Seismic Evaluation and Retrofit of

Existing Tall BuildingsStephen A. MahinDirector, PEER

Matt SchoettlerResearcher Engineer, PEER

Jiun-Wei LaiPostdoctoral Researcher, PEER

Shanshan WangGraduate Student Researcher, UC Berkeley

OutlineBackground

Objective

As-built building evaluation

Retrofit strategy--viscous dampers

Concluding remarks

Tall Building Initiative (Phase 1)

Task 1 – Establish the Tall Buildings Project Advisory CommitteeTask 2 – Develop consensus on performance objectivesTask 3 – Baseline assessment of dynamic response characteristics of tall buildingsTask 4 – Synthetically generated ground motionsTask 5 – Review and validation of synthetically generated ground motionsTask 6 – Guidelines on selection and modification of ground motionsTask 7 – Guidelines on modeling and acceptance valuesTask 8 – Input ground motions for tall buildings with subterranean levelsTask 9 – Presentations at conferences, workshops, seminarsTask 10 – Performance-based seismic design guidelines for tall buildingsTask 11 – Instrumentation (under development)Task 12 – Quantification of seismic performance levels of tall buildings

Focus o

n Design

and Analysis

of

New Buildings

Assessing and Reducing Risk in Existing Tall Buildings

Many older tall buildings exist in China, the US, and elsewhere around the world

Do they pose an unacceptable seismic risk?

What are the implications of damage to

community resilienceCan they be economically upgraded?

Vulnerable Pre-1994 steel buildings

Potential Vulnerabilities of Existing BuildingsGlobal Behavior

Low design forces

Few drift limits imposed

Irregularities common at base of structure

Local DetailsBrittle pre-Northridge beam to column connections

“Strong column-weak beam” not used

Capacity design principles not used

Column splices not able to develop member capacity

Panel zones not seismically designed

Loads not adequately transferred to foundations

OutlineBackground

Objective

As-built building evaluation

Retrofit strategy--viscous dampers

Concluding remarks

Tall Building Initiative (Phase 2) Objectives

Selecting representative tall buildings

Evaluation of selected buildings

Comprehensive structural analysis

Assessment of seismic performance using PBEE

conceptsRetrofit strategies

Examination of cost-effectiveness to achieve

targeted performanceDevelop the best practices and guidelines

OutlineBackground

Objective

As-built building evaluation

Retrofit strategy--viscous dampers

Concluding remarks

Trial Buildings Examined (so far)Model Building 1:35-story SteelMoment Resisting FrameBuilt in 1971

Model Building 2:40-story SteelMoment Resisting FrameBuilt in 1973

Nonlinear Response History Analysis using OpenSees

120 sets of 3 component ground motions covering 6 seismic hazard levels are being used (J. Baker, Stanford)

Monotonic Pushover Results

X-Dir. Y-Dir.P-Δ effects importantBrittle connection poses great danger

Global Response Three Representative GM records

Drift Ratio Envelope (BSE-1E, 20%/50yr)

Model Building 1, brittle model

Global ResponseThree Representative GM records

Drift Ratio Envelope (BSE-2E, 5%/50yr)

Model Building 1, brittle model

Response History (BSE-2E, GM20)

Model Building 1, brittle model

OutlineBackground

Objective

As-built building evaluation

Retrofit strategy--viscous dampers

Concluding remarks

Retrofit With Viscous Dampers

Plan viewN

E-W View N-S View

Preliminary Retrofits Results(BSE-1E)Drift Ratio Envelope (X-Dir.)

Model Building 1, ductile model

No residual drift

Preliminary Retrofits Results(BSE-1E)Floor Acceleration Envelope (X-Dir.)

Roof acceleration reduced by 70%

Damp out within 20 sec

Model Building 1, ductile model

Column Compression D/C Check

Peak D/C ratio >0.75

Preliminary Retrofits Results(BSE-1E)

Model Building 1, ductile model

Column line considered

OutlineBackground

Objective

As-built building evaluation

Retrofit strategy--viscous dampers

Concluding remarks

Concluding RemarksMany vulnerabilities for the existing high-rise

steel moment framesThe existing building under investigation has a

tendency to form weak stories in 5-10 storiesBeam-column connections render the building

even more vulnerable to earthquakesViscous dampers are effective in controlling peak

story drift ratio and reduce residual driftViscous dampers also result in smaller

acceleration and more rapid decay of responses

Future WorkSystematic optimization of damper distribution

and layout Explore other possible retrofit strategies

Use Performance Assessment Calculation Tool

(PACT) to assess the cost and Return on Investment

Question?