite rotations and translations

Abstract The kinematic problem of describing a position vector in an inertial frame of reference is considered. Large rotations and translations of an appertaining dynamic frame of reference are described using parameters which have easy physical interpretations. Derivations of transformation matrices and constraint equations are presented.

Jacobian matrices using the newmark direct integration scheme

Abstract The Newmark method for direct integration of nonlinear systems is considered. The differential equations of motion are written in a general form and the pertinent Jacobian matrices, which may be used as iteration matrices within the Newton-Raphson procedure, are presented. A numerical example is included.

A time integration method for simulations of tyre vehicles on flexible structures

Abstract The main objective was to perform simulations using a realistic model of a truck and trailer passing over a flexible bridge structure. A general multi-body dynamics program, DADS, and its tyre subroutines were interfaced with in-house finite element method (FEM) routines in order to create a vehicle/structure interaction computer program. Different time stepping algorithms from multi-body dynamics and FEM domains were integrated into a ‘common’ time stepping procedure. The bridge structure was simplified to a flat surface built up of shell finite elements. Tyre normal, lateral and longitudinal forces were applied to the bridge. Three driving cases were considered: straight forward driving with and without braking and a lane change manoeuvre. Braking moments were distributed to the wheel axles. The program ANSYS was used for post-processing of bridge stresses.