Vsl Damping Systems for Stay Cables

VSL DAMPING SYSTEMS FOR STAY CABLES

ASSESSMENT OFCABLE BEHAVIOURDESIGN AND FABRICATION INSTALLATION AND TUNINGRETROFITTING

Plaq Dampers(Bridges):Heavy Lifting 15/01/10 11:07 Page1

2 V S L D A M P I N G S Y S T E M S F O R S T A Y C A B L E S

Over 50 years of engineeringexperienceVSL’s specialist construction systems havebeen used throughout the world since 1956and have earned an excellent reputation forquality and reliability. This has made VSL arecognised leader in specialist constructionmethods and associated engineering works.

VSL - SPECIALIST KNOW-HOW FOR CABLE-STAYED STRUCTURES

A worldwide networkVSL provides solutions through its network oflocally-based subsidiaries who have access toa strong common technical support structure.Its clients work with a local partner whilebenefiting from the constant development andevolution of VSL’s technologies.

Vibration phenomenaVarious effects can lead to excitation of the cable and eventually to instability of the structure if thevibrations are not controlled. Stay cables are generally excited by aerodynamic forces acting on thecable, or by anchorage displacements caused by the action on the structure of dynamic forces suchas traffic. Wind excitations can be categorised in four families:

VSL Stay cable technologyStay cables are among the industry’s mostsophisticated technologies and so specialistexpertise is the key to providing viable,economical and reliable solutions. VSL hasbecome a world leader as stay cable contractorand offers an extensive range of servicesranging from design assistance through cablesupply and installation to full bridgeconstruction packages. This makes VSL aninvaluable partner for any bridge contractor orbridge owner, in particular when it comes toaddressing the specific challenges of cable-stayed bridges.

VORTEX SHEDDINGAlternating asymmetrical vortex detachment induces a lift force perpendicular to the direction of the wind andhence vibrations of the cable.

RAIN & WIND INTERACTIONWater rivulets forming at the surface of the inclined cable modify its aerodynamic profile resulting in anasymmetrical pressure distribution and hence a lift force perpendicular to the virtual wind velocity . Oscillation ofthe rivulet results in cyclic changes to the lift force and hence oscillation of the cable. This occurs typically atrelatively low wind velocities.

WAKE GALLOPINGVortices detached from upstream obstacle (adjacent cable) induce a pressure differential at the surface of thedownstream cable resulting in an alternating lift force perpendicular to the wind direction exciting thedownstream cable.

DRAG CRISISThe drag coefficient of the cable varies with the relativewind velocity. Variations of the relative wind velocity dueto movement of the cable parallel to the wind directioninduce cyclic fluctuations of the drag coefficient andhence the drag force exciting cable vibrations parallel tothe wind direction.


V S L D A M P I N G S Y S T E M S F O R S T A Y C A B L E S 3

VSL’s services: • Stability risk analysis• Proposal of mitigation measures,

including provision of dampers• Estimation of the behaviour of the

damper-equipped cables, taking intoaccount excitation by aerodynamic effectsand structural coupling

• Design, fabrication, workshop testing,supply and installation of a choice of twodifferent damping systems

• Full integration of two different dampertypes into the VSL SSI 2000 system eitherat the time of installation or forinstallation at a later stage (retrofitting)

• Retrofitting solutions for existing strandand parallel wire cables

• Fine-tuning of damping devices afterinstallation to match the actualcharacteristics of cable and structure

VSL SOLUTIONS FOR VIBRATION CONTROL

number of moveable parts. The two damperscomplement each other and allowimplementation of the most appropriatesolution, taking account of the characteristicsof the stay cables, the types of criticalvibrations and the required performance. Bothsystems can be used on the same structure.

Methods of controlling vibrationsModern cable-stayed structures have toaccommodate increased dynamic demands ontheir cables. VSL’s response to this requirementincludes use of the following devices:

Helical ribsThe stay pipes of the VSL SSI 2000 cable aresupplied with helical ribs. Their shape anddimensions have been optimised and validatedin wind tunnel tests for effective control of therisk of rain-wind induced vibrations whileminimising the increase in wind drag.

Additional damping devicesThe most versatile means of controllingvibrations is to increase the structuraldamping ratio of the cable by installingadditional damping devices. VSL offers twotypes of dampers: the VSL Friction damper andthe VSL Gensui damper. This permits selectionof the most appropriate damping behaviour tosuit the characteristics of each individualcable. The VSL Gensui damper is furtheravailable in two grades, «Standard» and«High performance». In addition to their high efficiency, the commonqualities of both damping systems are theiradaptability, their great durability and their lowmaintenance costs, achieved by minimising the

Modelling of vibration effectsToday’s recommendations for stay cablesissued by the various international bodiesrequire a project-specific assessment of thedynamic effects. However, the mechanisms ofdynamic excitation are particularly complexphenomena and their reliable prediction isdifficult. VSL uses analytical tools that havebeen developed in collaboration withinternationally-renowned experts in the field.These tools simulate the cable’s responseunder combined excitation effects whileapplying different stability criteria that allowestimation of the vibration risks and anyadditional damping required. VSL can therefore assist the designer inassessing the risk of cable vibrations and canpropose suitable mitigation measures basedon a modular approach.

Simulation of cable amplitudes for an undamped cableunder parametric excitation

The required additionaldamping ratio isdefined for eachstructure on a case-by-case basis and liestypically below 1%. The achievabledamping ratio can bedefined as a function of x/L, where x is theposition of the damperfrom the anchoragerelative to the cablelength L. The graphabove can be used to select the appropriate damper type depending on the achievable x/L. The damper is typically located close to the deck anchorage for improved accessibility andaesthetics. For special cases, an additional damper can be provided near the pylon anchorage.

TestingThe behaviour of VSL Dampers and thecorrelation of calculated and actual cableoscillations have been verified in severalfull-scale tests. The effectiveness of thedamper can be assessed by measuring thecable acceleration over time and comparingthe results with and without the damper.

Validation of damper modellingin a laboratory test


VSL DAMPERS – COMMON FEATURES WIT


Two systems cover the full range of requirementsIn general, VSL Friction dampers are more suitable forlong cables, whereas VSL Gensui dampers are moresuitable for short and medium length cables.The final selection of the most appropriate system ismade by VSL using analytical tools that have beendeveloped in collaboration with internationally-renowned experts in the field.

Two systems with the same external visual appearanceThe external components and shapes are identical forboth systems, even though they adopt differentapproaches to dissipating the vibration energy. Thisallows a consistent appearance along a bridge, evenwhere both systems are required.

Protective collarThe moving part of the damper connects

to the stay cable using a collar fitted with a protective neoprene ring to ensure lasting

protection of the cable’s main tensile element.

Stiff and integrated support The VSL Damper supports are integrated

into the guide pipe that is part of the mainstructure to ensure stiff support for maximum

performance. This forms the fixed element of the damper.

Modular dissipative element This is designed to allow inspection

and replacement on site using light tools and equipment, with minimum impact

on the bridge’s operation.

VSL Frictiondamper

The three main elements of both damping systems are:

Murom Bridge, Russia - 2009


WITH COMPLEMENTARY CHARACTERISTICS


Fully integrated for enhancedaestheticsThe compact and highly-efficient nature of thedampers allows installation close to the deckanchorage, where they are fully integrated intothe anti-vandalism pipe. This avoids themaffecting the visual appearance of the stay cable.Thanks to their identical cover designs, bothtypes of damper can be installed easily on thesame structure without introducing visualdifferences.

Easy maintenance The damper configurations provide easy accessfor inspection and maintenance operations anduse a minimal number of moving parts. All components can be dismantled and/orreplaced on site using light tools andequipment, with minimum impact on thebridge’s operation.

Versatile Both damper types are optimised for use asinternal dampers, fully protected inside theVSL SSI 2000 anti-vandalism pipe, but theycan also easily be adapted for retrofittingsolutions or use as external dampers ifrequired. They can be attached to both parallelstrand systems (PSS) or parallel wire systems(PWS). They can be fitted on new structures orretrofitted to existing ones.

VSL Gensui damper

Accurate adaptation tomeet damping

requirementsFor both dampers, the heart of the

damping system is made up ofmodular dissipative elements that arefully adaptable to all cable sizes. For VSL Friction dampers, theperformance can be fine-tuned onceinstalled by adjusting the friction forcewithout dismantling the damper.For the VSL Gensui damper, the numberof pads required is based on thedynamic characteristics of the cable.This allows easy adaptation to all cable

sizes whether for a new installation or aspart of a retrofitting solution.

Highly efficient Several comparative tests on full-scale cableshave demonstrated the exceptional efficiencyof the dampers. Their measured performancehas repeatedly exceeded the specifiedrequirements, as well as exceeding that ofother types of dampers.

Very durable Outstanding durability is achieved by movableparts and making use of dissipationmechanisms that employ stable solidmaterials instead of fluids.

Murom Bridge, Russia - 2009



THE VSL FRICTION DAMPER – HOW IT WDissipating energy through frictionThe VSL Friction damper applies the sameprinciples used in disc brakes, dissipating theenergy through friction generated between twofriction partners. The first is a pair of slidingdiscs which is connected via a collar to thetensile member of the stay. It is sandwiched bythe second partner, which consists of aspecially developed composite pad supportedby a pair of spring blades that are connectedto the external structure of the guide pipe.

High performance and outstanding efficiencyThe VSL Friction damper provides highperformance for critical cases or where thedamper has to be placed close to the deckanchorage relative to the overall length of the cable. The required stiffness of the spring blades togenerate the appropriate friction force dependson the dynamic characteristics of the cable.This allows easy adaptation to all cable sizes,whether for a new installation or as part of a retrofitting solution.

Unaffected by temperatureThe damper’s insensitivity to temperaturechanges results in friction characteristics thatare very stable throughout a wide temperaturerange. The chosen materials ensure well-controlled behaviour with no stick-slip effect atthe friction interface.

The force acting on the damper when it isactivated has initially to exceed the staticfriction before the friction partners startmoving relative to each other.

There is no movement of the damper arisingfrom the non-critical continuous vibration ofthe cable with the small displacements thattraffic and other effects cause on every cable-stayed structure.

This is a very efficient method for preventingexcessive wear of the friction surfaces withoutaffecting the damper’s performance.Inspections of installed dampers after severalyears of operation have clearly demonstratedthis beneficial effect.

Compositepad

Oscillatingmovement Sliding

disc

Collar

Springblades

Friction damper installed prior to covering with anti-vandalism protection



T WORKSPerfectly tuned for critical vibrationsVarious theoretical approaches have beendeveloped in an attempt to estimate theachievable damping of a stay cable equippedwith a damper installed near one anchorage.One result that is widely acknowledged showsthe existence of a limitation, and can beexpressed as (Kovacs 1982):

with x = damper distance to the closestanchorage, L = cable free length, ξ is theadditional damping ratio and δ is thelogarithmic decrement.However this approach does not consider thenon-linearity of the friction damper. While theyhelp to assess the overall performance of adamper system, it has been demonstrated infull-scale tests that these maximum ratios canbe surpassed by certain dampers on individualmodes of a modal analysis.The VSL Friction damper is one of the fewdampers in the market that has achieveddamping ratios exceeding the theoreticalmaximum values above. It has achieved anefficiency of 130% on the first mode duringtesting. The first mode is often considered themost critical mode as it can result in largeamplitudes and hence significantly affects the

comfort of the user at the same time as havingan Eigen frequency close to that of thestructure. This brings the risk of couplingeffects between the structure and cable, whichcould prove catastrophic.

Activated only when neededIn addition to this outstanding performance, thedamper’s characteristics are particularly suitedto stay cables. The initial effect of static frictionprevents activation of the damper under smallamplitudes. This prevents unnecessary wear onthe damper under vibrations that do not affectthe cable’s performance and that are consideredperfectly acceptable while protecting at the sametime the anchorage by filtering these movements.However, the VSL Friction damper achieves itsmaximum performance almost immediately oncethe cable vibration has reached a level that iscritical for the cable or the structure to which thedamper has been tuned. It achieves an extremely high damping ratio atthese critical amplitudes and dissipates thecable’s energy very efficiently to prevent anyfurther excitation and ensure that the cablenever reaches large amplitudes. These specificcharacteristics make the VSL Friction damperthe perfect solution for any cable, but inparticular for long cables on structures with ahigh risk of parametric excitation.

The example shows the calculated modal response of a specific stay cablesusceptible to rain-wind induced vibrations resulting in instability over atypical 10min period. The same cable equipped with a VSL Friction damperdoes not experience instability over the same period.

First mode modal component

Second mode modal component

Third mode modal component

Efficient at all vibration modes

The VSL Friction damper:durable thanks to its fewmoving parts

or


Based on the high performance measured during several full-scale tests, the VSL Frictiondamper can be considered amongst the most efficient passive damping devices for mitigation of stay cable vibration (Cable Dynamic Symposium Vienna 2007, proceedings, BehaviorComparison of Cable Dampers by Full-Scale Experiment, S.S. Ahn, J.H. Park, S.H. Lee, C.M. Park).Performances have surpassed 100% of the theoretical maximum damping that can be providedby a passive damping system to a stay cable.


THE VSL FRICTION DAMPER – HOW IT WClose to the anchorage:The low x/L ratio required by the VSL Frictiondamper and its compact body mean that it canbe positioned close to the deck anchorage andeasily integrated into the anti-vandalismprotection near deck level. The result is anaesthetically-pleasing solution that does notcompromise the performance. Aesthetics areoften important in the design of stay cableswhere they are the dominating feature oflandmark structures. Compared with externaldampers which require large mounting frames,the use of the VSL Friction damper as aninternal damper provides a visually-convincingsolution for any structure.

The damper is fully integrated into the anti-vandalism cover .

100%

1% Amplitude of vibrationexpressed as a ratio

of the cable length

Ratio

to m

axim

al th

eore

tical

dam

ping

ξm

ax=

x/(2

L)

Various viscous dampersavailable in the market]VSL Friction Damper



T WORKSAdjustable damping capacityThe contact force of the pads is controlled byelastic deformation of the spring blades.Varying the stiffness of the blades allowsadjustment of the damping characteristics foreach installation.

Further fine-tuning on site is carried out withoutdismantling the damper. The friction force isvaried through adjustment of the protrusion ofthe composite pads from the spring blades. Thematerials used as friction partners have beenselected for optimum energy dissipation, stablefriction behaviour and high durability.

Indicative dimensions of the VSL Friction dampers, for each anchorage size in the VSL SSI 2000 stay cable system brochure

Sucharskiego Bridge, Poland - 2001

type6-126-196-226-316-376-436-556-616-736-856-91

6-1096-127

L1 min DR*1500175019002100230025502650285030503150340035503950

L1 min DS*900

120013501550175020002050225024502600280030003350

L2**200200220220250250250280300300300400400

Ø1**430450470505545585610630650680700730740

Ø2 DR*219.1 x 6.3267 x 6.3

298.5 x 6.3323.9 x 6.3355.6 x 6.3406.4 x 8.8

419 x 10419 x 10508 x 11508 x 11

559 x 12.5559 x 12.5610 x 12.5

Ø2 DS*177.8 x 4.5219.1 x 6.3219.1 x 6.3244.5 x 6.3273 x 6.3

323.9 x 7.1323.9 x 7.1355.6 x 8

406.4 x 8.8406.4 x 8.8

457 x 10457 x 10508 x 11

* DR refers to the use of adjustable anchorage at the damper location level, while DS refers to the use of fixed anchorage.** These dimensions are given for indication only . They must be adapted according to specificities of the project.

Dimensions noted in millimetres.Other sizes available on request. For larger sizes please contact your local VSL representative.



THE VSL GENSUI DAMPER – HOW IT WOShear deformation of high damping rubberThe heart of the VSL Gensui damper is a highdamping rubber (HDR) material developed bySumitomo Rubber Industries Ltd. whichdissipates the kinetic energy of the vibrationby shear deformation of specially-designedpads. Each damper consists of a series ofpads mounted between a moveable collarattached to the tensile element of the staycable and a fixed support rigidly connected tothe guide pipe. The damper’s characteristics are adjusted foreach structure by varying the number of padsand choosing between two pad types, «highperformance» and «standard». The HDR canwithstand large shear deformations and canalso cater for significant deformation undertension or compression without affecting itsdamping properties. This allows longitudinalmovements between the cable and support tobe accommodated entirely within the HDR,which avoids the need for additional hinged orsliding interfaces that would affect thedamping ratio.

Durability without maintenanceThe high-damping rubber pads have a longdesign life and excellent fatigue resistance.Accelerated ageing tests have demonstrated alife expectancy of 60 years and the pads havesustained 10 million load cycles during fatiguetesting. The damper requires only minimal maintenanceduring its operating life: this allows it to beplaced if necessary even at the pylon, wheremaintenance access is difficult and expensive.

Gensui pads

Oscillatingmovement

Fixed supportconnected to

guide pipe

Moveablecollar

Incheon Bridge, Korea - 2009



WORKSIndependent of vibration mode and amplitudeThe behaviour of the VSL Gensui damper canbe modelled as a combined device consistingof a spring, a friction member and a viscouselement all contained in the HDR pad. Thismakes the damping performance of the VSL Gensui damper largely independent of thevibration mode and the amplitude. The time-displacement curves recorded from full-scaletests clearly illustrate this behaviour.

Time-displacement curves as recorded during VSL Gensui damper testing

The VSL Gensui damper performsbest on short to medium lengthstay cables or where compactsolutions are required.

First mode of vibration

Second mode of vibration

Third mode of vibration


The VSL Gensui damper can also be mounted as an external damperif required for special retrofitting applications.


THE VSL GENSUI DAMPER – HOW IT WOFull-scale test resultsAs well as laboratory testing, numerous full-scale in-situ tests have been carried out toassess the VSL Gensui damper’s performanceand the characteristics of the HDR componentunder varying environmental conditions. The pads have been subjected in laboratorytests to fatigue loading where they had tosustain 10Mio load cycles. To assess theirdurability, accelerated ageing tests have beencarried out, from which a life expectancy hasbeen derived that is in excess of 60 yearswithout deterioration of the mechanicalproperties.

The Gensui pad: the heart of the VSLGensui damper

Megami Chashi, Japan - 2006



WORKS

Indicative dimensions of the VSL Gensui dampers, for each anchorage size in the VSL SSI 2000 stay cable system brochure

type6-126-196-226-316-376-436-556-616-736-856-91

6-1096-127

L1 min DR*1500175019002100230025502650285030503150340035503950

L1 min DS*900

120013501550175020002050225024502600280030003350

L2200200200220220240240260260270270290300

Ø1**410430450470470490510530530560570600620

Ø2 DR*219.1 x 6.3267 x 6.3

298.5 x 6.3323.9 x 6.3355.6 x 6.3406.4 x 8.8

419 x 10419 x 10508 x 11508 x 11

559 x 12.5559 x 12.5610 x 12.5

Ø2 DS*177.8 x 4.5219.1 x 6.3219.1 x 6.3244.5 x 6.3273 x 6.3

323.9 x 7.1323.9 x 7.1355.6 x 8

406.4 x 8.8406.4 x 8.8

457 x 10457 x 10508 x 11

* DR refers to the use of adjustable anchorage at the damper location level, while DS refers to the use of fixed anchorage.** These dimensions are given for indication only . They must be adapted according to specificities of the project.

Dimensions noted in millimetres.Other sizes available on request. For larger sizes please contact your local VSL representative.

Highly deformableThe damper pads, including the interfacebetween the HDR and the mounting plates,have been designed to accommodate largedeformations without damage. The dissipatedenergy per shear cycle even increases withincreasing deformation. The maximumpermissible deformation is considerably inexcess of the actual movements occurring inthe damper. This results in very high durabilityand fatigue resistance.

Hysteresis of a VSL Gensuidamper «standard» pad under

varying shear displacement



APPLICATIONSUddevalla Bridge, Sweden - 1998

Neva Bridge, Russia - 2006

Canada Line Pier Bridge, Canada - 2008

Puente La Unidad, Mexico - 2003

Badajoz Bridge, Spain - 1996



VSL – guided by a strong QSE cultureVSL’s leading position is based on a rigorous andcommitted quality culture. The QSE (Quality, Safety,Environment) policy represents a major focus for everyservice provided. Local teams ensure co-ordination ofactions, encourage sharing of experience and promotebest practices, with the aim of continuously improvingperformance. In VSL’s culture, employees are vitallyimportant to the competitiveness and prosperity of thecompany. VSL is committed to maintaining the highestlevels of client satisfaction and personnel safety .

Changing the way we do businessFor VSL, sustainable development means striking abalance in its development model between the economicprofitability of its businesses and their social andenvironmental impacts. This commitment has beenformalised into the VSL Sustainable Developmentprogramme, which focuses on safety together with theuse of fewer scarce materials and less energy as well asthe production of less pollution and waste.

Creating sustainable solutions with VSL DampersVSL’s Dampers have been designed andconstructed as very durable systems.Moreover, their function in mitigatingvibration reduces the risk of material fatiguein the stays and in the bridge, which in turnreduces the need for maintenance andreplacement during the service life of thestructure. They make a vital contribution to increasing the life expectancy of thestructures where they are installed.

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Copyright 2010, VSL International Ltd. Printed in France – patented.

The information set forth in this brochure including technical andengineering data is presented for general information only. While everyeffort has been made to insure its accuracy, this information should not beused or relied upon for any specific application without independentprofessional examination and verification of its accuracy , suitability andapplicability. Anyone using this material assumes any and all liabilityresulting from such use. VSL disclaims any and all express or impliedwarranties of merchantability fitness for any general or particular purpose orfreedom from infringement of any patent, trademark, or copyright in regardto the information or products contained or referred to herein. Nothingherein contained shall be construed as granting a license, express orimplied under any patents.

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Vsl Damping Systems for Stay Cables