Base Isolation

Base Isolation

Conventional Construction Practice assumes Fixed Base Structures

The Dynamic Characteristics of Fixed Base Structures are determined by the general characteristics of the structural system

The Designer can only slightly adjust those Dynamic Characteristics

As a matter of fact, most common Building Structures have unfavorable Dynamic Characteristics that result in increased Seismic Response

Earthquake Performance of Fixed Base Structures

Conventional Fixed Base Structures can not be realistically designed to remain elastic in large seismic events (more so

in regions of high seismicity)

Common practice is to design them so that they experience damage in a controlled manner and have large inelastic

displacements potential

Unconventional Earthquake Protective Systems

Base Isolation is the most common System

Earthquake Protective Systems

Passive Protective Systems Hybrid Protective Systems Active Protective Systems

Tuned Mass Damper

Energy Dissipation

Base Isolation

Active Isolation

Semi-Active Isolation

Semi-Active Mass Damping

Active Mass Damping

Active Bracing

Adaptive Control

The Concept of Base Isolation

Fixed Base

Base IsolatedPeriod

Res

pons

e

Significantly Increase the Period of the Structure and the

Damping so that the Response is

Significantly Reduced

Base Isolation in Buildings

Original Structure Isolated Structure

Isolation at foundation level


Isolator Components Between the Foundation and Superstructure

An Isolation Interface is formed



How exactly does Base Isolation Work?

• Isolators have large deformation potential

allowing for large drift on the Isolation Interface

Most types of Isolators exhibit nonlinear behavior

Lengthening of the Structure’s Period and increased Damping that result in a large scale decrease of the

Seismic Response

Force – Displacement Relationship at the Isolation Interface

actual hysteretic behavior viscoelastic idealization

Response of Base Isolated Buildings versus Fixed Base Response

Drift on Isolation Interface

Reduced Superstructure Deformations

for Base Isolated Structure

Most Common Types of Isolation Components (Isolators)

Isolation Devices

Elastomeric Isolators Sliding Isolators

Natural Rubber Bearings

Low-Damping Rubber Bearings

Lead-Plug Bearings

High-Damping Rubber Bearings

Resilient Friction System

Friction Pendulum System

Elastomeric Isolators – Lead Core Rubber Bearings

Sliding Isolators – Friction Pendulum System

Superstructure

Foundation

Friction Pendulum System

Oakland City Hall

First high-rise government office building in the United

States Tallest Building on the West Coast at the time of its construction in 1914 -

324 feet tall Riveted Steel Frame with

infill masonry walls of brick, granite and terracotta

Continuous Reinforced Concrete Mat foundation

Oakland City Hall

Building was severely damaged during the 1989 Loma Prieta earthquake

Building is listed on the historic register – Retrofit had to preserve the interior architecture and the historic fabric of the building

Both conventional fixed-base and base isolation retrofit concepts were studied

The most economical and effective method was determined to be base isolation

Oakland City Hall

Isolation System: 111 rubber

isolation bearings 36 of them with

lead cores

Fixed Base Base Isolated

Earthquake Response

Oakland City Hall

Critical Construction Issues: Temporary lateral bracing during construction period to

safeguard against possible earthquake occurrence Symmetric work sequence was important to reduce the

possibility of torsional response in the event of an earthquake

Vertical column displacement during jacking was limited to around 0.10 inches to prevent damage to

superstructure finishes

San Francisco City Hall Built in 1915 Today recognized as

one of the most notable examples of classical architecture in the U.S.

Nearly 300 feet tall Steel Frame with

concrete slabs Spread interior

footings – Strip perimeter foundation

San Francisco City Hall

The original design of the building incorporates a “soft story” approach at the main floor, intended to dissipate energy

This alongside other discontinuities in the structural system make the dynamic characteristics of the building unfavorable

The Building was severely damaged during the 1989 Loma Prieta earthquake


Four Retrofit Strategies were considered

Base Isolation

Fixed Base / Concrete Shear Walls

Fixed Base / Steel Braced Frames

Fixed Base / Steel Moment Frames

Flexible Story




Based on the results of the analysis, the Base Isolation Scheme was selected as best meeting the retrofit objectives and

providing the most favorable performance for the least relative cost

Base Isolation is considered a particularly effective strategy when applied to massive (and

rather stiff) structures

Documents

Base Isolation