A simple strategy to tune the lateral response of unbonded Fiber Reinforced Elastomeric Isolators (FREIs)

Chris Tran,Andrea Calabrese,Michalis F Vassiliou,Simone Galano

doi:10.1016/j.engstruct.2020.111128

Abstract

The study of Fiber Reinforced Elastomeric Isolators (FREIs) has gained momentum over the last decade for their potential to be adopted directly under the foundation of masonry buildings with no need for costly rigid diaphragms and transfer structures, which are required for the application of conventional base isolation (BI) devices. While the possibility of base isolating masonry structures with no diaphragms at the isolation level is fascinating, this requires FREIs with a tunable response in all 3 directions (i.e., devices that can be engineered to perform in a specific way in each direction of loading). It is known that unbonded FREIs exhibit softening and instability under large displacements. While softening of the bearings is beneficial because it reduces the force demand on structural elements, instability is not. This is particularly true as the unstable branch of FREIs is followed by a sharp hardening response. With the aim of contributing (i) towards the development of tunable FREIs and (ii) towards the development of FREIs with tunable hardening under large lateral deformations, this manuscript describes the findings of a wide set of Finite Element Analyses (FEAs) performed to assess the effectiveness of modifying the stiffness and the lateral response of FREIs through lateral holes. Results of the analyses show that, a simple to implement strategy can be used to tune the lateral response of FREIs to a desired hardening and lateral displacement capacity. The findings of this work constitute a step towards the development of scalable technologies for the isolation of non-engineered buildings in developing regions of the world.

Highlights

Seismic isolation (SI) involves the installation of highly flexible bearings at the base of the structure to decouple the structure from the ground motion (e.g.[1,2])
The study of Fiber Reinforced Elastomeric Isolators (FREIs) has gained momentum over the last decade for their potential to be adopted directly under the foundation of masonry buildings with no need for costly rigid diaphragms and transfer structures, which are required for the application of conventional base isolation (BI) devices
With the aim of contributing (i) towards the development of tunable FREIs and (ii) towards the development of FREIs with tunable hardening under large lateral deformations, this manuscript describes the findings of a wide set of Finite Element Analyses (FEAs) performed to assess the effectiveness of modifying the stiffness and the lateral response of FREIs through lateral holes

Summary

Introduction

Seismic isolation (SI) involves the installation of highly flexible (or slippery) bearings at the base of the structure to decouple (isolate) the structure from the ground motion (e.g.[1,2]). In an effort to reduce the cost of conventional rubber-based bearings, Kelly [4] suggested that the heavy steel reinforcement can be replaced by a flexible fiber reinforcement, to construct the so called “Fiber Reinforced Elastomeric Isolators (FREIs)”. The concept on which this research is based is that, by creating a pattern of square holes in FREIs, it should be possible (i) to achieve the desired orthotropic behavior, (ii) to modify the behavior of slender FREIs so to have hardening past the peak response with no instability, (iii) to reduce the stiffness in the out of plane direction allowing the use of these devices directly under masonry walls in absence of strong wall beams and diaphragm.

Finite element analysis

Verification of the model without holes

Lateral response of FREIs with horizontal holes

Findings

Conclusions