ISO 23618:2022 and Seismic Isolation Devices – A Comprehensive Introduction

Earthquakes remain among the most destructive natural hazards for built environments. Conventional seismic design philosophy accepts structural damage as long as collapse is prevented. However, in recent decades society has demanded better resilience: buildings should not only protect life, but also remain operational immediately after earthquakes.

To meet this goal, engineers worldwide have developed seismic isolation systems. Rather than strengthening every structural member, isolation aims to decouple the structure from ground shaking, reducing transmitted forces. This concept, once novel, has matured into global practice, with thousands of buildings, bridges, and industrial facilities protected.

In 2022, ISO published ISO 23618:2022 – Bases for design of structures - General principles on seismically isolated structures. This international standard consolidates decades of knowledge into a coherent framework, covering design, analysis, construction, and maintenance. It highlights the role of rubber bearings, sliding devices, and dampers as essential isolation products.

This document provides a detailed introduction to ISO 23618, with emphasis on isolation devices such as high-damping rubber bearings (HDRBs), lead-rubber bearings (LRBs), and viscous dampers. It also situates ISO 23618 alongside European and American standards, and provides case studies, performance discussions, and future directions.

ISO 23618 applies to horizontal seismic isolation of buildings and certain structures. It excludes vertical isolation, LNG tanks, and most bridges (though principles may be adapted). The standard stresses that isolation is not an add-on, but a system-level strategy that must be integrated at the earliest design stage.

The philosophy rests on three pillars:
1). Lengthening of structural period – shifting the natural period to 2–3 s or more, reducing accelerations.
2). Increasing damping – dissipating seismic energy to limit displacement.
3). Ensuring re-centring – returning to original position after shaking.
 

The isolation layer lies between superstructure and substructure, sustaining vertical loads, permitting horizontal displacement up to 600 mm, and providing rotational flexibility. The substructure must be stiff and strong, deformation should concentrate at the isolators, and clearance gaps (moats) must prevent pounding. Performance is defined for SLS, ULS, and MCE conditions.

ISO 23618 allows equivalent linear analysis (effective stiffness and damping) and nonlinear time-history analysis. Device properties are calibrated from tests.

Devices include elastomeric bearings (HDRBs, LRBs), sliding pendulums, linear guides, and dampers. Each must satisfy load capacity, displacement, energy dissipation, durability, and quality control.
5.1 High-Damping Rubber Bearings (HDRBs)
HDRBs consist of rubber and steel laminations. Modified rubber compounds provide 15–18% damping. They support vertical loads up to tens of MN, allow horizontal shear modulus ~0.4–1.0 MPa, and are durable. Common in schools, offices, and towers.
5.2 Lead-Rubber Bearings (LRBs)
LRBs incorporate a central lead plug that yields at 10–12 MPa, dissipating energy. Damping is 10–20%. Reliable, tunable, and widely applied in hospitals and emergency centers. Example: Wellington Hospital, New Zealand.
5.3 Curved Surface Sliders (Pendulum Systems)
Spherical sliding systems generate restoring force via gravity. Period depends on radius of curvature, independent of mass. Friction coefficients 0.02–0.06 provide damping. Displacement capacity up to 600 mm or more. Used in airports and stadiums.
5.4 Supplemental Dampers
Viscous dampers dissipate energy through velocity-dependent fluid resistance. Hysteretic dampers use yielding steel. Friction dampers slide under controlled friction. These enhance isolation where damping is insufficient.

ISO 23618 complements regional codes:
- EN 15129:2009 + AC:2010 (Europe) – Anti-seismic devices standard covering bearings, sliders, dampers.
- ASCE/SEI 7-22 (USA) – Design loads including Chapter 17 on isolation.
- AASHTO LRFD Guide Specifications (2014, USA) – Seismic isolation design for bridges.
ISO aligns conceptually with both, focusing on performance-based design.

Hospitals: Kaiser Permanente Hospital (California) uses pendulum isolation.
Bridges: Akashi Kaikyō Bridge (Japan) with viscous dampers.
Heritage: Salt Lake City & County Building (Utah, USA) retrofitted with isolation.
Nuclear: Japanese plants use isolation for critical safety.

ISO 23618 requires type tests (full-scale cyclic), routine factory tests, construction supervision, and maintenance inspections. EN 15129 adds CE-marking and Factory Production Control. US standards demand qualification testing.

Wind effects – isolators must not displace under service wind. Fire – elastomer needs protection. Mid-storey isolation – practical for tall buildings, requiring careful diaphragm and load path design.

Benefits: Reduced accelerations (50–70%), protection of equipment, operational continuity, retrofit adaptability.
Limitations: Higher cost (5–10%), space requirements for moat, maintenance, less effective on soft soils.

Hybrid systems combine rubber and sliders. Smart dampers (magnetorheological fluids) enable semi-active control. Tall building mid-storey isolation is emerging. Devices are being designed for multi-hazard resilience (earthquake + wind + fire).

ISO 23618:2022 codifies global best practices in seismic isolation. Its philosophy emphasizes extending period, adding damping, and ensuring recentering. By linking with ISO 22762 for elastomeric bearings and aligning with EN 15129 and ASCE 7, it fosters global harmonization. Isolation is not just a structural solution, but a resilience strategy for society.