021-1700-016e 09a Aerodynamics Low COWI

7/28/2019 021-1700-016e 09a Aerodynamics Low COWI

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Bridge Aerodynamics


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COWI Services

COWI has been working with

aerodynamics o structures or

more than 40 years. We have

gained considerable experience in

design and prediction methods,

balancing theory, experiments and

computer simulations to obtain the

best results with regard to struc-

tural saety and human comort.

COWI works with a large

number o internationally estab-

lished wind tunnel laboratoriesworldwide on testing o bridges and

bridge members. We also operate

relevant computer codes developed

in-house and thoroughly calibrated

against wind tunnel test and eld

data.

Examples o COWIs services

within the eld o bridge aerody-

namics are presented in the ollow-

ing pages.

COWIs services are relevant or

completed bridges and bridges

under construction.

Bridge Aerodynamics

COWI Expertise

COWI is an international design

consultant with a market leading

position in bridge, tunnel and

marine engineering. COWI pos-

sesses a wide range o expertise

within core disciplines o bridge

aerodynamics:

Establishmentofdesignbasis

Planning,designandinterpreta -

tion o wind and turbulence site

measurements

WindclimatemeasurementsAeroelasticanalysisandcomputer

simulation o wind eects

Design,supervisionandinterpre -

tation o wind tunnel tests

Structuralmodellingofstaticand

dynamic wind loads

Optimisationofbridgedesign

with respect to wind

Designofcountermeasuressuch

as tuned mass dampers

Troubleshooting

Staycablevibrationassessment

and damping.

COWIsISO9001certication

covers bridge aerodynamics.

Aerodynamicphenomena

Cable oscillations

Cable oscillation amplitudes

should be kept at a minimum

to avoid atigue problems and

problems related to user

comort.

Rain/wind testingo stay cables

Flutter instability

Aeroelasticinstability(divergent

motion o the deck) must be

conrmed not to occur at wind

speeds oreseen within the design

lie o the bridge.

0 10 20 30 40 50 60 70 80 90 100-25

-20

-15

-10

-5

0

5

10

15

20

25Pitch(deg)

Divergent motion(twist) o bridge

girder


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Buffeting

Wind turbulence gives rise to a

dynamic wind load - the buet-

ing wind load - that orms a

signicant part o the design

wind loading on the bridge.

Measurement mast, powerspectrum, turbulence intensities,bueting response

0 5 10 15 20

0.5

0.6

0.7

0.8

0.9

1

1.2

1.5

2

Vertical vortex excitationresponse. Note car partly

hidden by undulatingroadway

Vortex excitation

Vortex shedding excitation o

the girder or the pylons is

important or human comort

and atigue lie and can

urthermore induce detrimental

large-amplitude cable oscilla-

tions due to internal resonance.

Pressure distribution at vortex sheddingrequency, w/o and with guide vanes

Flow

Flow

Stonecutters Bridge in Hong Kong with a main span o1018 m is a bridge where wind eects has been veryimportant or the design

Wind speed

Wind speed

Intensity

Response

Spectrum

Frequency


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4 Computermodelling andanalysis

Static wind load

Staticwindloadcoefcientsfor

girders and pylons are important

input parameters to the structural

designofabridge.Theycanbe

computed very quickly based only on

the two-dimensional cross section

geometry(includingrails,barriers

etc.) using COWIs in-house computer

codeDVMFLOW.Thisisavery

useul tool in the early design phasebeore wind tunnel tests are per-

ormed, or smaller bridges where

tests are not carried out, and when

evaluating wind tunnel test results. In

addition,DVMFLOWsimulations

allows or quick sensitivity analyses

when design changes are considered.

DVMFLOWisdevelopedspeci-

cally or computation o fow around

two-dimensional blu cross sections

and is based on the discrete vortex

method.Thegrid-freenatureofthe

computational scheme allows ast and

easy computation o fows around

stationary or moving bodies.

Static wind load coecientsC

D, C

Land C

Mas unctions

o wind incidence angle.DVMFLOW results (points)compared to wind tunnel testresults (lines)

Stability analysis

Assessmentofutterstabilityofa

bridge girder and the associated

critical futter wind speed can easily

be carried out once the motion

dependent aerodynamic coecients

or the girder cross section are

known.Again,DVMFLOWoffersa

ast and ecient way o obtaining

these coecients at an early stage in

thedesignprocess.Thegirdercross

section is subjected to a number o

orced vertical bending and twisting

motions rom which the aerodynamic

coefcientsarederived.Apartfrom

the cross-section geometry, the input

to these simulations consist o modal

mass and stiness o the bridge girder

andthecentreofrotation.Forthe

subsequent analysis, also the eigen-

modes and eigenrequencies must be

known.Thesecanbeobtainedfroma

structural dynamic analysis usingCOWIs in-house bridge modelling

softwareIBDAS,orsimilarFE

models.

Simulated fow and time traceo aeroelastic response o the1st Tacoma Narrows Bridgeduring the catastrophic event

(19 m/s)

Example o DVMFLOWgeometric modelrepresentation, with crashbarriers

CM

CD

CL

-1.5

-1.25

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

-15 -10 -5 0 5 10 15

time

Moment

Angular response


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Vortex shedding excitation

Thevortexsheddingperformanceof

a bridge girder or a pylon can also

be assessed computationally using

DVMFLOW.ByFFTanalysisofthe

static lit coecient time trace, the

dominant vortex shedding requency

canbefound.Asimulationwiththe

cross section elastically suspended in

the wind fow is then carried out

assuming lock-in between the vortex

shedding requency and the struc-

turalfrequency.Theoutputisthe

response time trace rom which thepeak response is determined.

Buffeting

Thestaticanddynamicwindloadona

bridge is calculated using COWIs

in-house integrated bridge analysis and

designsoftwareIBDAS.Turbulence

intensities, the spectral distribution o

the turbulence, the coherence o

turbulence along the bridge structure

and the mean wind speed prole are all

used in the dynamic bueting calcula-

tions.Theaerodynamicadmittanceof

the deck cross section can also be

included in case it is known rom wind

tunneltests.Thebasicoutputfroma

wind load simulation are defections

and sectional orces.IBDAS analysis o bueting

Modeshape, simulated fow andvertical response time trace atlock-in

CFD

Fordetailedanalysisoftheow

and pressure elds around bridge

members,computationalFluid

Dynamicscanbeapplied.3D

geometries can be studied and

turbulence eects included.COWIusesStarCCM+

or this purpose.

100

50

0

-50

-100

timeResponse amplitude (mm)

Simulated pressure eld and streamlines on girder surace


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Wind tunnel testing orms an integral

part o the design and analysis o most

long span bridges, and is oten a

requirement in many codes and national

standards.

In order to obtain the best possible

results based on available resources, it is

necessary to careully plan and execute

the tests and be aware o the inherent

shortcomings and pitalls o physical

modelling.

COWI can oer a ull range o

services within the eld o wind tunnel

testing rom planning over participationand supervision to interpretation o

results in relation to design.

Wind tunnel testing

Tower model, Stonecutters Bridge, Hong Kong.At VELUX, Denmark

Full bridge model (cantilevered), StonecuttersBridge, Hong Kong. At FORCE, Denmark

Section model, Osteroy Bridge, Norway.At VELUX, Denmark

Stonecutters Bridge girder.At NRC Canada

Section model, Xihoumen Bridge, PR China.At BMT, UK


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Full bridge model, ChacaoBridge, Chile. At FORCE,

Denmark

Section model construction,Sutong Bridge, China. At

FORCE, Denmark

Shenzen Western CorridorBridge. Full bridge model,

at BLWTL, Canada


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InternationalCodesofPractice

oten gives an insucient guide to

design loads or bridges at specic

sites.Forlongspanbridgesitis

common practice to establish a dedi-

catedDesignBasisreectingthe

specic environmental conditions at

thebridgesite.Thisprocedureis

based on available data rom the

area and new measurements

Design basis

Assessment o site specic windclimate and synthesis or designersapplication

Turbulence intensity

0

0.05

0.1

0.15

0.2

0.25

-1000 -800 -600 -400 -200 0 200 400 600 800 1000

distance along span

Iu,I

w

Iu (N)

Iw (N)

Iu (land)

Iw (land)

Iu (NE)

Iw (NE)

Terrain model and selected measurements

designed specically or the purpose

combined with theoretical knowl-

edgeandpastexperience.Single

point site measurements are ex-

treapolated to the bridge line by

means at terrain model wind tunnel

tests.


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9Designoptimisation andtrouble-shooting

Animportantelementofbridge

aerodynamics is optimisation o

bridge design with respect to wind

effects.Theoptimisationiscarried

out in close collaboration with the

bridge designers and structural

analysts in order to achieve the best

over-all bridge design.

Three designs proposed to achieve better futterstability o a plate girder. Question: Whichdesign will achieve the best aerodynamic

perormance taking into account stability, windloading and vortex shedding?

Vortex shedding, Osteroy Norway. Short sectionalguide vanes were mounted to suppress vortex sheddingoscillations (see inlaid photo). These were ound towork better than continuous guide vanes

Vertical vortex shedding oscillations o

thegirderofStorebltBridgewere

mitigated by mounting o guide vanes at

locationsforowseparation.The

eciency o guide vanes were conrmed

through wind tunnel testing, and proven

through operational experience.

Measured vertical displacement,3rdvertical mode

Guide vanes mounted atlocations or fow separation

The guide vanes eliminates the vortexshedding response

0.00

0.02

0.04

0.06

0.08

0 0.5 1 1.5 2 2.5 3

U/fB

1:60, Re = 1.5 10

No guide vanes

Guide vanes

RMS displacement / deck height

.5

130

Displacement y (m)

Sec. 130/0

33 33.1 33.2 33.3 33.4 33.5 33.6 33.7 33.8 33.9 34-0.6

-0.4

-0.2

0

0.2

0.4

0.6

Data recorded: 04-May-1998


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Cable systems see a wide applica-

tion in civil engineering structures

such as bridges, guyed masts,

suspended roos and power trans-

mission lines. Owing to their long

spans and extremely low damping,

cable systems are easily set into

vibration by the wind oten in

combination with rain or ice/snow.

Strandedcablesusedintransmission

lines and or bridge parapets mayencounter excitation by the wind

because o cross wind aerodynamic

orces created by the stranded

surace texture.

Asitisimpracticalorvirtually

impossible to eliminate the excita-

tion caused by the wind, cable

vibrations are most readily miti-

gated by introducing some orm o

mechanical damping to the cable

system.

Based on 20 years o experience,

COWI oers a ull range o services

including diagnostics o cable

vibrations, design and analysis o

damping systems and acceptance

tests.

Dampingof cablestructures

Storeblt Bridge:Stockbridge dampers ormitigation o hand ropevibrations

Measurement o damper characteristicsor cable damping diagramme

resund Bridge: Stay mounted TMDor mitigation o rain/ice vibrations

0 2 .105

4 .105

6 .105

8 .105

1 .106

0

0.02

0.04

0.06

0.08

Damping coefficient C [Ns/m]

Modaldamping,

log.

dec.,

Stroke(m)

A


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11COWI in bridgeaerodynamicsscience

Tacoma

Fornearlysixdecadestheunderlying

aerodynamic mechanisms or the

Tacomacollapsewerenotunder-

stood by the bridge engineering

community. In year 2000 COWI

engineers unveiled the migration o

a large vortex structure across the

deck as the source o the instability.

Thisinstabilitymechanismwas

proven in water tunnel experimentsas well as in numerical simulations.

TodaythemigratingTacoma

vortex is recognised as being the source

o torsion futter well known in plate

girderbridges.TheTacomavortex

wasacknowledgedinASCEs2001

publication(IntheWakeofTacoma).

Tacoma vortex. Water tunnel test (let)and DVMFLOW simulation (right)

Aerodynamicstabilityoftendeter-

mines the maximum achievable span

length or cable supported bridges

and suspension bridges in particular.

Gibraltar

Contemporary designs or box girder

and truss suspension bridges may be

builttospanlengthsof1500m-

2000 m without encountering

aerodynamic instability at typical

design wind speeds, but in case

longer spans are called or, special

deck and cable designs are needed to

tacklethestabilityproblem.Adesign

studyforaGibraltarStraitcrossing

called or suspended span lengths in

therange3500m-5000m.For

these structures a COWI research

project developed a twin deck

structure which were able to ulll

the requirement to aerodynamicstability while keeping aerodynamic

induced deck twist to a minimum.

3500 m spans or the proposedGibraltar Fixed Link


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21-1700-016e-09a

rintedinDenmarkbyKailow

COWI Group

Head ofce

COWI A/S

Parallelvej 2DK-2800 Kongens Lyngby

Denmark

Tel.: +45 45 97 22 11

Fax: +45 45 97 22 12

E-mail: [email protected]

Internet: www.cowi.com

Contact:

Allan Larsen

Senior Specialist, Aerodynamics

and Structural Dynamics

Major Bridges

[email protected]

Sanne Poulin

Senior Engineer, AerodynamicsMajor Bridges

[email protected]

COWI is a leading

northern European

consulting group. We

provide state-o-the-

art services within the

elds o engineering,

environmental science

and economics with

due consideration or

the environment and

society. COWI is a

leader within its elds

because COWIs 4000

employees are leaderswithin theirs.

Hga Kusten Bridge, Sweden

Osteroy Bridge, Norway

www.cowi.com

021-1700-016e 09a Aerodynamics Low COWI

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