Tuned-Mass Damper Design-A Case Study

7/22/2019 Tuned-Mass Damper Design-A Case Study

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Tuned-Mass Damper DesignA Case Study

Dr. James (Jay) Lamb

Structural Engenuity, Inc.

(972) 247-9250 x212

[email protected]
http://www.structuralengenuity.com/http://www.structuralengenuity.com/


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Tuned-Mass Damper DesignCase Study, 2

Agenda

What is a Tuned-Mass Damper?

Case Study: TETRA Technologies Project

Initial Site SurveyWill a Tuned-Mass Damper Work?

Tuned-Mass Damper Design and Analysis

Prototype Testing

Installation and Performance Verification

Summary and Conclusions


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Tuned-Mass Dampers

m

kf

Spring

TMD

2

1

Mass and Coil Spring PendulumMass and Flexure

L

g

fTMD2

1

mL

EIfTMD 3

48

2

1

A tuned-mass damper is a mass-spring-damper system that is attached to a structure

to reduce the amplitude of undesirable motion

The mass, spring stiffness, and damping factor must be tuned relative to the existing

structures dominant mode (frequencyfModefTMD) responsible for the motion

The location on the structure where the TMD(s) is/are attached is critical

TMDs can have many different forms depending upon the application:

A very compact form of TMD; ideal

for space-limited applications or

when concealment is criticalProbably the least expensive form of

TMD; can be tailored for almost any

application

Ideal for low-frequency

applications like tall

buildings or flexiblewalkways

L

L


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TETRA Technologies CaCl2 Plant

SEI Asked to Perform a Site Vibration Survey, Identify the Cause of

the Problem, and Provide Recommendations for Possible Solutions

Control room swayed side-

to-side immediately when

plant opened

Motion persists throughout

the day and night

Staff irritated by level ofmotion and complained to

management

Original engineers tried and

failed to solve the problem

Control Room

Site Overview


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Site SurveyProblem Diagnosis (1/2)

SEI Measured and Identified all Significant Sources of Vibration;

The 3.5-Hz Motion is the Primary Source of the Staffs Discomfort

Motion

Measured vibration data at foundation,

along a column, and in the control room

Power Spectrum

Control Room and Structural Frame


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Site SurveyProblem Diagnosis (2/2)

Vibration Near 3.5 Hz is 3 Times Higher than the Human-Comfort Limit;

Ground-Borne Vibration Excites the Sway Mode of Structural Frame

Need to Reduce Vibration by 70%Tuned-Mass Damper is Practical Solution

Human Perception Criteria

Front-to-Back

Criteria

0.005-g

Limit

Measured Control Room Vibration (3.5 Hz)

Limit = 0.005 g

Data filtered around 3.5 Hz


7/16Tuned-Mass Damper DesignCase Study, 7

Structural Dynamics Model of Existing Building

Finite Element ModelControl Room Mass

(both sides)

12 ft

28 ft

24 ft

18 ft

Structural member properties taken fromexisting-structure drawings

Mass of cables and pipes (not shown) at each

level estimated from photographs

Mass of prefabricated control room (not shown)

obtained from manufacturer; additional mass of

fit-out estimated

Structural Model has Same Stiffness and Mass Properties as Existing Structure;

Only 3 Bays Modeled Because They Act Independently in East/West Direction



Model Validation via Frequency Response

Frame Sway Mode (3.5 Hz) Frequency Response

3.5 Hz

Model Parameters Adjusted so Models Sway Mode Matches the Measured

Motion of 3.5 HzModel can now be Used to Design/Assess TMDs

Motion at top (control

room) is magnified by

factor of 85 relative to

motion of foundation

Ratio of control room motion

to foundation motion

|H(f)|



TMD Conceptual Design

Simple Design of Flexure-Based TMD Minimizes Cost and Performance

can be Verified During Prototype Testing Before Final Installation

Damping

in joint

FlexureBars

Mass

Attach to

Existing Bldg

Flexure-type (cantilever) TMD is

appropriate for this structure

Constrained-layer damping is

incorporated into joint

Flexure bars must be stiffer to

compensate for joint flexibility

East/West flexural mode (fTMD)required to be 3.4 Hz ( 3.5 Hz)

Place 3 TMDs on the columns

supporting the control room

Idealized

Model



Optimum TMD Performance

ReinforcementTMD and Bldg

Move in Phase

CancellationTMD Opposes Bldg

Motion

DampingTMD and Bldg

90 Out of Phase

Reduction in Vibration(Increases With TMD Mass)

TMD

Mass

TMD

Mass

BldgBldg

Original Bldg

Bldg with TMD

TMD Has No Effect at Frequencies Belowor Above the Tuning Frequency

Analysis Indicates TMDs Reduce Vibration by 90% of Initial

Level Near 3.5 HzReal Performance will not be this Good

In-Phase Mode Out-of-Phase Mode

|H(f)|



TMD Internal Damping Optimization

Maximum

Reduction

Original Bldg

Out-of-phase modeIn-phase mode

There is an Optimal Level of Internal Damping, but 8% to 16% Critical

Damping Usually Yields a Robust Range for Very Good Performance

Frequency Response: Effect of TMD Damping

If TMD damping is too low, bothpeaks for the in-phase and out-

of-phase modes will be present

Optimal damping produces a

nearly flat curve

If damping is too high, the twomodes merge into a single peak

TMDs made of steel or alum-

inum usually require additional

damping be incorporated

|H(f)|


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Vibration Mitigation Effectiveness = TMD Mass

Select TMD Mass to Achieve Desired Mitigation Over Narrow Band (1 Hz)

Need to Reduce Vibration by at Least 70% HereUse 1500 lbm/TMD

Results for optimized dampingfor each TMD mass

Results (1-Hz Bandwidth):190 lbm 47% reduction

375 lbm 55% reduction

750 lbm 63% reduction

1500 lbm 71% reduction3000 lbm 78% reduction

Selection of bandwidth is

somewhat arbitrary

Increment of improvement in

vibration mitigation diminishes

with increasing mass

Frequency Response: Effect of TMD Mass

Determine vibration

reduction over band for

broadband excitation

High

Low

High

Low

f

f

f

fm

dffH

dffHR

2

0

2

)(

)(1

|H(f)|

fLow fHigh


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Prototype TMD Testing45-in Long Flexure

41-in Long Flexure

31-in Long Flexure

Constrained-Layer

Damping

SBR Rubber Layers and a Flexure Bar of 37.5 inches Identified

as Best Combination and Provides About 12% Damping

SEI Tested Various Combinations of TMD Flexure Bar Lengths and

Constrained-Layer Damping Materials to Find Best Combination


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Installation and Performance Assessment

Tuned-Mass Dampers Successfully Reduce the Vibration in the Control Room

Below 0.005-g Limit; Staff Report Environment is Significantly Better Now

TMDs Installed on Structure Before/After Vibration

SEI tested the TMDs after installation to verify the tuning. Data were also

acquired in the control room for comparison with the original motion


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Summary and Conclusions

Tuned-mass dampers can be fabricated in many different forms

based on the physical and aesthetic constraints of the application

Tuned-mass dampers are viable vibration mitigation solutions whenthe motion is caused by a low-damped mode of the structure

Design process for tuned-mass dampers

Site Survey: Measure the frequency and magnitude of the undesirable motion

Analyze/Design: Develop model of existing structure and determine the TMDmass and placement of TMD(s) to achieve vibration mitigation requirement

Test: Perform prototype testing of the TMD to fine-tune the design

Install/Verify: Measure the motion of the TMD(s) on the structure to confirmperformance and that the mitigation objective was achieved

Expect 70% to 80% reduction in the vibration after installation

Weight of TMD is typically about 5% to 10% of effective weight ofmode responsible for the motion


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Questions? Contact SEI

Please contact us with any vibration mitigation issues you

have and let us help you to resolve them

Structural Engenuity, Inc.

Dr. James (Jay) L. Lamb

Office: (972) 247-9250

Cell: (214) 412-8388

[email protected]

www.structuralengenuity.com
mailto:[email protected]:[email protected]

Tuned-Mass Damper Design-A Case Study

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