Base isolation is an earthquake protection system that involves installing seismic isolation bearings between a structure's foundation and superstructure. This significantly increases the natural period of the structure and adds damping, reducing the seismic response. Common base isolation bearings include elastomeric isolators and sliding isolators. Two examples given are the historic Oakland City Hall and San Francisco City Hall, which were retrofitted with base isolation systems after being damaged in the 1989 Loma Prieta earthquake. This helped preserve their historic integrity while improving seismic performance.

1. Base Isolation

2. 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

3. 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

4. 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

5. The Concept of Base Isolation
Fixed Base
Base Isolated
Period
Response
Significantly Increase
the Period of the
Structure and the
Damping so that the
Response is
Significantly Reduced

6. Base Isolation in Buildings
Original Structure Isolated Structure
Isolation at foundation level

7. Base Isolation in Buildings
Isolator Components Between the Foundation and Superstructure
An Isolation Interface is formed

8. Base Isolation in Buildings

9. Base Isolation in Buildings

10. 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

11. Force – Displacement Relationship
at the Isolation Interface
actual hysteretic behavior viscoelastic idealization

12. Response of Base Isolated Buildings
versus Fixed Base Response
Drift on Isolation Interface
Reduced
Superstructure
Deformations
for Base
Isolated
Structure

13. 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

14. Elastomeric Isolators
– Lead Core Rubber Bearings

16. Sliding Isolators
– Friction Pendulum System
Superstructure
Foundation

17. Friction Pendulum System

18. 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

19. 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

20. Oakland City Hall
Isolation System:
 111 rubber
isolation bearings
 36 of them with
lead cores
Fixed Base Base Isolated
Earthquake Response

21. 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

22. 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

23. 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

24. 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

25. San Francisco City Hall

26. San Francisco City Hall

27. 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