Download - Base isolation for Engineers
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Base Isolation
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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
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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
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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
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The Concept of Base IsolationFixed Base
Base IsolatedPeriod
Res
pons
e Significantly Increase the Period of the Structure and the
Damping so that the Response is
Significantly Reduced
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Base Isolation in Buildings
Original Structure Isolated Structure
Isolation at foundation level
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Base Isolation in Buildings
Isolator Components Between the Foundation and Superstructure
An Isolation Interface is formed
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Base Isolation in Buildings
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Base Isolation in Buildings
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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
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Force – Displacement Relationship at the Isolation Interface
actual hysteretic behavior viscoelastic idealization
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Response of Base Isolated Buildings versus Fixed Base Response
Drift on Isolation Interface
Reduced Superstructure Deformations
for Base Isolated
Structure
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Most Common Types of Isolation Components (Isolators)
Isolation DevicesElastomeric Isolators Sliding Isolators
Natural Rubber Bearings
Low-Damping Rubber Bearings
Lead-Plug Bearings
High-Damping Rubber Bearings
Resilient Friction System
Friction Pendulum System
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Elastomeric Isolators – Lead Core Rubber Bearings
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Sliding Isolators – Friction Pendulum System
Superstructure
Foundation
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Friction Pendulum System
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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
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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
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Oakland City Hall
Isolation System: 111 rubber
isolation bearings 36 of them with
lead cores
Fixed Base Base Isolated
Earthquake Response
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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
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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
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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
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San Francisco City Hall
Four Retrofit Strategies were considered
Base Isolation
Fixed Base / Concrete Shear Walls
Fixed Base / Steel Braced Frames
Fixed Base / Steel Moment Frames
Flexible Story
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San Francisco City Hall
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San Francisco City Hall
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San Francisco City Hall
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