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 Base Isolation is the most common System Earthquake Protective Systems Passive Protective Systems Tuned Mass Damper Energy Dissipation Base Isolation Hybrid Protective Systems Active Isolation Semi-Active Isolation Semi-Active Mass Damping Active Protective Systems Active Mass Damping Active Bracing Adaptive Control

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

Base Isolation in Buildings Original StructureIsolated 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

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

Force – Displacement Relationship at the Isolation Interface actual hysteretic behaviorviscoelastic idealization

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

Most Common Types of Isolation Components (Isolators) Isolation Devices Elastomeric Isolators Natural Rubber Bearings Low-Damping Rubber Bearings Lead-Plug Bearings High-Damping Rubber Bearings Sliding Isolators Resilient Friction System Friction Pendulum System

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 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 Isolation System: 111 rubber isolation bearings 36 of them with lead cores Fixed BaseBase Isolated Earthquake Response

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

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