Long span bridges & dynamic structures wind engineering & architectural aerodynamics

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Tel: +44 20 8614 4400 Fax: +44 20 8943 3224 Email: enquiries@bmtfm.com Website: www.bmtfm.com

long span bridges are highly wind sensitive structures. due to their relative flexibility and behaviour as line-like structures they are prone to wind driven dynamic resonance.

the specification of design wind speeds for structural,

serviceability and environmental design requires careful

analysis of long-term wind statistics. Bmt has access to

global wind record databases, which provide long-term

wind statistics for extreme wind events including synoptic

and non-synoptic wind patterns including e.g. typhoon,

shamal, and thunderstorm.

By application of sophisticated industry-standard wind

models of the atmospheric boundary layer and extreme

value statistical analysis of storm records,

site-specific wind climate models are generated for each

development that can also provide detailed resolution of

wind directionality and probability of occurrence.

where required, generic wind models are complemented

through detailed topography models and non-synoptic

wind pattern modelling. wind climate studies are

accepted throughout the world by local regulatory

authorities as a basis of design and deliver a robust basis

for reduction of inherent conservatism in code specified

design windspeeds.

Key services

• wind climate analysis

• structural load & dynamic response analysis

• aerodynamic stability analysis

• aerodynamic solution development

• damping system simulation

Key Benefits

• cost-effective and safe structural design

• code compliance verification

• interactive solution development

• iterative design optimisation

Wind Climate analysis

over the course of their design, bridge designers are

required to verify aerodynamic stability over the full

design wind speed range not only with respect to vortex

shedding phenomena but also with respect to divergent

instabilities such as flutter and galloping.

this applies not only to the deck of the bridge but also

to supporting structures such as pylons. with ever-

increasing span of bridge decks and the implementation

of innovative design concepts such as multiple deck

arrangements and airfoil like cross-sectional shaping,

bridge designers are challenged to make reliable

predictions for dynamic wind loading and dynamic

stability for these structures, which are not well described

by applicable codes of practice.

in the case of footbridges, complex structural

arrangements in which there are non-linear interactions

between the deck and supporting structures such as

arches and pylons together with architecturally styled

barrier designs means that these structures require

specialist wind studies for cost-effective and safe design.

due their relative flexibility and aerodynamic shape,

masts and so-called ‘special structures’ such as spires or

antennas - like long span bridges, are highly susceptible

to wind-driven instability, which in the case of divergent

instabilities can lead to structural failure in low-cycle

fatigue.

accordingly, in the design of these structures it is

imperative to thoroughly study dynamic wind interactions,

which can often be mitigated through aerodynamic shape

changes and/or adjustment of structural parameters such

as damping.

detailed study of these types of structures employs

testing and wind modelling methodology that is

analogous to that used for long-span bridge deck

aerodynamics. Hurricane Topography Model

in cases where the aerodynamically-driving portion

of the structure either is of constant shape or can be

approximated at such, section model testing in an

aeronautical wind tunnel provides a technically robust,

validated and cost-effective approach to quantification of

steady aerodynamic load coefficients for a full range of

wind angles by direct measurement using dynamic force

transducers.

seCtion model studies

Bridge Section Model

noaa/sCienCe PHoto liBRaRy

Tel: +44 20 8614 4400 Fax: +44 20 8943 3224 Email: enquiries@bmtfm.com Website: www.bmtfm.com

Full aeRoelastiC model studies

in cases where the shape of the structure is highly three-dimensional,

or where there are complex structural interactions between linked

subsystems of the structural systems (e.g. bridge deck and support

pylons) full aeroelastic modelling of the complete structure at small

scale in simulated boundary layer flow can provide robust study of

fluid / structure interactions that govern critical wind loading and

aerodynamic stability scenarios. though this modelling approach is

significantly more involved, in terms of complexity of the wind tunnel

models that it requires, it is nevertheless a well-proven technique for

assessment of effects such as buffeting as well as dynamic instabilities,

especially when applied in conjunction with section model testing. Bmt

has substantial experience and capabilities in this specialised form of

modelling, having applied it to numerous long span bridges and special

structures.

seCtion model studies Cont

in addition, section model studies can be carried out

using multi-degree of freedom dynamic spring mass

damper systems, which allow the dynamic properties of

the full scale structure to be imposed on the wind tunnel

model at model scale such that critical wind speeds for

occurrence of vortex shedding and divergent instability

can be directly assessed in the wind tunnel through

measurement of dynamic responses using purposely

placed accelerometers.

section model studies are ideally suited for solution

development studies as they allow cost-effective and

time-efficient iterative testing of different aerodynamic

shapes and ranges of structural parameters e.g.

damping ratios, frequency ratios and dynamic mass.

this in turn provides designers with an opportunity

to use the wind tunnel as an interactive tool to deliver

aerodynamically optimised design solutions that comply

with performance targets.

ComPutational Fluid dynamiCs (CFd)

Bmt maintains a resourceful in-house numerical-modelling group that

specialises in the application of state-of-the-art cfd to wind engineering,

based on extensive research and development. there is an increasing

level of confidence that cfd can, at an early stage in the design of

long-span bridge design, be used to provide preliminary assessments

of the 2d aerodynamic characteristics of bridge decks, to provide initial

design guidance ahead of detailed design stages, when validation of key

performance parameters through wind tunnel testing must be regarded as

mandatory.

Section Model Mast

Load Coefficient vs. Wind Angle Dynamic Response

Vortex Strakes

Aeroelastic Road Bridge Aeroelastic Bridge Pylon

Bridge Pylon (linear mode model)

Aeroelastic Sculpture

CFD

Aeroelastic Footbridge

For further information contact: Tel: +44 20 8614 4400 Fax: +44 20 8943 3224Email: enquiries@bmtfm.com Website: www.bmtfm.com

BMT Fluid Mechanics Limited, 67 Stanton Avenue, Teddington, Middlesex, TW11 0JY, UK