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COWI Expertise
COWI is an international design consultant with a market leading position in bridge, tunnel and marine engineering. COWI possesses a wide range of expertise within core disciplines of bridge aerodynamics:
• Establishment of design basis
• Planning, design and interpreta tion of wind and turbulence site measurements
• Wind climate measurements
• Aeroelastic analysis and computer simulation of wind effects
• Design, supervision and interpre tation of wind tunnel tests
• Structural modelling of static and dynamic wind loads
• Optimisation of bridge design with respect to wind
• Design of countermeasures such as tuned mass dampers
• Troubleshooting
• Stay cable vibration assessment and damping.
COWI Services
COWI has been working with aerodynamics of structures for 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 structural safety and human comfort.
COWI works with a large number of internationally established wind tunnel laboratories worldwide on testing of bridges and bridge members. We also operate relevant computer codes developed in-house and thoroughly calibrated against wind tunnel test and field data.
Examples of COWI’s services within the field of bridge aerodynamics are presented in the following pages.
COWI’s services are relevant for completed bridges and bridges under construction.
Cable oscillations
Cable oscillation amplitudes should be kept at a minimum to avoid fatigue problems and problems related to user comfort.
Rain/wind testing of stay cables
COWI’s ISO 9001 certification covers bridge aerodynamics.
-25
0
-20
-15
-10
-5
0
5
10
15
20
25
Pitch(deg)
10 20 30 40 50 60 70 80 90 100
Flutter instability
Aeroelastic instability (divergent motion of the deck) must be confirmed not to occur at wind speeds foreseen within the design life of the bridge.
Divergent motion
(twist) of bridge girder

Flow
Vortex excitation
Vortex shedding excitation of the girder or the pylons is important for human comfort and fatigue life and can furthermore induce detrimental large-amplitude cable oscillations due to internal resonance.
Flow
Vertical vortex excitation response. Note car partly hidden by undulating roadway
Pressure distribution at vortex shedding frequency, w/o and with guide vanes
Stonecutters Bridge in Hong Kong with a main span of
1018 m is a bridge where wind effects has been very important for the design
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Buffeting
Wind turbulence gives rise to a dynamic wind load - the buffeting wind load - that forms a significant part of the design wind loading on the bridge.
Frequency
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Ratio of Propeller Wind Speeds versus Mean Wind Speed
1.5
1.2
1
0.9
0.8
0.7
0.6
0.5
0 5 10 15 20
Wind speed
Wind speed
Measurement mast, power spectrum, turbulence intensities, buffeting response

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Static wind load
Static wind load coefficients for girders and pylons are important input parameters to the structural design of a bridge. They can be computed very quickly based only on the two-dimensional cross section geometry (including rails, barriers etc.) using COWI’s in-house computer code DVMFLOW. This is a very useful tool in the early design phase before wind tunnel tests are performed, for smaller bridges where tests are not carried out, and when evaluating wind tunnel test results. In addition, DVMFLOW simulations allows for quick sensitivity analyses when design changes are considered.
DVMFLOW is developed specifi cally for computation of flow around two-dimensional bluff cross sections and is based on the discrete vortex method. The “grid-free” nature of the computational scheme allows fast and easy computation of flows around stationary or moving bodies.
Example of DVMFLOW geometric model representation, with crash barriers
1
0.75
0.5
0.25
0
-0.25
C
D
C
M
-0.5
-0.75
-1
-1.25
-1.5
-15˚
C
L
-10˚ -5˚ 0˚ 5˚ 10˚ 15˚
Static wind load coefficients
C
D
, C
L
and C
M
as functions of wind incidence angle.
DVMFLOW results (points) compared to wind tunnel test results (lines)
Simulated flow and time trace of aeroelastic response of the
1st Tacoma Narrows Bridge during the catastrophic event
(19 m/s)
Stability analysis
Assessment of flutter stability of a bridge girder and the associated critical flutter wind speed can easily be carried out once the motion dependent aerodynamic coefficients for the girder cross section are known. Again, DVMFLOW offers a fast and efficient way of obtaining these coefficients at an early stage in the design process. The girder cross section is subjected to a number of forced vertical bending and twisting motions from which the aerodynamic coefficients are derived. Apart from the cross-section geometry, the input to these simulations consist of modal mass and stiffness of the bridge girder and the centre of rotation. For the subsequent analysis, also the eigenmodes and eigenfrequencies must be known. These can be obtained from a structural dynamic analysis using
COWI’s in-house bridge modelling software IBDAS, or similar FE models.
Moment
Angular response time

Vortex shedding excitation
The vortex shedding performance of a bridge girder or a pylon can also be assessed computationally using
DVMFLOW. By FFT analysis of the static lift coefficient time trace, the dominant vortex shedding frequency can be found. A simulation with the cross section elastically suspended in the wind flow is then carried out assuming lock-in between the vortex shedding frequency and the structural frequency. The output is the response time trace from which the peak response is determined.
IBDAS analysis of buffeting
CFD
For detailed analysis of the flow and pressure fields around bridge members, computational Fluid
Dynamics can be applied. 3D geometries can be studied and turbulence effects included.
COWI uses Star CCM+ for this purpose.
Modeshape, simulated flow and vertical response time trace at lock-in
100
50
0
-50
-100
Response amplitude (mm)
Buffeting
The static and dynamic wind load on a bridge is calculated using COWI’s in-house integrated bridge analysis and design software IBDAS. Turbulence intensities, the spectral distribution of the turbulence, the coherence of turbulence along the bridge structure and the mean wind speed profile are all used in the dynamic buffeting calculations. The aerodynamic admittance of the deck cross section can also be included in case it is known from wind tunnel tests. The basic output from a wind load simulation are deflections and sectional forces.
Simulated pressure field and stream lines on girder surface time
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Wind tunnel testing forms an integral part of the design and analysis of most long span bridges, and is often a requirement in many codes and national standards.
In order to obtain the best possible results based on available resources, it is necessary to carefully plan and execute the tests and be aware of the inherent shortcomings and pitfalls of physical modelling.
COWI can offer a full range of services within the field of wind tunnel testing from planning over participation and supervision to interpretation of results in relation to design.
Tower model, Stonecutters Bridge, Hong Kong.
At VELUX, Denmark
Full bridge model (cantilevered), Stonecutters
Bridge, Hong Kong. At FORCE, Denmark
Stonecutters Bridge girder.
At NRC Canada
Section model, Osteroy Bridge, Norway.
At VELUX, Denmark
Section model, Xihoumen Bridge, PR China.
At BMT, UK

Full bridge model, Chacao
Bridge, Chile. At FORCE,
Denmark
Section model construction,
Sutong Bridge, China. At
FORCE, Denmark
Shenzen Western Corridor
Bridge. Full bridge model, at BLWTL, Canada
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International Codes of Practice often gives an insufficient guide to design loads for bridges at specific sites. For long span bridges it is common practice to establish a dedicated Design Basis reflecting the specific environmental conditions at the bridge site. This procedure is based on available data from the area and new measurements designed specifically for the purpose combined with theoretical knowledge and past experience. Single point site measurements are extreapolated to the bridge line by means at terrain model wind tunnel tests.
Terrain model and selected measurements
Assessment of site specific wind climate and synthesis for designers’ application
Turbulence intensity
0.25
0.2
0.15
0.1
0.05
0
-1000 -800 -600 -400 -200 0 200 400 distance along span
600 800 1000
Iu (N)
Iw (N)
Iu (land)
Iw (land)
Iu (NE)
Iw (NE)

An important element of bridge aerodynamics is optimisation of bridge design with respect to wind effects. The optimisation is carried 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 flutter stability of a plate girder. Question: Which design will achieve the best aerodynamic performance taking into account stability, wind loading and vortex shedding?
Vortex shedding, Osteroy Norway. Short sectional guide vanes were mounted to suppress vortex shedding oscillations (see inlaid photo). These were found to work better than continuous guide vanes
Vertical vortex shedding oscillations of the girder of Storebælt Bridge were mitigated by mounting of guide vanes at locations for flow separation. The efficiency of guide vanes were confirmed through wind tunnel testing, and proven through operational experience.
Guide vanes mounted at locations for flow separation
Displacement y (m)
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
33 33.1
33.2
33.3
Data recorded : 04-May-1998
Sec. 130/0
S
33.4
33.5
33.6
33.7
33.8
33.9
34
130
Measured vertical displacement,
3 rd vertical mode
RMS displacement / deck height
0.08
0.06
0.04
0.02
0.00
0 0.5
1 1.5
U/fB
The guide vanes eliminates the vortex shedding response
2
No guide vanes
Guide vanes
.
5
2.5
3
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Cable systems see a wide application in civil engineering structures such as bridges, guyed masts, suspended roofs and power transmission lines. Owing to their long spans and extremely low damping, cable systems are easily set into vibration by the wind often in combination with rain or ice/snow.
Stranded cables used in transmission lines and for bridge parapets may encounter excitation by the wind because of cross wind aerodynamic forces created by the stranded surface texture.
As it is impractical or virtually impossible to eliminate the excitation caused by the wind, cable vibrations are most readily mitigated by introducing some form of mechanical damping to the cable system.
Based on 20 years of experience,
COWI offers a full range of services including diagnostics of cable vibrations, design and analysis of damping systems and acceptance tests.
Öresund Bridge: Stay mounted TMD for mitigation of rain/ice vibrations
Storebælt Bridge:
Stockbridge dampers for mitigation of hand rope vibrations
0.08
0.06
0.04
0.02
A  
0
0 2 .10
5 4 .10
5 6 .10
5
Damping coefficient C [Ns/m]
8 .10
5
Measurement of damper characteristics for cable damping diagramme
1 .10
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Tacoma
For nearly six decades the underlying aerodynamic mechanisms for the
Tacoma collapse were not under stood by the bridge engineering community. In year 2000 COWI engineers unveiled the migration of a large vortex structure across the deck as the source of the instability.
This instability mechanism was proven in water tunnel experiments as well as in numerical simulations.
Today the migrating “Tacoma” vortex is recognised as being the source of torsion flutter well known in plate girder bridges. The Tacoma vortex was acknowledged in ASCE’s 2001 publication (In the Wake of Tacoma).
Tacoma vortex. Water tunnel test (left) and DVMFLOW simulation (right)
Aerodynamic stability often deter mines the maximum achievable span length for cable supported bridges and suspension bridges in particular.
3500 m spans for the proposed
Gibraltar Fixed Link
Gibraltar
Contemporary designs for box girder and truss suspension bridges may be built to span lengths of 1500 m -
2000 m without encountering aerodynamic instability at typical design wind speeds, but in case longer spans are called for, special deck and cable designs are needed to tackle the stability problem. A design study for a Gibraltar Strait crossing called for suspended span lengths in the range 3500 m - 5000 m. For these structures a COWI research project developed a twin deck structure which were able to fulfill the requirement to aerodynamic stability while keeping aerodynamic induced deck twist to a minimum.
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Höga Kusten Bridge, Sweden
Osteroy Bridge, Norway
COWI is a leading northern European consulting group. We provide state-of-theart services within the fields of engineering, environmental science and economics with due consideration for the environment and society. COWI is a leader within its fields because COWI’s 4000 employees are leaders within theirs.
COWI Group
Head office
COWI A/S
Parallelvej 2
DK-2800 Kongens Lyngby
Denmark
Tel.: +45 45 97 22 11
Fax: +45 45 97 22 12
E-mail: cowi@cowi.com
Internet: www.cowi.com
Contact:
Allan Larsen
Senior Specialist, Aerodynamics and Structural Dynamics
Major Bridges aln@cowi.com
Sanne Poulin
Senior Engineer, Aerodynamics
Major Bridges sami@cowi.com