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BBR VT International Ltd is the Technical Headquarters and Business Development Centre ofthe BBR Network located in Switzerland. The shareholders of BBR VT International Ltd are: BBRHolding Ltd (Switzerland), a subsidiary of the Tectus Group (Switzerland); KB Spennteknikk AS(Norway), BBR Polska Sp. z o.o. (Poland) and VORSPANN-TECHNIK GmbH & Co. KG (Austria/ Germany), all members of the KB Group (Norway); BBR Pretensados y Técnicas Especiales,S.L. (Spain), a member of the FCC Group (Spain).
The BBR Network is recognized as the leading group of specialized engineeringcontractors in the field of post-tensioning, stay cable and related constructionengineering. The innovation and technical excellence, brought together in 1944by its three Swiss founders – Antonio Brandestini, Max Birkenmaier and MirkoRobin Ros – continues, more than 60 years later, in that same ethos andenterprising style.From technical headquarters in Switzerland, the BBR Network reaches outaround the globe and has at its disposal some of the most talented engineersand technicians, as well as the very latest internationally approved technology.
The Global BBR NetworkWithin the Global BBR Network, established traditions and strong local roots arecombined with the latest thinking and leading edge technology. BBR grants eachlocal BBR Network member access to the latest technical knowledge andresources – and facilitates the exchange of information on a broad scale andwithin international partnering alliances. Such global alliances and co-operationscreate local competitive advantages in dealing with, for example, efficienttendering, availability of specialists and specialized equipment or transfer oftechnical know-how.
Activities of the NetworkAll BBR Network members are well-respected within their local businesscommunities and have built strong connections in their respective regions. Theyare all structured differently to suit the local market and offer a variety ofconstruction services, in addition to the traditional core business of post-tensioning.
BBR TechnologiesBBR technologies have been applied to a vast array of different structures – such as bridges, buildings, cryogenic LNG tanks, dams, marine structures,nuclear power stations, retaining walls, tanks, silos, towers, tunnels, wastewatertreatment plants, water reservoirs and wind farms. The BBR brands andtrademarks – CONA, BBRV, HiAm, DINA, SWIF and CONNAECT – arerecognized worldwide.The BBR Network has a track record of excellence and innovative approaches– with thousands of structures built using BBR technologies. While BBR’shistory goes back over 60 years, the BBR Network is focused on constructingthe future – with professionalism, innovation and the very latest technology.
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Although BBR is mostly famous for wire stay cables, we were
actually also the inventors of strand and carbon stay cables – and
carried out the world’s first projects using high fatigue resistant
wire, strand and carbon stay cables ... we are the company who
started it all!
When you read the pages which follow, you’ll discover that BBR Stay Cable
Technology is simply the best there is – excellent performance, flexible product
range, easy to install.
Our Swiss roots are deeply embedded in technological development and, down
the years, our engineers have constantly striven to produce the most advanced
products and technology. Today, this combines with a strong international
network – the BBR Network of Experts – who first listen, then advise and
deliver best-in-class solutions to customers around the globe.
In many ways, our story is just beginning – we had the longest experience in
the 20th Century … and you can bet on us having the longest in the 21st
Century too!
Innovation Excellence Experience
2 Cable-stayed structures
7 Superior strand stay cabletechnology
10 Flexible options
12 BBR HiAm CONA®
(dimensions)
15 Testing, design & detailing
16 Classical considerations
19 Cable damping
21 Pushing classical boundaries
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What is the first thing that you think about when you see a cable-stayed structure? Is it
perhaps the strength of the technology supporting that structure – or maybe it is the
sheer elegance that stay cables bring to the landscape or city skyline?
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Some of the most dramatically beautiful architectural designs and technically excellent
feats of engineering provide a reliable service, on a daily basis, to thousands of people
around the world. Many of these creations have been realized with the use of BBR
Technologies ..........
“By far the best proof is experience.”Sir Francis Bacon
English author, courtier, & philosopher 1561 – 1626
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Cable-stayed structures............ for decades, BBR has offered the very best, state-of-the-art
technology for cable stayed structures and today this is backed by over
60 years of specialist technical know-how.
BBR Stay Cable Technology has beenapplied to over 400 major structuresaround the world. While many cablesuppliers built their first major cablesupportedstructure in the late 1970sand early 1980s, BBR Stay CableTechnology wasused for the first time inthe late 1950s and, since those days,BBR has followed on with milestone-after-milestone and continues to set thestandard in the field of stay cables.
Stay cable applications
BBR Stay Cable Technology can be used forthe following applications:
Cable-stayed bridges – built in rapidlyincreasing numbers since 1950 andespecially suitable for medium- to long-spanbridges from 100 to 1,000 m, wheretechnical and economic factors dictate thissolution. For smaller bridges, otherparameters may be decisive in the choice ofa cable-stayed solution – such as reduceddepth of deck, construction methodologyand aesthetics. BBR Stay Cable Technology isthe ideal choice for the cables.
Arch bridges – where BBR Stay CableTechnology is the solution of choice for thehangers.
Roofs – of grandstands, stadiums, aircrafthangars and other lightweight wide-spanstructures are an ideal application for BBRStay Cable Technology.
Towers – for communication facilities,chimneys and antennas, as well as windpower stations can be stabilized using BBRStay Cable Technology.
Advantages of using stay cables
Cable-stayed structures have a greatpotential when it comes to meeting theincreasing demand for long span structures.Their advantages lie mainly in increasedaerodynamic stability, reduced costs for theabutments and faster, easier constructionand light overall structures. However, themost important factor in ensuring thedurability and performance of a cable-stayedstructure is the stay cable system.
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International specifications
Specifications for stay cables have, in thepast, always been covered by guidelines andrecommendations, historically the mostpopular has been PTI – Post-tensioningInstitute (USA), Recommendation for StayCable Design, Testing and Installation.
There are other less common and minornational recommendations such as, forexample, from SETRA/CIP in France.National recommendations cover onlylocally available materials and constructionpractices and are limited to the knowledgeof local suppliers, which lead to anunjustified and incorrect treatment of staycable systems as a whole. To ensureinternational and legally correct tenders,such national recommendations should not
be considered – they may occasionally onlybe used as complementary guidelines. Today,the state-of-the-art and internationallyversatile recommendation is fib –International Federation for StructuralConcrete, Bulletin 30, Acceptance of StayCable Systems using Prestressing Steels.
Importance of high fatigueresistance
Stay cables are subjected to high tensileforces and, given the fact that cablesupported structures are typically very lightstructures, the stay cables are subjected tohigh stress variations, thus high fatigueresistant stay cables are of greatimportance.
Photo: Kevin Lazarz
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Typical loadings of a stay cable
Besides maximum axial stresses in a stay
cable under service conditions, ultimate
state conditions and fatigue loadings, a series
of other loads must be considered at the
design stage – these include construction
loads, accidental loads and bending effects.
An additional factor is the durability of stay
cables and the most modern stay cable
systems have been developed and tested
(fib) with provisions leading to an intended
working life of the stay of 100 years and
more.
Research & development
Extensive research, testing and development
efforts place BBR at the forefront in the
field of post-tensioning and stay cable
applications. To assure the highest qualityproduct, all of the system components aresubjected to the most stringent testing andquality assurance procedures, based oninternationally recognized codes andrecommendations.
Beware of imitations
There has been much talk about counterfeitcomponents – copies of BBR Stay CableTechnology which ultimately risk lives anddo not guarantee the required performancefor the owners. There are indeed many staycable systems on the market which, despitesome of them looking very much like BBRsystems or even bearing our trademarks,they actually have no relation to the originaland genuine BBR technology. For stay cables,it is not only the technology itself which hasto fulfill the highest requirements, but alsothe installation of the cables on site mustconform to these standards and beexecuted only by trained professionals. Ifyou are in any doubt about a product orservice which is offered, seek advice fromBBR VT International Ltd. u
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The BBR HiAm CONA® parallel
strand stay cable system is the best
product in the international market
place. It has the highest capacity,
most compact and widest range
of anchorages available ..........
Superior strand staycable technology
BBR HiAm CONA
Key Benefits
• Tendon capacity 200 – 60,000 kN
• Superior fatigue resistance
• Advanced water tightness
• High corrosion protection
• Compact anchorage and cable
• Simple installation
• Easy maintenance
Superior strand stay cable technology................ developed, tested and continuously maintained by BBR engineers in
Switzerland, the BBR HiAm CONA parallel strand stay cable system is being used by
the BBR Network around the world. Combined with the installation expertise of the
BBR Network – backed by the Engineering and Special Projects Team from the Swiss
Headquarters – this system is simply unrivaled anywhere on the planet.
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Strong & sleek
Its superior fatigue resistance – ‘HiAm’stands for high amplitude fatigue resistance– makes it attractive for the mostchallenging of projects and thus it appealsto engineers and clients alike. Designersand architects have welcomed, inparticular, the compactness of theanchorage and cable system, as it allowsthem greater scope to produce astructure which has sleeker lines andwhich appeals to the visual senses of allwho use and gaze upon it.
Certification
The BBR HiAm CONA stay cable system isdeemed approved and in compliance withthe fib as well as the corresponding PTI andSETRA recommendations.
Local knowledge – internationalexpertise
The BBR HiAm CONA Stay Cable systemis exclusively installed by teams of certifiedBBR PT Specialist Companies. Cable-stayed bridges are highly-specialisedengineering projects, requiring local know-
how and specific engineering knowledge.
Therefore the local project management is
typically handled by the local BBR
Network member, while all stay cable
specification, engineering system
component manufacturing and
procurement is handled by the Special
Projects Team from the Swiss-based BBR
Headquarters.
Stay cable configuration
BBR HiAm CONA cables are made up of acompacted bundle of a predeterminednumber of parallel seven-wire strandsenclosed in a co-extruded (carbon blackinternal and colored external) ultra-violetresistant high-density polyethylene (HDPE)sheath of circular cross-section. The individualstrands generally have a diameter of
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15.7 mm (0.62”), are of low relaxation grade,with nominal cross-sectional area of 150 mm2
and a minimum Guaranteed Ultimate TensileStrength (GUTS) of 1,860 MPa. The strandsare galvanized, corrosion inhibited andindividually sheathed with a continuous andwear resistant HDPE coating, providing eachstrand with an individual multilayer protectionsystem with three nested barriers.Alternatively, coated strands with acorresponding corrosion protection systemmay also be used.
Anchorage configuration
In the anchorage zone of the BBR HiAmCONA cables, the strand bundle passes adeviator and spreads out towards the BBRHiAm CONA socket, where each strand isindividually guided, sealed leak tight and
locked in the anchor heads with speciallydesigned high fatigue resistant BBR HiAmCONA grips. Ring nuts screwed onto theanchor heads transfer the cable loads bycontact pressure onto the supporting bearingplates. Alternatively, the anchor heads maytransfer the loads directly onto the bearingplate. All anchorage components of the BBRHiAm CONA system have been designed fora stress range greater than 300 MPa and towithstand the ultimate breaking load of thestrand bundle with adequate safety.
Bending damper & counteringcable vibration
In the socket of the anchorage, each strandis individually protected with a proprietarybending damper. Bending effects in cablesmay be introduced from excessive
construction tolerances and cable vibrations.Supplemental internal or alternativelyexternal high damping devices protect thestay cable from vibrations. Another effectivecountermeasure against wind and rain-induced vibrations is the use of a helical ribon the outside of the cable surface.
Installation
The installation of the BBR HiAm CONAsystem is typically performed on site usingthe strand-by-strand installation method,which is comprised of four basic steps:
u Installation of the upper (pylon) andlower (deck) HiAm CONA anchorage.
u The preassembled stay cable sheath ishung between the two anchorages usingtwo master strands. The stay cable sheathis now used as a guide passage fromanchorage-to-anchorage.
u The strand is positioned at deck level andpulled up through the stay pipe and theupper anchorage and inserted into thelower anchorage.
u Each strand is tensioned immediatelyafter installation, using the BBRISOSTRESS tensioning method, ensuringan equal stress distribution among thestrands of an individual cable.
Alternatively to the single-strand installationmethod, fully or partially prefabricatedcables can be installed and tensioned.
Single strand replacement
Each individual strand installed in the BBRHiAm CONA cable system can be re-stressed at any time during or after theinstallation, allowing not only for a re-stressing, but also for the selective removal,inspection, replacement or addition ofindividual strands. u
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Flexible optionsThe BBR HiAm CONA stay cable system is designed to fit every project,
so different options are available.
Ultimate flexibility – standard and compact
All BBR anchorages can be combined with each other meaning that, for example, astandard BBR HiAm CONA Nut Head can be used for the stressing anchorage and aCompact BBR HiAm CONA Uni Head can be used for the dead end of the cable. Inaddition, all details of the anchoring, force bearing elements and sealing are identicalfor both the standard and the Compact version of the anchorage, thus theperformance is absolutely identical. Use of a BBR Regulator under the BBR HiAmCONA Uni Head anchorage can transform the otherwise dead end anchorage intoan adjustable anchorage with any required adjustability.
BBR HiAm CONA Uni Head non-adjustable anchorage
The end of the stay cable from which thetensioning is performed – the stressing endof the cable – is fitted with adjustable BBRHiAm CONA Nut Head anchorages andthe opposite end, or the dead end of thecable, is typically fitted with BBR HiAmCONA Uni Head anchorages.
Standard configuration
The standard configuration of theadjustable and dead anchorage requiresidentical openings in the bearing plate –thus, if the structure is designed with thisphilosophy and assumptions, the stressingand the dead end orientation of the cableis interchangeable at any time during thedesign stage of the cable-stayed structure.
BBR HiAm CONA Nut Head anchorage(A): Adjustable anchorage, with a typicaladjustability of 0, 60 or 120 mm. Theadjustability can be modified toaccommodate any regulation specification.This anchorage is required at the stressingend of the cable and may also be requiredat the dead end anchorage if fullyprefabricated cables are installed – or if theanchorage detail in the structure does notpermit an installation of the anchoragefrom the back face of the bearing plate.
BBR HiAm CONA Uni Head anchorage(F): Non-adjustable anchorage with identicalkey dimensions compared to the BBRHiAm CONA Nut Head anchorage with 0mm adjustability. This anchorage should beused if the same anchorage details at thestressing and dead end are desired and ifthe anchorages can be installed from theback face of the bearing plate.
Compact configuration
In addition to the standard configuration, aCompact version is offered for both theBBR HiAm CONA Nut Head and the BBRHiAm CONA Uni Head anchorage. TheCompact version suits smaller openings inthe bearing plate, compared to thestandard configuration. All Compact BBRHiAm CONA anchorages requireinstallation from the back face of thebearing plate.
Compact BBR HiAm CONA Nut Headanchorage (CA): Adjustable anchorage,with a typical adjustability of 0, 60 or 120 mm. Use of the Compact Nut HeadAnchorage is only recommended forspecial applications, such as cablereplacements with given openings in thestructure or other constraints which dictateusage of the compact anchorage.
Compact BBR HiAm CONA Uni Headanchorage (CF): Non-adjustable anchoragewith reduced dimensions compared to thestandard Uni Head.
BBR HiAm CONA Nut Headanchorage 120 mm adjustability
BBR HiAm CONA Nut Headanchorage 0 mm adjustability
Anchorage options
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Wind induces static and dynamic effects oncable-stays and should therefore be takeninto account during design. The static dragforce of wind on a stay cable causessignificant transversal stresses on the pylon,particularly on large cable-stayed bridges.The drag force Fd [N/m] is given by:
Where ρA [1.25 kg/m3] is the density of air, U [m/s] is the wind velocity , Ds [m] is theouter cable diameter and CD is the dragcoefficient.
As the formula above indicates, thepredominant factor is wind velocity as itgoes in square. For instance, the drag forceincreases by 78% when U rises from 30 m/sto 40 m/s – assuming the other factorsremain stable.
In a classic case of circular stay pipes, thevalue of the drag coefficient depends on thewind velocity, or more specifically on theReynolds number Re, and the roughness ofthe outer casing.
Three basic ranges of CD can be observed:
u Subcritical range at low wind velocitywhere Re is below 2·105: High dragcoefficient of 1.20
u Critical range where Re is between 2·105 and 8·105: The drag coefficientdrops significantly
u Supercritical range at high wind velocitywhere Re is above 8·105: Low dragcoefficient of 0.50-0.60.
Cable stays are commonly in thesupercritical range in strong winds. A CD of0.50 for a smooth BBR stay pipe and a dragcoefficient of 0.55-0.60 for a BBR stay pipewith helical rib can be achieved in windtunnel tests. Nevertheless, the effects ofextreme winds are often calculated byadopting a CD of 0.70-0.80 to be on the
safe side and to allow for the possibleevolution of surface roughness (dirt build-up, etc) over time.
Reduced wind loads can be achieved byreducing the outer cable diameter.
For long span bridges, where the cable staydrag is a preponderant factor, the installationof compact BBR stay pipes should beevaluated. The compact system enables thedrag force to be reduced by 20% comparedto the standard system. This system requiresspecial material and installation techniqueson site. BBR’s first application of compactstay pipes was in 2000, when the 475 mlong Rama VIII Bridge in Bangkok, Thailandwas erected.
BBR HiAm CONAcompact strand
stay pipe
BBR HiAm CONAstandard strand
stay pipe
BBR HiEx CONA® Parallel strandcable system for extradosed bridges
The BBR HiEx CONA cable system forextradosed bridges is a further developmentof the BBR HiAm CONA stay cable systemand has been fine-tuned for the specificrequirements of extradosed bridges. Unlikestay cables, the cables of an extradosed bridgecan be subject to stresses at a higher level(55% GUTS), which is reflected in specific testswhich have been executed on the BBR HiExCONA cable system, such as the Category Afatigue and static tests according to SETRA.
Standard and compact stay pipe options
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In the anchorage zone of the BBR HiAmCONA stay cables, the strands are bundledat the deviator and within a transition lengthspread out towards the BBR HiAm CONAsocket. Depending on the chosenconfiguration – guide deviator, free deviatoror damper – a different transition length isrequired.
Guide deviator
Guide deviators have historically beenused, with good experience, to support thestay cable laterally and to limit transversedisplacements of the stay cables. As aconsequence, they protect anchoragesfrom the effects of transverse loads, whichare transferred into the structure at thelocation of the guide deviator. When usinga guide deviator, the minimum requiredtransition length is denoted by GDL, seepages 12 & 13.
Free deviator
Use of a guide deviator is not necessary ifconstruction tolerances and anchoragerotations under the governing service andultimate limit states are moderate andbelow the applicable limit of nationalregulations, PTI or fib (e.g. ± 0.3° and ± 1.4°). In such cases where a free deviatoris used, consideration should be given toinstalling a BBR Square Damper to avoid anypossibly large additional anchorage rotationscaused by cable vibrations. When using afree deviator, the minimum requiredtransition length is denoted by DVL withthe option of adjustment for differentanchorage rotations, see pages 12 & 13.
BBR Square Damper
If a BBR Square Damper is installed to addsupplemental damping to the stay cable, thetransition length must be adjusted so thatthe transversal movement at the damperlocation – due to service loads, wind,temperature and cable vibration – can beintroduced safely into the anchorage. Whena standard BBR Square Damper is used, theminimum required transition length, takinginto account the maximum free amplitudeof the damper (80mm), is denoted by SDL,see pages 12 & 13. This distance, however,might be increased due to significantstructural rotations at the location of theanchorage or to provide enoughsupplemental damping.
For special applications, an additional BBRBending Damper outside of the socket ofthe BBR HiAm CONA anchorage can beconsidered, which allows for higherrotations and a minimal transition length.
Please read this section in conjunctionwith the section on “Testing, design anddetailing” later in this brochure – orcontact BBR VT International Ltd fordetails. u
Flexible options
Transition length options
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BBR HiAm CONAwith BBR Square Damper
BBR HiAm CONAguide deviator
BBR HiAm CONAfree deviator
Testing, design & detailingDesigners, builders and owners of cable-stayed structures all need to be certain they have specified
components which will deliver the level of performance they seek. ...
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Stay cable anchorageperformance testing
The PTI specifies that strand stay cablesshould withstand certification testing of2·106 load cycles with a stress range of 159 MPa at an upper load of 45% of theGuaranteed Ultimate Tensile Strength(GUTS) of the tensile elements. The morerecent international fib recommendationscall for a fatigue stress range of 200 MPa. Inaddition, anchorage rotations of 0.6° areintroduced during the test – to simulateconstruction tolerances and local bendingeffects from live loads and cable vibrations.Eventually, the tendon is loaded to failureand the tensile resistance subsequent to thefatigue test must be greater than 95%GUTS.
Service Limit State (SLS) design
The cross-section of a stay cable is typicallysized such that the maximum axial stress inthe stay cable under service conditions(SLS) does not exceed the specified limits.In the past, the maximum axial stress was
usually limited to 45% GUTS. Due to themore stringent testing requirements, asspecified by fib, higher axial stresses of up to50% GUTS are nowadays consideredpermissible, but only for the latestgeneration stay cable systems.
Loadings of stay cables during constructionor cable replacement should not introduceinelastic deformations in the stay cablesystem, and a verification of axial stressesagainst permissible stresses is oftensufficient. The permissible axial stressesduring construction and stay cablereplacement was historically limited to 55%GUTS but, with more advanced materialsand certification requirements, the limits arenowadays set at 60% GUTS.
Ultimate Limit State (ULS)design
When verifying the ultimate limit state(ULS), GUTS of the tensile elements can beconsidered as the characteristic tensilestrength of the stay cable system – theresistance factors, in accordance withnational standards, should then be appliedto find the design strength. If such resistancefactors for stay cables are not provided innational codes, one may use a resistancefactor of 1.35 for stay cables tested withangular rotation and of 1.50 for stay cablestested without angular rotation.
Classical considerations.... Consequently, there are a number of classical technical details
concerning testing, design and detailing which have to be considered.
Fatigue and subsequent tensile test
Typical loads in stay cables
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Fatigue Limit State (FLS) design
Cable-stayed structures are typically lightstructures and the stay cables thereforeexperience high stress variations – so highfatigue resistant stay cables are of greatimportance. The fatigue design of stay cableshas to consider the relevant fatigue loads, inaccordance with national standards appliedto the particular structure, to determine thefatigue relevant stress range in the stay cables– and then compare it with the fatigueperformance of the stay cable system.
In the simplest case, the relevant fatigueload is a specific truck (axle load) and thestress variations in the stay cable created bythis loading which are then compared witha reduced stay cable fatigue test resistance,whereas the reduction depends on nationalregulations. In an actual design situation,fatigue verification may need to beperformed at a number of load cycles –other than 2·106 load cycles, where‘Wöhler-Curves’ (S-N curves) can be used.
Fire and impact
Bridges are well-ventilated and aretherefore rarely exposed to hightemperatures in the event of a fire. If a truck
were to catch fire on a cable-stayed bridge,the resultant blaze would normally beunlikely to affect more than one stay cableat a time – except where stay cables areclosely grouped, for example, back stays.Structural stability is thus not generally aproblem. However, some bridges arelocated in special environments – such asnear fuel depots or oil refineries – wherethey will be frequently be used by fueltrucks. In such cases, improved fireresistance of stay cables may be justified to
avoid loss of main tensile elements in theevent of a fire.Typical fire or impact designconsiderations establish that the failure ofone single stay cable should not lead tofailure of the entire cable-stayed structure.The designer should also take into accountthe dynamic effects caused by the breakageof the stay. Additional measures might berequired for grouped stay cables, wherestructural impact barriers might providesuitable protection.
Replaceability of stay cables
Stay cable systems should be replaceable –this is particularly important for bridges. Atan early stage, a decision should be taken asto whether the stay cables of the structureare going to be replaceable – eitherindividually, or several at a time. It shouldalso be specified whether replacement isfeasible under full, reduced or zero trafficload. Typically for highway bridges, individualstay cable replacement should be factoredinto the design – under reduced traffic load,meaning closure of the nearest traffic lane.
Durability
Modern stay cables have a multiple layercorrosion protection system and have toundergo severe corrosion and leak tightnesstesting. Today, modern stay cables – whichhave been fully tested to the latestprovisions – have a projected service life of100 years.
S-N fatigue curves
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Classical considerations Construction tolerances
In order to comply with the assumptions ofthe PTI and fib for flexural effects nearanchorages, the designer should specify aninstallation tolerance of the bearing platesand recess pipes of 0.3° (5 mrad) aroundthe theoretical axis of the stay cable.
Transversal loads
Stays in cable-supported structuresessentially carry tensile loads. However,although minimal in comparison with axialloads, transverse loads from differentsources also act on the stay cables. Maincauses of transverse loads are:
u construction tolerances andmisalignments
u change of cable sag caused byconstruction and traffic loads
u rotation of the anchorage points due toloadings on the structure
u wind loads on the cables
u temperature changes.
As already mentioned, guide deviators haveoften been used to support the stay cable
laterally and to limit transverse
displacements of the stay cables - thus,
protecting the stay cable anchorages from
the effects of transverse loads.The
transverse support provided by the guide
deviator to the stay cable causes a kink in
the geometry of the stay cable.
Consequently, the cable exerts a transversal
force to the guide deviator and the guide
deviator to the structure. These transverse
forces are the product of angular kink and
axial stay cable force, usually the permanent
stay force. As guidance for preliminary
design of the structure supporting the guide
deviator, an angular kink of 1.4° (25 mrad) is
suggested as a reasonable assumption,
which leads to a transversal load in the
order of 2.5% of the cable force.
Bending
Stay cables are characterized, in comparison
with other structural elements, by possessing
a very great slenderness. This characteristic
makes them very flexible under distributed
normal loading to their axial configuration –
and almost precludes the appearance of
bending stresses in their free length.
However, stay cables might locally sufferbending stresses at anchorages or whenpassing over a saddle. In the first case, theorigin of bending stresses has already beendiscussed in the section dealing withtransversal load and, in the second case,bending stresses arise when the cable isforced to follow the saddle’s curvature. Inboth situations, bending stresses might be ofthe same order of magnitude as the axialstresses and may require specific analysis.The maximum fixed end index bendingstress σB [MPa] in stay cables at theanchorage location might be evaluated bythe following equation:
Where α [rad] is the angular deviation ofthe stay cable with respect to thepermanent position and EP [MPa] and σA
[MPa] are the Young Modulus and the axialstress in the steel respectively. Evaluating thisequation shows that for relatively smalldeviation angles, the overall stress level(axial stress + bending stress) might exceedthe allowable limit. Consequently, it is alwaysrecommended that suitable and fully testedprovisions are made to minimize thebending stresses occurring at theanchorages. u
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Even newly constructed cable-stayedbridges have experienced quite severevibrations. Several cable vibrationmechanisms have been identified andcharacterized, with the four most commonphenomena being vortex shedding,galloping, parametric excitation – deck/pylonand cable interaction – and wind and rain-induced vibrations. The short-termconsequence of cable vibration iscomplaints from the public – bridge users –the long-term consequences are reducedsafety or even failure of complete cablescaused by a rapid accumulation of bendingfatigue stress cycles at the anchorages.
Special measures
Structural elements have a certain level ofinherent “self-” damping – which,conservatively, for strand stay cables can beassumed as 0.8% logarithmic decrement.The inherent damping of a stay cable is themaximum rate at which the cable dissipatesthe energy which makes it oscillate. Oftenthe inherent damping is not sufficient todamp the stay cables and then it isnecessary to add passive supplementaldamping. Additionally to the supplementaldamping, the installation of special measures– like surface treatment of the cable andcross-ties – might improve the response ofthe stay cable against vibrations.
Supplemental damping
Supplemental damping devices add dampingto the cable – hence achieving sufficienttotal damping as an efficient measure againstcable vibrations. If the cable begins tovibrate, with the movement of the cable atthe position where the cable is attached tothe damping device, energy is dissipatedthrough the damper to stabilize the cable.Active damping devices are also available,but they require external power sources,high maintenance and should therefore onlybe considered for repairs and retrofits.The theoretical Maximum PassiveSupplemental Damping is independent from
Cable dampingDespite the wide use of cable-stayed bridges, there are still several
areas of great concern, especially the effects and elimination of cable
vibration phenomena.
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Specification
The Required Supplemental Damping should be specified by the designer for a particular stay cable arrangement, stay pipe configuration(diameter, with or without surface treatment) and damper location. A sufficient factor of safety, in the order of load factors applied instructural engineering, must be achieved between the Required Supplemental Damping and the Maximum Theoretical SupplementalDamping:
δReq.Sup, Required Supplemental Damping [%] < δMax.Sup, Maximum Passive Supplemental Damping [%]
Cable damping
“The goal of the engineeringcommunity is to give birth tohealthy structures withoutplanned implementation ofpace-makers before thestructures are even born.”
the particular damping device (internal or
external, oil, friction, viscous etc) and a
measure of the maximum possible damping
that the “perfect damper” can provide to
the stay cable. The key parameter is the
distance at which the damper is placed from
the anchorage – a suitable position for a
damper should be considered by the
designer at an early stage. It is often
suggested that dampers should be placed at
2.5% of the cable length.
Evaluating susceptibility
A preliminary evaluation of the susceptibilityof a stay cable to vibration can beperformed using the Scruton Number. Ingeneral, it is recommended that the ScrutonNumber be kept as high as possible andvalues greater than 10 are often suggested.For unusual cable geometries, multiplecables within very close proximity or, ifformation of ice on the cable surface isprobable, careful case-by-case evaluations ofthe above-mentioned limits arerecommended.
Damper installation
Dampers are usually installed once staycables are structurally active and carry thepermanent loads of the structure. Afterinstallation, factors – such as service loads,traffic, wind and temperature – modify thegeometry of the entire structure andconsequently induce relative rotationsbetween the structure and the stay cable,which result in transversal and longitudinalmovements at the damper location. Thesemovements are often larger than thoseimposed on the damper and on theanchorages by possible cable vibrations.
To ensure good damping performance,durability and safety, the damper, stay cablesand the anchorages have to be seen as anintegrated system which has to be analyzed,designed and detailed as a whole.Consequently, both the stay cable and thedamping device should be provided by thesame company.
Where, Sc is the Scruton Number, ρA [1.25kg/m3] is the density of air, DS [m] is the outer diameter of the stay cable, mS [kg/m] is the linearweight of stay cable, ρI is the inherent damping and LD [m] and LS [m] are the distance from the anchorage to the damper and the length ofthe stay cable respectively. u
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Pushing classical boundaries– BBR HiAm CONA®
While the recommendations made by the PTI and fib are
well-established and respected, BBR has chosen to go still
further and has tested its products using even
more stringent criteria.
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Pushing classical boundaries – BBR HiAm CONA®
Fatigue, tensile resistance anddesign values
The BBR HiAm CONA stay cable systemhas been subjected to extensive testing,according to state-of-the-art fib testingprovisions. Remarkably, the system has alsobeen tested with parameters exceeding thetraditional requirements of fib and the PTI –for example, to upper stress limitsexceeding 45% GUTS during fatigue tests. Inaddition, the system has been designed for afatigue stress-range of greater than 300 MPa. Given the more stringent testingperformed on the system – and upon thejudgment of the designer – different designvalues might be considered using the BBRHiAm CONA system (see table below).
Durability
In addition to fatigue and tensile tests, leaktightness tests according to the latestprovisions have been performed on theBBR HiAm CONA system, so that it isreasonable to assume a 100-year service life– and more.
Bending
The BBR HiAm CONA® system isequipped with a special bending damperand it can therefore be assumed that thefixed end index bending stress, explainedearlier, can be reduced by 90%.Furthermore, the index bending stress – inthe wires of the strands inside theanchorage and socket – remain constantand essentially independent of the axialstress and anchorage rotation. The systemwith these index bending stresses has beentested in fatigue with a subsequent tensileresistance greater 95% GUTS.
Due to the high bending stress resistanceof the BBR HiAm CONA® anchorage, theuse of a guide deviator is not necessary ifconstruction tolerances and anchoragerotations under the governing service limitstates are moderate and below theapplicable limit of national regulations, PTIor fib (e.g. 0.3° and 1.4°). If a freedeviator is going to be used, considerationshould be given to installing a BBR SquareDamper – to avoid any large anchoragerotations due to cable vibration. Inaddition, under maximum anchoragerotations of the stay cable, possible impactpoints with the structure have to bereviewed. For bridges, the anchoragerotations at the pylons are often relativelysmall, thus a guide deviator might not berequired, especially if a BBR SquareDamper is installed at the deck anchorageto control cable vibrations.
Countering cable vibration
BBR offers an effective countermeasureagainst wind and rain-induced vibrationsby fitting the outside cable surface with ahelical rib. The helical fillet helps to preventthe formation of the coherent waterrivulets, which are responsible for thecable vibrations and therefore mitigatesthe excitation at its source. Using BBRStrand Stay Pipes with helical rib, theminimum required Scruton number toprevent cable vibration may be reduced toa value as low as 5. Additionally, thedamping requirement can further berelaxed by choosing a Compact BBRStrand Stay Pipe (see page 11).
The BBR Square Damper is a superiorsupplemental passive damping device,which is based on friction. The device canbe used as an internal damper, where it isinstalled inside the steel guide pipe oralternatively as an external damper,attached to the cable free length using adamper housing and external brace. If thetransversal force at the damper location,generated by cable vibration, exceeds the
PTI fib BBR
Fatigue and tensile test
Upper load 45% 45 ... 55% GUTS
Stress range 159 200 200 MPa
Tensile resistance 95% 95% GUTS
Design requirements
Service lmit state:
Service load 45% ... 50% 50 ... 60% GUTS
Short term 55% ... 60% 60 ... 70% GUTS
Ultimate limit state:
Resistance factor 1.35 ... 1.5 1.2 ... 1.35 _
Fatigue limit state:
Design limit 125 (NR) 170 ... 200 MPa
National regulation (NR)
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characteristic friction force of the damper,energy is dissipated and the cable isstabilized. The basic characteristics of theBBR Square Damper are:
u The damper is not activated at low andnon-critical cable vibration amplitudes, toavoid constant working of the damperand therefore minimizes maintenancerequirements
u The damping efficiency is independent ofthe acceleration and mode of cablevibration
u The damper, by itself, achieves theMaximum Passive Supplemental Dampingconsidered for a “perfect damper”, thusthe safety factors relating to RequiredSupplemental Damping can be reduced
u Free longitudinal movement and freerotation of the stay cable at the damperlocation is provided, allowing fortemperature elongation and forcevariations of the stay cable withoutconstraints
u The damping characteristics can easily beadjusted at any time.
Due to its simple design, high efficiency, easyadjustability and low maintenancerequirements, the BBR Square Damper is
superior compared to other dampingdevices, such as oil or gas dampers whichrequire frequent maintenance andreplacement. u
“Everything should be made as simpleas possible, but not simpler”Albert Einstein
BBR Square Damper
Having reached this page, you can certainly be in no doubt as to our
commitment to the finest technology and our enthusiasm for
delivering our projects.
Our six decades of experience has resulted in BBR stay cable
technology being applied to over 400 cable supported structures
and, in the process, we have continued to refine and enhance our
range. The result is that we can supply simply the best technology
available – the BBR HiAm CONA system.
Technology does not however develop by itself - all through the
years, we have been fortunate enough to have attracted some of the
best engineers in the business. It is their dedication which has
maintained the BBR reputation - and continues to do so today.
Our well-established worldwide network is supported in the
development of cable-stayed structures by our Special Projects Team
who will help to specify and procure the systems required. So, local
knowledge synchronises with international know-how to realise
projects – some large, some smaller, but always technically excellent
and dramatically beautiful!
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And finally ....
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“Without continual growth and progress, suchwords as improvement, achievement, and
success have no meaning.”Benjamin Franklin,
American Statesman, scientist, philosopher, printer, writer & inventor,1706-1790
BBR VT International Ltd
Bahnstrasse 23
8603 Schwerzenbach (ZH)
Switzerland
Tel +41 44 806 80 60
Fax +41 44 806 80 50
www.bbrnetwork.com
Copyright B
BR
VT
Inte
rnational 03.2
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