nchrp --idea project 141: signal head vibration …sp.bridges.transportation.org/documents/2013...

Reducing Fatigue in Traffic Signal Reducing Fatigue in Traffic Signal

Support Structures through the use Support Structures through the use

of a Signal Head Vibration Absorberof a Signal Head Vibration Absorber

NCHRPNCHRP--IDEA Project 141: Signal Head IDEA Project 141: Signal Head

Advanced Hazards Mitigation Lab Advanced Hazards Mitigation Lab –– Civil EngineeringCivil Engineering

NCHRPNCHRP--IDEA Project 141: Signal Head IDEA Project 141: Signal Head

Vibration AbsorberVibration Absorber

Richard ChristensonRichard Christenson

Associate ProfessorAssociate Professor

University of ConnecticutUniversity of Connecticut

NCHRPNCHRP--IDEA Project 141IDEA Project 141

“Reducing Fatigue in Wind-Excited Traffic

Signal Support Structures using Smart

Damping Techniques”

Objective: Develop and demonstrate the use

of a signal head vibration absorber (SHVA)

to reduce fatigue in traffic signal support

structures exposed to wind excitation.

FixedFixed

SHVASHVA

Reduce acceleration from 0.5 g to 0.06 g

(~3.5”-0.4”) in 2.75 sec (from 300 sec)

Increase damping from 0.2% to 10.1%.

98% reduction in steady state vibrations

Damped Vibration AbsorberDamped Vibration Absorber

Modeling the traffic pole as a single-degree-

of-freedom (sdof) spring-mass system

∆+=2ω

mm ∆−

1ωmk =

Consider the addition of a small spring-mass-

damper (ka, ma , ca ) system to the larger

structural system (k, m)

),sin()( tFxkxkkxcxcxm oaaaaaa ω=−++−+ &&&&

0=+−+− aaaaaaaa xkxkxcxcxm &&&&

We are interested in a solution of the forced

vibration – dynamic amplification

( ) ( )( ) ( ) ( )( ) 222

22222 λγζλ −+

( ) ( ) ( )( ) 2222222221112 λµγλγλλµζλ −−−++−

== mma /µ

== aan mk /2

==Ω mkn /2

=Ω= na /ωλ

== nωωγ /

== KPx ost /

=Ω= )2/( nmcζ

Mass ratio= Absorber mass/Main mass

Natural frequency of absorber

Natural frequency of main system

Frequency ratio

Forced frequency ratio

Static deflection of system

critical damping

Plotting the dynamic amplification shows there are

two points independent of damping (P and Q)

ζζζζ=0

ζζζζ=∞

ζζζζ=0

1. Optimal absorber freq. adjust amplitude of P & Q

to equal heights (Den Hartog)

The magnitude of P & Q is

µ/211 +=X

=Ω= na /ωλ Frequency ratio

== mma /µ Mass ratio

2. Optimal damping is to adjust the slope of the curve

to zero at P & Q (Seto)

In practice choose the average of the two values

( ) optζµ

µζ =

µ/21 +=stX

=Ω= )2/( nmcζ critical damping

ζζζζ=0 ζζζζ=∞

Consider the 35’ traffic signal support structure

ζζζζ=0.70

ζζζζ=0.085ζζζζ=0.05

ζζζζ=0.01

ζζζζ=0.20

Previous Mitigation DevicesPrevious Mitigation Devices

An effective vibration mitigation device can

decrease the amplitude and number of cycles,

extending the service life of these structures.

A number of other types of mitigation devices

have been proposed previously (McManus et

al. 2003; Cook et al. 1998; Hamilton et al.

2000; Pulipaka et al., 1998)

Type of

Dampers

Variation % Critical

damping

% Increase Disadvantage

damper

Traditional 8.71 32 Different natural

frequency requires

separate tuningStockbridge 0.42 1.5

Batten 1.82 6.7

Liquid

damper

Horizontal 0.38 1.4 Ineffective

U- tube 0.40 1.5

Friction

damper

6.49 23.9 Unattractive

damper

Elastome

ric pads

Pad at mast arm 0.28 1.9 Ineffective

Pad at mast arm and

0.43 2.9

Pad at base 0.39 2.6

Strut 2.4-6.0 16-40Requires luminary

extension

Contd.

Type of

Dampers

Variation % Critical

damping

damper

Traditional 8.71 32 Different natural

frequency requires

separate tuningStockbridge 0.42 1.5

Batten 1.82 6.7

Liquid

damper

Horizontal 0.38 1.4 Ineffective

U- tube 0.40 1.5

Friction

damper

6.49 23.9 Unattractive

damper

Elastome

ric pads

Pad at mast arm 0.28 1.9 Ineffective

Pad at mast arm and

0.43 2.9

Pad at base 0.39 2.6

Strut 2.4-6.0 16-40Requires luminary

extension

Contd.

Dampers Variation % Critical

damping

Impact

dampers

Vertical Spring/mass

impact dampers

6.79 25 High cost

Spring/mass liquid

impact dampers

6.12 22.5 High cost

Hapco Impact

damper

0.31 2.1 Ineffective

Flat –bar impact

damper

0.3-0.37 2.0-2.5 Ineffective

dampers damper

Shot-Put impact

damper:

0.20-0.29 1.3-1.9 Ineffective

Strand impact

damper

0.97-1.4 6.5-9.3 Large size and

Alcoa Dumbbell

damper

Dampers Variation % Critical

damping

Impact

dampers

Vertical Spring/mass

impact dampers

6.79 25 High cost

Spring/mass liquid

impact dampers

6.12 22.5 High cost

Hapco Impact

damper

Flat –bar impact

damper

0.3-0.37 2.0-2.5 Ineffective

dampers damper

Shot-Put impact

damper:

0.20-0.29 1.3-1.9 Ineffective

Strand impact

damper

0.97-1.4 6.5-9.3 Large size and

Alcoa Dumbbell

damper

Signal Head Vibration AbsorberSignal Head Vibration Absorber

• In the original SHVA configuration

the moving mass, ma, is the signal

head itself, suspended by a spring

and an employing eddy current

damper

• The original SHVA fits inside of the

signal head

SpringSpring

The spring rate is equal to 4.12 lb/inch with a free length of 15”

Maximum solid length of 3.3”

The spring has outside diameter of 3.25”

of 3.25”

Made of 0.187" thick stainless steel wire

2bdBtC

Eddy Current DamperEddy Current Damper

Eddy Current Damper (Seto 1979)

c= damping coefficient (N s/m)

ρ= peculiar resistance (Ω m)

Co= modification co efficient

bd= area of magnetic flux (m2)

B= magnetic –flux density (T)

t= thickness of conductor

Completed (January 2011)

Final report available: http://onlinepubs.trb.org/onlinepubs/idea/

finalreports/highway/NCHRP141_Final_Report.pdf

Short video describing SHVA: mms://159.247.0.209/mediapoint/Uconn/

NCHRP_Idea_141_v4.wmv

Continued StudiesContinued Studies

Quantify and understand the robustness of a

SHVA performance to mistuning

Redesign SHVA based on feedback

Field testing in actual wind conditions

Robustness of SHVARobustness of SHVA

SHVA based on principles of tuned mass damper – required to be tuned (frequency) to the dynamic property of main structure.

Individual tuning may not be practical, yet frequency of traffic signal supports range from

Individual tuning may not be practical, yet frequency of traffic signal supports range from 0.7 Hz to 1.4 Hz.

Performance of a single SHVA applied to 2 dynamically dissimilar traffic signal support structures is examined

Two traffic poles erected in the laboratory

35 foot mast arm 55 foot mast arm

Dissimilar StructuresDissimilar Structures

Two traffic poles erected in the laboratory

35 foot mast arm 55 foot mast arm

Dissimilar StructuresDissimilar Structures

Dynamic Characteristics

ωωωω1 = 1.1 Hzξ= 0.2%

Meff = 229 lbs

Dynamic Characteristics

ωωωω1 = 0.8 Hzξ= 0.2%

Meff = 263 lbs

Movable Member

Test SetupTest Setup

accelerometersstrain sensors

Effective Mass of Structure

RobustnessRobustness

• Mass and damping effects not only optimal

performance but robustness to mistuning

eduction (

ncontr

m = 70 lbs

m = 100 lbs

0.8 1 1.2 1.4 1.6 1.8 275

Frequency of Mast Arm (Hz)

eduction (

ncontr

m = 10 lbs

m = 30 lbs

m = 50 lbs

m = 70 lbs

eduction (

s. uncontr

ξξξξ = 5%

ξξξξ = 10%

ξξξξ = 20%

0.8 1 1.2 1.4 1.6 1.8 2

Frequency of Mast Arm (Hz)

eduction (

s. uncontr

ξξξξ = 0.2%

ξξξξ = 1%

ξξξξ = 5%

• SHVA has both

– large mass of the movable member (30-100 lbs);

– effective damping of the eddy current damper

(durable, low maintenance, small & large

amplitude)

Experimental EvaluationExperimental Evaluation

• To simulate wind excitation a linear shaker

used to apply sinusoidal force at mast arm tip

– weight is 13 lbs

– sine wave (±2 cm, 0-5 Hz)

Experimental EvaluationExperimental Evaluation

• To simulate wind excitation a linear shaker

used to apply sinusoidal force at mast arm tip

– weight is 13 lbs

• BDI STS-WiFi system to

measure and collect

strain and accel. data

Peak acceleration decreased

from 1.42 to 0.21 m/s2

35’ 35’ Tip Tip Acceleration ResponseAcceleration Response

85% 85%

stiction in bearings

Peak strain decreased

from 68 µε to 9.7 µε

35’ 35’ Strain Strain ResponseResponse

86% 86%

Increase Damping from 0.2 to 9.6% of critical

Peak acceleration decreased

from 2.2 to 0.4 m/s2

82% 82%

stiction in bearings

Peak strain decreased

from 61 µε to 12 µε

555’ 5’ Strain Strain ResponseResponse

80% 80%

Increase Damping from 0.2 to 6.8% of critical

• A single SHVA can effectively reduce vibration

in both a 35 ft and 55 ft long mast arm

– relatively large additional mass

– effective damping

• Stiction in bearings can limit performance

• A single SHVA can be used to effectively

mitigate the wind induced vibration of

multiple traffic signal support structures

reducing acceleration and strain by >80%

Redesigned DeviceRedesigned Device

New design to remove spring and damper

from inside signal head to outside

• In the new SHVA configuration the

moving mass, ma, is the signal

head itself, suspended by springs

inside of the two rails and

employing eddy current dampers

at the 4 bearings

• The SHVA fits between available

signal brackets and the signal head

Counterweights used to reduce

friction in bearings to

accommodate different signal

head configurations

Extension of design to 5-head

and horizontal signal head

configurations

Laboratory tests on 35’ mast arm validate

performance

Impulse response provides information on

steady state response

ξξξξ = 0.25%

Laboratory tests on 35’ mast arm validate

performance (0.2% to 5%)

ξξξξ = 0.25%

ξξξξ = 5%

Field Testing in ConnecticutField Testing in Connecticut

Manchester, Connecticut

Manchester, Connecticut – August 2012

Free vibration tests show damping increased

from 0.2% to 8.5%

Qualitative results from traffic camera show

pole steady relative to others at intersection

Field Testing in TexasField Testing in Texas

UConn supplement to “Development of Design

Guidelines and Mitigation Strategies for Wind-

Induced Traffic Signal Structure Vibrations” (Project

No. 0-6649), PI Delong Zuo (Texas Tech University)

Objectives:

experimentally validate the SHVA for reducing

wind induced vibration; and

assist in establishment of guidelines of design

and implementation of SHVA devices on traffic

mast arms

October 2012 initial visit and installation of 3-head

units; January 2013 installed 5-head SHVA unit

October 2012 initial visit and installation of 3-head

units; January 2013 installed 5-head SHVA unit

ConclusionsConclusions

• NCHRP-IDEA Project 141 successfully

demonstrated performance of SHVA concept

• Mass and damping of SHVA provide

robustness to mistuning

• SHVA redesigned to use existing signals and

brackets with straight forward installation

• Field testing underway to verify performance

and durability of design

AcknowledgementAcknowledgement

• NCHRP-IDEA program (Project 141)

• Connecticut Department of Transportation

• Texas Department of Transportation

• Texas Tech University

• University of Connecticut

• Town of Manchester, CT

• Traffic Structures Subcommittee

nchrp --idea project 141: signal head vibration …sp.bridges.transportation.org/documents/2013...

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