Behaviour of machine foundations subjected to vertical dynamic loading

Abstract

The vertical dynamic behaviour of machine foundations subjected to vertical dynamic loading was investigated for surface and embedded foundations. The responses of these machine foundations were determined using analytical and numerical solutions ranging from simple to complex. An accurate prediction of impedance functions for the foundation system is a key step in the design procedures. The prediction accuracy depends on how close the modelling procedures are to reality. The subject of soil dynamics is complex. At times, the choice of the analysis model is based on the experience of the engineer with the model. The chosen model may or may not result in an optimal, efficient, and accurate design. The current advancement in manufacturing technology calls for machine foundation systems with high performance, availability and reliability. The analysis and design of such complex, large and sensitive machine foundations requires good understanding of their dynamic behaviour. The aim of this thesis is to investigate and evaluate the most accurate analytical and numerical models for determining the dynamic behaviour of surface and embedded machine foundations. Surface and embedded footings were cast at the experimental station at the University of Pretoria. The vertical dynamic behaviour of these foundations was determined by vertical harmonic loading. The measured impedance functions were compared with predicted responses obtained from analytical solution of the Winkler model, elastic half-space theory, simplified Lysmer (1965) model, Veletsos and Verbic (1973) models, the Dyna5 program and numerical solution of finite element method (Abaqus). The dynamic responses of the surface foundation predicted by the analytical solution proposed by Veletsos and Verbic (1973) soil with mass, compared reasonably well with the results obtained from field-measured data. The measured impedance functions of the embedded foundation were compared with the predicted results ascertained using the analytical solution proposed by Novak and Beredugo (1972), Dyna5 program and numerical solution of the finite element method (Abaqus). It is shown that embedment increases stiffness, natural frequency, natural frequency ratio, and damping ratio. The embedment reduces resonant amplitude, resonant amplitude ratio and has an insignificant effect on resonant frequency and resonant frequency ratio. The measured dynamic responses compared favourably with the results predicted by the finite element method (Abaqus). The conclusion is that the analytical model proposed by Veletsos and Verbic (1973) soil with mass, and the finite element method (Abaqus) can be used to accurately predict the dynamic response of surface and embedded machine foundations respectively.