Transcript Base Isolation Conventional Construction Practice assumes Fixed Base Structures  The Dynamic Characteristics of Fixed Base Structures are determined by the general characteristics.

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
Earthquake Protective Systems
Passive Protective Systems
Hybrid Protective Systems
Active Protective Systems
Tuned Mass Damper
Active Isolation
Active Mass Damping
Energy Dissipation
Semi-Active Isolation
Active Bracing
Base Isolation
Semi-Active Mass Damping
Adaptive Control
Base Isolation is the most common System
The Concept of Base Isolation
Fixed Base
Significantly Increase
the Period of the
Structure and the
Damping so that the
Response is
Significantly Reduced
Period
Base Isolated
Base Isolation in Buildings
Original Structure
Isolated Structure
Isolation at foundation level
Base Isolation in Buildings
Isolator Components Between the Foundation and Superstructure
An Isolation Interface is formed
Base Isolation in Buildings
Base Isolation in Buildings
How exactly does Base Isolation
Work?
• Isolators have large
Most types of
Isolators
deformation
potential
exhibit nonlinear
allowing
for large behavior
drift on
the Isolation Interface
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
Reduced
Superstructure
Deformations
for Base
Isolated
Structure
Drift on Isolation Interface
Most Common Types of Isolation
Components (Isolators)
Isolation Devices
Elastomeric Isolators
Sliding Isolators
Natural Rubber Bearings
Resilient Friction System
Low-Damping Rubber Bearings
Friction Pendulum System
Lead-Plug Bearings
High-Damping Rubber Bearings
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
Earthquake Response
Isolation System:
 111 rubber
isolation bearings
 36 of them with
lead cores
Fixed Base
Base Isolated
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
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
San Francisco City Hall
San Francisco City Hall
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