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Base Isolation

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  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

  5. 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

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

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

  13. Most Common Types of Isolation Components (Isolators)

  14. Elastomeric Isolators – Lead Core Rubber Bearings

  15. Sliding Isolators – Friction Pendulum System Superstructure Foundation

  16. Friction Pendulum System

  17. 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

  18. 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

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

  20. 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

  21. 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

  22. 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

  23. San Francisco City Hall

  24. San Francisco City Hall

  25. San Francisco City Hall

  26. 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