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Dynamic Performance Investigation of Base Isolated Structures

By Ather K. Sharif


This thesis concerns an investigation of the dynamic performance of buildings which are base isolated to control vibration and groundborne noise from man made sources, and in particular railways.

With a view to preparing an International Standard it examines the wider issues of Base Isolation and in particular the dynamic performance using a number of case studies.

This research includes a unique experimental investigation to determine the actual insertion loss for Base Isolation, by comparing the performance of a test room in its un-isolated then isolated case, investigating the performance of a range of isolators. It also compares the performance of complete buildings in their un-isolated and isolated state.

MDOF models are shown to provide key features in the behaviour of Base Isolated buildings, representing a necessary alternative to the common use of SDOF models. Simple bar models used to examine the axial modes of a column are shown to correlate better with site measurements when storey loading is included. Modal density is introduced as a relevant parameter in the evaluation of likely performance.

Case studies include an investigation of ambient, impact and forced response testing. Measurements show that Base Isolated buildings respond more strongly to internal services. The investigation uniquely provides the inertance of isolated and un-isolated columns, which are otherwise identical. It also provides the point inertance of identical pile caps, below an un-isolated and isolated column, with results compared to theoretical predictions.

It examines the use of Vibration Dose Values from BS6472(1992) as a means to justify a decision to implement Base Isolation. With measurements directly on railway platforms it highlights that current guidance is inconsistent with the subjective impression of perception, and is used to justify a re-examination of the standard.

The research concludes with a draft International Standard, which has been submitted to ISO.