seismic isolation bearings

D.S. Brown Company Seismic Isolation Bearings

Seismic Isolation Bearings1. Basic Theory2. Isolation Devices

Benefits of Seismic Isolation3. Compared to standard

design methods4. Comparison between

LRB and other devices

Overview

Seismic isolation technology involves:Placing the isolators between the superstructure and substructures in place of regular bearings

Basic Theory

Seismic isolation technology involves:Insertion of devices (isolators) that provide:

• Period shift – by means of low lateral stiffness bearings

• Added damping – hysteretic, friction, viscous

• Displacement capacity – by means of rubber or slidingAll in one, self-contained, maintenance-free unitReplacement not needed after seismic event(s)

Basic Theory

Increasing Period of Vibration

Acce

lera

tion

Typical Fixed-Base Structure

Typical Isolated Structure

Basic TheoryA. Shifting the fundamental period of the structure

Idealized Displacement Response Spectrum

Increasing Period of Vibration

Disp

lace

men

t

Typical Fixed-Base Structure

Typical Isolated Structure

With Added Damping

Idealized Displacement Response Spectrum

Basic TheoryB. Adding damping to control displacements

– Maintenance-Free– Good for Life-Span of Structure– No need for Replacement after

Earthquake– Proven Performance (shaking-

table + real earthquakes)

Description of the TechnologyMost Commonly used Isolator System:Elastomeric Isolators with Lead CoresAdvantages

Applications• Function Critical Structures

- Lifeline bridges, hospitals, emergency and first response structures

• Critical Operational Structures- Data storage centers- Utilities- Communication towers- Military or scientific structures- Piers

• Historical or High-Value Structures- Historical bridges and buildings- Museums, mints, national treasures- Petroleum, gas, chemical structures

In the USA and worldwide, LRB are most often selected

D.S. Brown:• Over 80 Bridge projects since 1995• Thousands of Isolators• Fabricating and Testing isolators since 1986• Design, Fabrication and Testing since 1995

LRB has the Longest Track Record worldwideIn earthquakes, structures with LRB performed very well

Where does D.S. Brown fit in?

Typical Configuration

Fabricating Multiple Isolators at Once

Finished Lead Rubber Isolators

A. Why try to reduce seismic forces to the substructures?

• Reduce or eliminate strengthening during retrofit

• Reduce substructure dimensions

• Reduce footing and pile requirements

• All of the above

Each of these benefits will SAVE MONEY and TIME.

Reduce construction time and avoid traffic restrictions.

Benefits of Seismic Isolation

B. Why redistribute seismic and service loads in bridges?

• Seismic loads can be sent to strongest substructures

• Lateral service loads can be redistributed

This SAVES MONEY and construction TIME. Use existing

capacity and send more force to stronger locations in order

to eliminate or minimize retrofit of weaker substructures.


C. Why deal with something new when we can design a code compliant bridge without isolators?

Because isolators actually reduce substructure forces whereas traditional design methods do not. An isolated structure will perform better because the isolation bearings will:

• Reduce or eliminate inelastic deformation

• Reduce damage from earthquake

• Permit immediate use after earthquake

Reduce or eliminate repair COSTS and TIME


Summary:

• Can reduce the cost of construction

• Avoid retrofit and reduce traffic disruptions

• Eliminate damage and reduce losses from earthquakes

Uninterrupted operation after an earthquake

Revenue from bridge tolls

Use by emergency vehicles

Use by construction companies to reach affected areas


Golden Gate Bridge, CA South ApproachLRB Isolation Bearings

Lead-Rubber Isolators are “special” Elastomeric Bearings• Design approach was very different from Methods A & B• AASHTO LRFD design is now similar to AASHTO Isolation GS• Detailed design calculations required for each isolator• Shear testing is done on each job, typically 100% tested

As a result, isolators have:• Higher capacity for axial loads and shear displacement• Higher cost

How are Isolators Different from “Bearings”?

Q.C. Test: Combined Compression & Shear

Dynamic Testing in Compression & Shear

Spectrum of Isolation ProjectsVariables include: Axial Load, Seismic Displacement, Plan Dimensions, Height of Isolators, Economy

LRB is Suitable and Economical for Most Projects

Extremely High LoadVery Large DisplacementSevere Height LimitationVery Low Stiffness

Extremely Small LoadVery Large DisplacementVery Low StiffnessSevere Height Limits

Applicability of LRB

Advantages of LRB Isolators over Other Systems

HISTORYSIMPLICITYRELIABILITYTOLERANCE OF IMPERFECTIONS• Long History based on Steel-Laminated Bearings• Similar Materials (NR, shims) as Steel Laminated• First Isolation Systems in US were LRB & HDR• Proven Performance in Actual Earthquakes

How does LRB compare to other systems?

1978

Lead

-Rub

ber Is

olator

Paten

ted in

US

1985

DIS L

icens

ed to

sell L

RB in

US

Firs

t Isola

ted B

ridge

in U

S (LR

B)

Fi

rst Is

olated

Buil

ding i

n US (

HDR)

“Erad

iquak

e” D

evice

Paten

ted

1986 1987

Frict

ion Pe

ndulu

m Paten

ted

Approx. 300 Isolated Structures in USAApprox. 2000 Structures Isolated Worldwide

Present1995

Kobe

Earth

quak

e

General Timeline of Seismic Isolation

• No Wear or Leakage Concerns• No Moving Parts• No Maintenance or Replacement Parts Required• Protected on All Sides by ½” Integral Cover Rubber

How does LRB compare to other systems?Advantages of LRB Isolators over Other Systems

HISTORYSIMPLICITYRELIABILITYTOLERANCE OF IMPERFECTIONS

• No Need to Replace or Repair after an Earthquake• Isolator Properties are Unchanged after Many Cycles



Direction 1

• Installation the Same as an Elastomeric Bearing• Sacrificial Cover Rubber• Extra Capacity for Load, Rotation, Displacement• Will Support Axial Load after “Failure”



Tolerance for Imperfections: Example

Tolerance for Imperfections: ExampleAs Installed by Contractor & Opened to Traffic

After Placing Grout

• The Contractor had already removed jacks, falsework, etc.

• The Owner wanted to know if the Isolators are OK

D.S. Brown needed to find out how will they perform under axial and shear loads.


DSB fabricated 2 identical isolators and tested them:• In the un-deformed state, and• After deforming the masonry plates as seen in the bridge


This Damage Caused No Appreciable Change in Performance

-100

-75

-50

-25

0

25

50

75

100

-3 -2 -1 0 1 2 3

Shear Displacement (inches)

Shea

r Fo

rce

(kip

s)

Prior to Deformation of Masonry Plate

After Deformation of Masonry Plate

Corporate Office300 East Cherry StreetNorth Baltimore, OH

45872

Websitewww.dsbrown.com

http://www.dsbrown.com/

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