implementation of base isolation in retrofit applications · forell / elsesser engineers, inc....

Forell / Elsesser Engineers, Inc.

100th Anniversary Earthquake Conference

Commemorating the 1906 San Francisco Earthquake

Implementation of Base Isolation Implementation of Base Isolation in Retrofit Applicationsin Retrofit Applications

Presented bySimin Naaseh, SE, Principal Mason Walters, SE, Principal

Forell/Elsesser Engineers, Inc.April 17th, 2006


Impetus for Base Isolation of Existing Buildings

Enhanced seismic protectionEnhanced seismic protection

Reduced extent of Reduced extent of superstructure retrofitsuperstructure retrofit

Protection of historic fabricProtection of historic fabric

Protection of valuable contentsProtection of valuable contents

Cost advantageCost advantage


Technical Issues Unique to Base Isolation Retrofit

Isolation system is designed Isolation system is designed to meet certain thresholdsto meet certain thresholds

Limited existing structure Limited existing structure capacitycapacity

Unique dynamic response of Unique dynamic response of the structurethe structure

Limits on floor accelerationLimits on floor acceleration

Limits on moat dimensionLimits on moat dimension


Base Isolation RetrofitPlanning and Design Challenges

Accommodating the isolator Accommodating the isolator spacespace

Accommodating the moatAccommodating the moat

Retrofitting the building Retrofitting the building appendages to accommodate appendages to accommodate free movement free movement

Retrofitting stairs and elevators Retrofitting stairs and elevators to accommodate large to accommodate large displacementsdisplacements


Base Isolation RetrofitConstruction Challenges

Minimize building vulnerability Minimize building vulnerability during constructionduring construction

Minimize impact on brittle Minimize impact on brittle elementselements

Isolator installation sequenceIsolator installation sequence


Seismic Retrofit of San Francisco City Hall


Historic Rotunda


Major Earthquake Response Issues


Fixed Base Response - Shear Walls


Major Earthquake Response Issues


Fixed Base Response


Base Isolated Response


Sequence of Construction


New Foundation


Column Jacking


Column Cutting


Installation of Base Isolators


Perimeter Shoring


Perimeter Side Beams


Installed Isolators at Perimeter


New Ground Floor


New Shear Walls


Berkeley Civic Center


C

Isolators at Basement

Section Looking West


C

Basement Wall / Jacking Pier Construction Sequence


C

Column Jacking / Isolator Installation Construction Sequence


Existing Condition


Install Foundation Reinforcing


New Foundation Built


Install Formwork for Jacking Piers Around Existing Columns


Jacking Pier Construction


Jacking Pier Built


Temporary Jacks Installed


Existing Column Cut


Isolator Installed


Isolator Locking Plates Installed


Steel Bracing Installed


Locking Plates Removed


Isolators Installed


Base Isolation Response at Perimeter Wall


Design an ConstructionProblems and Solutions


Construction Problems and Solutions

Sequence of operationsSequence of operations

Symmetry and balanceSymmetry and balance

Cutting and jackingCutting and jacking

PreloadingPreloading

Temporary lateral strengthTemporary lateral strength


Structure Types

Unreinforced Unreinforced masonry bearing wallmasonry bearing wall

Steel / masonry infillSteel / masonry infill

Reinforced concreteReinforced concrete


Example: Unreinforced Masonry Bearing Wall Building


Salt Lake City and County Building


Bearing Wall Sequence


Isolator Installed at Perimeter


Example: Steel Framed Building

with Masonry Infill


Oakland City Hall


Identification of Existing Building Properties

Archaic materials and infill systemsArchaic materials and infill systems

Structure’s Modal frequenciesStructure’s Modal frequencies

Forced/Ambient Vibration TestsForced/Ambient Vibration Tests

Historic DataHistoric Data

Old Old AccelerogramsAccelerograms

Masonry infill example Masonry infill example


Identification of Existing Building Properties

Tests of Existing MasonryTests of Existing Masonry

InIn--situ cyclic testssitu cyclic tests

Obtain shear stressObtain shear stress--strain relation, Gstrain relation, G

Obtain diagonal cracking strain,Obtain diagonal cracking strain, ggcrcr

Use G for elastic modeling of structureUse G for elastic modeling of structure

Use Use ggcrcr to determine the limits of to determine the limits of acceptable interacceptable inter--story driftstory drift


In-Situ Masonry Test


Stress Versus Deflection


Local Wall Model


ETABS Model

(Non-Linear ANSR)


Jacking of Existing Column


Column Cutting


Example: Reinforced Concrete

Frame Buildings


Pasadena City Hall


Building Section Showing Load Transfer

8

3

(E) Column

7

2

59

6

1(E) Footing (to be Removed)

1

(E) Wall

(E) Basement Grade

1 Shoring

2 Foundation

3

4

5

6

7

8

9

(E) Wall

Moat Cover

4

Grade Beam & S.O.G. Friction Pendulum Isolator PT Threadbar(Clamping Reinforcing)

Concrete Pedestal

Temporary Shoring

PT Conc. Transfer Girder

Concrete Slab


Installation of Isolators


Utah State Capital Building


Overall Basement

Plan


Big Idea

Rotunda base must be isolated with Rotunda base must be isolated with remainder of buildingremainder of building

Main problem Main problem –– getting isolators “beneath” getting isolators “beneath” rotunda footingrotunda footing

“Below” and “Above” approaches studied“Below” and “Above” approaches studied

“Above” approach safer to build, requires “Above” approach safer to build, requires less excavationless excavation


Big Idea

Existing concrete has relatively good Existing concrete has relatively good quality, strengthquality, strength

Maintain footing Maintain footing –– make it work as part of make it work as part of new transfer girdernew transfer girder

“Composite”“Composite”

Allow new transfer structure to span to Allow new transfer structure to span to isolators on new foundations (fig)isolators on new foundations (fig)

New foundation can be built ahead of New foundation can be built ahead of transfer transfer –– outside existing footingoutside existing footing


Existing Footing

How to Achieve Composite Action and Transfer Load?

Stress Girders


Locate and Mark PT Ducts on Flange

How to Achieve Composite Action and Transfer Load?


Rotunda Footing


100th Anniversary Earthquake Conference

Commemorating the 1906 San Francisco Earthquake

Implementation of Base Isolation Implementation of Base Isolation in Retrofit Applicationsin Retrofit Applications

Presented bySimin Naaseh, SE, Principal Mason Walters, SE, Principal

Forell/Elsesser Engineers, Inc.April 17th, 2006

implementation of base isolation in retrofit applications · forell / elsesser engineers, inc....

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