Base Isolation Method
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
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Base Isolation is the most common System
The Concept of Base Isolation
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
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Isolator Components between the Foundation and Superstructure
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Fig: An Isolation Interface is formed
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How exactly does Base Isolation Work?
Most types of Isolators exhibit nonlinear behavior
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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
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Response of Base Isolated Buildings versus Fixed Base Response
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Most Common Types of Isolation Components (Isolators)
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Elastomeric Isolators –
Lead Core Rubber Bearings
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Sliding Isolators
– Friction Pendulum System
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Friction Pendulum System
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Examples of Buildings on Base Isolators
1. 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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  • 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
Isolation System:
  • 111 rubber isolation bearings
  •  36 of them with lead cores
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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
2. 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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  • 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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  • 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