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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

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conventional construction practice assumes fixed base structures
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
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
Unconventional Earthquake Protective Systems

Base Isolation is the most common System

the concept of base isolation
The Concept of Base Isolation

Fixed Base

Significantly Increase the Period of the Structure and the Damping so that the Response is Significantly Reduced

Response

Period

Base Isolated

base isolation in buildings
Base Isolation in Buildings

Original Structure

Isolated Structure

Isolation at foundation level

base isolation in buildings7
Base Isolation in Buildings

Isolator Components Between the Foundation and Superstructure

An Isolation Interface is formed

how exactly does base isolation work
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

response of base isolated buildings versus fixed base response

Drift on Isolation Interface

Response of Base Isolated Buildings versus Fixed Base Response

Reduced Superstructure Deformations for Base Isolated Structure

oakland city hall
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 hall19
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 hall20
Oakland City Hall

Earthquake Response

Isolation System:

  • 111 rubber isolation bearings
  • 36 of them with lead cores

Fixed Base

Base Isolated

oakland city hall21
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
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 hall23
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 hall27
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