021-1700-016e 09a Aerodynamics Low COWI
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Transcript of 021-1700-016e 09a Aerodynamics Low COWI
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Bridge Aerodynamics
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2
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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6
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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8
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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0
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
Sanne Poulin
Senior Engineer, AerodynamicsMajor Bridges
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