A performance-based design framework for base-isolated buildings against ground-borne vibration
Creators: name: Sanitate, Giuseppe
Publisher: name: Apollo - University of Cambridge Repository
Year: 2020
Subjects: ground-borne vibration
soil-structure interaction
SSI
base isolation
building vibration
added-foundation effect
kinematic interaction
added-building effect
base-isolated building
power-flow analysis
insertion gain
IG
power flow insertion gain
PFIG
power flow

description: \noindent Ground-borne vibration in buildings is a recurrent design concern for new developments in the vicinity of surface or underground railways, a common question being whether or not a building necessitates some form of vibration mitigation. This question is commonly tackled by comprehensive numerical modelling of the source of vibration (train excitation), the propagation path (the ground) and the receiver (the building), allowing interaction between the different parts of the complex system with the main focus on the vibration and re-radiated noise levels in the building. Although numerical modelling may be necessary for absolute predictions, it is computationally expensive and not suitable for design purposes. In such a context, different configurations of the foundation and the building may be investigated in order to minimise vibration levels. A widespread measure of mitigation is base-isolation: the building is mounted on an isolation system composed of either rubber bearings or steel springs. The goal is to de-couple the building from the soil-foundation system in order to reduce the levels of vibration and re-radiated noise in its interior. Despite innovations in the bearings themselves, it is not yet clear how best to evaluate the isolation performance, which relates to the level of attenuation provided. In this dissertation, the Power~Flow Insertion~Gain~($PFIG$) is adopted as the main metric for the isolation performance. %It provides an understanding of the change in mean-vibrational-power values, associated to a given domain of the building, in two different design configurations: with and without the isolation system. This metric is adopted because of its inherent consideration of both the axial and the bending response of the building by means of a scalar quantity that leads to a meaningful and global comparison of the change in vibration levels. In the context of design, it is desirable to consider simplified methods and models that are able to capture the underlying physics of the problem and to provide an estimation of the isolation performance in terms of the $PFIG$. A new design analysis framework is presented that can be adopted for this purpose with reference to a staged approach, which relates inherently to the topic of dynamic soil-structure-interaction~(SSI). The design analysis framework considers the free-field vibration represented by incident plane wave-fields, in the form of P-, SV- or Rayleigh waves. A series of simplified methods and/or models are then adopted to account for the \textit{added-foundation effect}~(AFE) and the \textit{added-building effect}~(ABE) associated with the construction of the foundation and the building respectively. The design analysis framework introduced here is conceptually general, but it is contextualised to the case of portal-frame buildings and surface foundations. The different assumptions related to the design approach are systematically tested against a rigorous approach that involves the modelling of the building and the soil-foundation system by means of the Dynamic~Stiffness~Method~(DSM) and the Boundary~Element~Method~(BEM) respectively. The frequency-dependent value of the $PFIG$ obtained by the design and the rigorous frameworks, for different incident wave-fields, is in good agreement. The design framework can be then used to inform the design of a base-isolated building, based on the attainable isolation performance, at a limited computational cost. For the specific case of a purely vertical input motion at the base of the building (i.e. normally incident P-wave), the design framework leads to an approximate closed-form expression for the $PFIG$. The latter has conceptual significance and, together with the design framework presented in this dissertation, may form the basis for future guidelines on the design of base-isolated buildings against ground-borne vibration.
description: The author gratefully acknowledges the ﬁnancial support of WSP (https://www.wsp.com/en-GB) and EPSRC (RG75686) for the doctoral studentship.