setra cable stays

Laboratoire Central

des Ponts et Chaussees

Driving research accross networks

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Recommendations of French interministerialcommission on Prestressing

~"~F~REPUBLIOUE PIANCAISE

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2002

Issued by the Service d'Etudes Techniques des Routes et AutoroutesCentre des Techniques des Ouvrages d'Art46, avenue Aristide Briand -BP 100 -92225 BAGNEUX CEDEX -FranceTel. 33 (0)1 46 11 31 53 -Fax 33 (0)1 46 11 3355 -www.setra.eQuipement.gouv.fr

CIP recommendations on cable stavs

CONTENTS

1115

Article 2.1 Evolution of cable-stay technology Article 2.2 Operation and required qualities of cable stays

18

2022

24

Article 3.1 Inventory of cable-stay ageing factors Article 3.2 Effects of mechanical and environmental factors

Article 3.3 Choice of materials Article 3.4 Replaceability

25

2831

36

Article 4.1 Dynamic par.ameters of cable stays Article 4.2 Physical phenomena inducing vibration Article 4.3 Remedial actions Article 4.4 Specifications to prevent cable-stay vibration

CHAPTER 5. STATIC BEHAVIOUR OE CABLE STA~

~

39

39

4247

51

53

Article 5.1 Introduction Article 5.2 Linear model of a cable stay Article 5.3 Approximate effect of cable-stay selfweight.

Article 5.4 Catenary model Article 5.5 Model of inextensible sagging cable Article 5.6 Modelling a real cable stay

57

58

63

63

Article 6.1 Cable stay deviated at a saddle Article 6.2 Taking account of the flexural stiffness of a cable at its anchorage.

Article 6.3 Vibrations in the free length of a cable stay Article 6.4 Cable bending and durability ..""""'..."'.""'."""""""""""""".'

~PTER

7. CABLE-STAY MECHANICS DURING CONSTRUCTION 67

Article 7.1 Preloading of cable stays 67

3

CIP recommendations on cable stavs ,.

Article 7.2 Intrinsic characterization of cable-stay preloading 71

Article 7.3 Calculating the instantaneous tension of cable stays 77

Article 7.4 Strand-by-strand tensioning 78

CHAPTER 8. DYNAMIC BEHAVIOUR OF CABLE STAYS 81--

Article 8.1 Taut-string model.. '..." ' '...'.' ' ' ' 81

Article 8.2 Vibration modes of a sagging cable stay 83

Article 8.3 Excitation by lateral displacement of an anchorage 91

Article 8.4 Parametric excitation by longitudinal displacement 93

Article 9.1 Common general requirements ."..."'.."'...' ' 99

Article 9.2 PSC category: parallel strand cable stays 103

Article 9.3 PWC category: parallel- wire cable stays 108

Article 9.4 MLS category: multi-layer-strand cable stays , 111

Article 9.5 Collective external barrier " ' ' 115

Article 9.6 Other kinds of main tensile element 119

CHAPTER 10. CABLE-STAY ANCHORAGE 121--

Article 10.1 Functions of a cable-stay anchorage 121

Article 10.2 General provisions common to all anchorage types 122

Article 10.3 Classification of anchorages 125

Article 10.4 Type C anchorages for parallel-strand cable .stays 126

Article 10.5 Type S anchorages for parallel-strand cable stays 130

Article 10.6 Type B or B+R anchorages for parallel-wire cable stays 131

Article 10.7 Type F+R anchorages for MLS cable stays 133

CHAPTER 11. QUALIFICATION TESTING OF A CABLE-STAY SYSTEM 139

Article 11.1 General 139

Article 11.2 Mechanical qualification of cable stays 140

Article 11.3 Qualification of cable-stay watertightness ' ' 147

CHAPTER 12. CABLE-STAY INSTAllATION 151

Article 12.1 Organizational aspects 151

Article 12.2 Supply 152

Article 12.3 Manufacture of cable stays 153

Article 12.4 Erection of cable stays 156

Article 12.5 Tensioning and adjustment 158

Article 12.6 Permanent corrosion protection 162

CHAPTER 13. MONITORING AND MAINTENANCE OF CABLE STAYS 165

Article 13.1 Principles and objectives of cable-stay maintenance 165

Article 13.2 Monitoring and maintenance 165

Article 13.3 Cable-stay adjustment 167

Article 13.4 Cable-stay replacement 168

4


CHAPTER 14. CABLE-STAY DESIGN AND VERIFICATION RULES 173

Article 14.1 General 173

Article 14.2 Actions and combinations of actions 173

Article 14.3 Cable-stay strength. ' 176

Article 14.4 Ultimate limit states 179

Article 14.5 Serviceability limit states.. 180

Article 14.6 Verifications of fatigue 182

Article 14.7 Saddles 184

Article 14.8 Extradosed prestressing tendons 184

CHAPTER 15. REFERENCES 189

Article 15.1 Standards 189

Article 15.2 Bibliograpical references ' ' ".' ". 191

CHAPTER 16. DEFINITIONS AND NOTATIONS 193

Article 16.1 Glossary , 193

Article 16.2 Notation , 197

~

CIP recommendations on cable stays

1.FOREWORD

Early in 1997, the French Interministerial Commission on Prestressing (CommissionInterministerielle de la Precontrainte -CIP) set up a working group to study the technologicalproblems involved in cable stays and to establish an approval procedure similar to thatimplemented for prestressing systems.

The working group drafted these Recommendations, a state-of-the-art review advising on thedesign, qualification, and implementation of cable-stay systems. It calls on the experience acquiredwith cable-stayed bridges in France and elsewhere in the last thirty years or so. This experienceincludes large cable-stayed bridges such as the Brotonne Bridge and the Pont de NormandieBridge in France, the Second Severn Crossing in the UK, and the Vasco de Gama Bridge inPortugal, but also involves a wide range of smaller bridges.

The cable technology described in these Recommendations principally concerns cable-stayedbridges, the cables of which are characterized by large variations in tension, fatigue effects, anddirect exposure to the elements. More generally, it is hoped the recommendations will be of use forall cables exposed to climatic aggression, particularly to the ties of bowstring bridges, extradosedor intradosed prestressing tendons, and cables used in any stayed civil engineering structures,such as stadium roofs, masts, etc.

On the other hand, the applications of interconnected cable networks are beyond the scope ofthese Recommendations which do not, therefore, deal with cabled spaceframe structures orsuspension-bridge technology. In addition, cable-stay saddles are addressed only in the form of afew recommendations on design, but using them is advised against, because of their effect on thedurability of cable stays and because of maintenance and replacement difficulties.

These Recommendations are broken down into four parts:.Part 1 (Chapters 2 to 8) is a review of current scientific knowledge in the matter. It takes the

form of a manual which can be referred to by designers and which substantiates the choicesrecommended in the subsequent parts.

.Part 2 (Chapters 9 and 10) describes the cable-stay systems commonly used at the moment,and gives recommendations on the technology that can achieve the greatest durability.

.Part 3 (Chapters 11 to 13) is the benchmark for approval and implementation of cable-staysystems that the CIP required.

.Part 4 (Chapter 14) presents limit-state cable-stay design rules.

Texts in standard type are recommendations.Texts in italics are comments.Texts in small type are descriptions or examples.

7


MEMBERS OF THE CIP CABLE-STAYS WORKING GROUP

Chairman:

Robert Chaussin (Roads Department, Ministry of Public Works)

Yves Bournand (VSL)

Alain Chabert (LCPC)

Louis Demilecamps (GTM)

Andre Demonte (ISPAT -Trefileurope)

Pierre Jartoux (Freyssinet International)

Patrick Laboure (ISPAT -Trefileurope)

Dominique Le Gall (Baudin Chateauneuf)

Benoit Lecinq (SETRA 1)

Daniel Lefaucheur (SETRA)

Claude Neant (ETIC -BBR)

The following also helped in the drafting of the Recommendations:

Michel Marchetti (Formule Informatique)

Michel Virlogeux (Consulting Engineer)

These Recommendations were co-ordinated by Jocelyne Jacob (SETRA) and Benoit Lecinq

Drawings by Philippe Jullien and Louis Risterucci (SETRA).

Translation by Alex Greenland.

Photo credits:Cover photos:1, 5, 9 (Freyssinet) -2 (Etic) -3, 4, 10 (SETRA) -6 (VSL) -7 (Fontainunion) -8 (GTM)

Photos 43, 51, 55 to 57: EticPhotos 6, 11, 35: FontainunionPhotos 8,16,17,18,20,22 to 24,26,31,33,34,41,44,45,48,54,58,59,61 to 63: FreyssinetPhotos 30, 39, 40, 46, 49, 50: GTMPhotos 13 to 15, 27 to 29: LCPCPhotos 1 to 5,7,9,10,12,19,21,25,36 to 38, 42, 47, 52, 53, 60: SETRAPhoto 32: VSL

1 Benoit Lecinq has joined the Freyssinet International group since these Recommendations were published.

8


Analysis of the fatigue strength of cable stays must consider two complementary factors:.pure tensile stresses due to imposed loads, whose amplitude is much greater than in the case

of prestressing tendons;.flexural stresses at anchorages, due principally to cable-stay vibration, but also to relative

deformation of the cable stables and the bridge structure. These stresses are negligible in thecase of prestressing tendons.

2.2.3 Environmental aaaressionContrary to prestressing tendons, cable stays are directly exposed to environmental aggression:rain, wind, ultraviolet radiation, freeze-thaw cycles, etc. (see Chapter 3).

2.2.4 Corollary on the desian of cable staysCable stays, which are the key factor in the stability of cable-stayed bridges, must provide the bestpossible operational guarantees. Their durability must be analyzed very rigorously at the initialdesign stage or at the stage of qualification of a cable-stay system.

However, the cable stays will remain the most vulnerable elements of the structure, and there willremain some imponderables in the appreciation of their durability. Moreover, the possibility ofdamage due to a road accident cannot be excluded.

For these reasons the design of cable stays must allow for their rapid replacement, without harmfulconsequences to the structure or serious disruption to traffic. All the protective arrangements mustguarantee that inspection, adjustment, and maintenance can be carried out to attain the requiredlifetime or to determine the need for replacement.

2.2.5 Cateaories of utilizationHigh-performance cable-stay systems are appreciably more expensive than prestressing systems.In order to restrict these extra costs for structures where the cables are less heavily loaded, asecond category of utilization has been defined (see Chapters 11 and 14).

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Photo 28: Wear at point of contact (contact betweencrossed wires in a cable stay)

Photo 29: Crack at point of contact (wire in aninner layer of a cross-layed multi-layer-strand

cable)

6.4.3.2 Consequences for cable design

The external parameters that limit initiation and propagation of fatigue cracks are:

..

limitation of the maximum axial stress in service;increase in inter-wire contact areas, by increasing the lay length, by prestressing cables to ahigher tension or by plastification of contact areas, or by preferring linear contact to pointcontact;limitation of variations of curvature and of the maximum curvature, by increasing the radius ofsaddles or by reducing the angles of deviation in anchorages;limitation of flexural stress variations, by damping vibration due to wind or traffic;reduction in coefficients of inter-wire friction, by using a lubricant whose effect can bemaintained, or, on the contrary, by preventing any relative movement between wires and thuseliminating fretting fatigue and fretting corrosion phenomena.

The experiments performed by Waterhouse, Patzak, and Siegert show that the 100 million cyclefatigue limit of multi-layer strands with bright or galvanized wires loaded to 50% of their breakingstress is about 100 MPa.For shorter lifetimes involving contact fatigue phenomena, the fatigue strength at 2 million cyclescan attain 120 to 150 MPa. These values do not take account of the presence of an effectivelubricant which might durably maintain the coefficient of inter-wire friction below 0.2 (value belowwhich the fatigue strength increases).

6.4.4 Conclusion: detailinaIn practice the following detailing is recommended

.

Abandon saddles and replace them by anchorages. If saddles are used all the same, ensurethey provide a sufficiently large radius of curvature (see Article 14.7).Eliminate any unnecessary metal-on-metal contact between cables and parts of anchorages orsaddles or deviators.Use flexible materials in zones of deviation: nylon, polychloroprene, zinc, aluminium alloy. Usea flexible guide to attenuate or even eliminate free bending of the cable where it leaves theanchorage, on both the bridge deck and the pylon. .

Inject flexible lubricants to reduce the coefficient of inter-wire friction, and use cables madefrom galvanized wire. Galvanization is primarily associated with corrosion-protection of steel,but it also reduces coefficients of friction and the contact pressure between wires: the zinc isflattened and partially extruded around the edges of the contact areas.

.66

Photo 30: Roof of Stade de France stadium on temporary supports

Conversely, if the deck of a concrete cable-stayed bridge were built on falsework, and if the deckwere encastered into the pylon, when the falsework was removed there would be substantial sagof the main span, resulting in an unacceptable negative moment near the pylon (Figure 20). Thesolution for preventing this situation of course involves pretensioning the cable stays with a jackbefore removing the falsework. This means we are dealing with active structural elements.

This is why, in most cases, cable-stayed structures require pre-tensioning. They are highlystatically indeterminate, and pre-loading the cable stays by pre-tensioning is nothing other thanintroduction into the structure of a set of self-balanced forces. These forces-two equal andopposite forces at the two anchorages-induce no boundary forces overall, but are balanced bythe distribution of forces in the structure (bending moments in the deck and pylon in the case of acable-stayed bridge). This distribution of preloading forces enables the structure to take the effectsof permanent loads with only very little bending.

It is therefore natural to regard a cable stay as a preloaded2 structural element. Adjusting a cablestay then involves applying preloading of a given intensity.

7.1.2 Cable-stay adiustment from the desian point of viewDuring the design of a cable-stayed bridge, finding the right adjustment involves optimizingadjustments of cable-stay tension in order to achieve the following objectives:

.allowable stresses in the cable stays and in the structure, both during construction and aftercommissioning, under variable loads;

.if at all possible, zero or very low bending moment in the structure under permanent loads(selfweight and any prestressing of concrete) in order to limit redistribution due to creep and tofacilitate mid-span jointing.

~

2 Or 'prestressed', but in civil engineering this term is often considered to

using tendons, as within a bridge deck, for instance.

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CIP recommendations on cable stays

Photo 32: Uddevafla Bridge under construction

7.2.4 Deck saaAnother restriction on using the initial tension for cable-stay adjustment comes to light in the caseof flexible structures: the tension is determined by the pendulum rule (see § 7.1.2) and the verticalcomponent is practically the same irrespective of cable length. This method results in irregulartension from stay to stay, an overtensioned cable stay being following by an undertensioned one,and soon.

A means of overcoming this problem has been envisaged in the case of very flexible decks; itinvolves tensioning the cable stay until the correct deck sag is obtained at the end of the cablestay. This method cannot be used when the structure is rigid, especially not for adjusting the firstcable stays of a cantilever end-fixed to a pylon. Nor is it very satisfactory to use both methodstogether, depending on whether a rigid or flexible part of the structure is being dealt with.

Finally, deck sag does not directly characterize cable-stay preloading; on the contrary, it introducesunfortunate confusion between geometrical adjustment and adjustment of cable-stay preloading.

7.2.5 Cable lenath under no tension (neutrallenath)

Cable-stay preloading is entirely determined by the following three things:.definition of a reference state (most commonly this involves the bridge geometry, which is

defined by drawings and which is used to define the design model),.the distance I between anchorages fixed to the structure, when the structure is in its reference

state,.the neutral length 10 of the untensioned cable stay, which is shorter than distance I.It is theoretically possible to adjust cable stays on the basis of their neutral length, by accuratelymeasuring cable-stay length 10 when they are made, and then tensioning them by appropriatemeans to tie the anchorages into the structure. This method is not affected by actual construction

74

The neutral length of cable 10 is the length of cable measured between two anchorages when thecable is not tensioned and rests on a support which cancels out the effects of selfweight. Likecable mass, 10 is an intrinsic quantity that is independent of the conditions to which the cable stay issubject in the works.

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.protective metal coating of zinc or standard zinc/aluminium alloy applied at coverage of between190 al:ld 350 g/m2 (mean thickness of 26 to 40 11m approximately); .

.strength class fclass 1770 MPa or 1860 MPa;

.strain under maximum load Agt at least 3.5%;

.modulus of elasticity of the bundle of parallel strands of about 195 GPa :t 5%;

.very low relaxation: no more than 2.5% at 1000 hours at 0.7 Fm ( at 20°);

.category B of French standard NF A 35-035 (revised 2000), i.e. MTEs with special capacitiesmeeting the following test conditions:~ fatigue strength: 2 million cycles with maximum stress of 0.45 FOUTS and stress variation of

300 MPa;~ deflected tensile strength coefficient of no more than 20%.

The nominal values and tolerances apply to coated products, i.e. they include for the metalcoating. The strand lengths commonly produced can have welds made on individual wires beforedrawing, but may not be welded during or after drawing.

Photo 35: Detail of strand

The project specifications may lay down more stringent requirements, particularly with respect tothe protective metal coating, within the scope given in § 9.1.2.1.

9.2.2 Individually sheathed multi-strand cable stavsA sheathed, waxed/greased galvanized strand is a product made especially for cable-stayapplications. The individual sheath is made by extruding high-density polyethylene (HOPE) directlyonto the strand previously coated with an infilling material.

The use of strands sheathed by threading a preformed sheath over an MTE is prohibited forpermanent cable stays.

Experimental French standard NF XPA 35-037 (currently being drafted) contains most of the

following requirements.

9.2.2.1 Individual sheath

The individual sheath is an very important factor in cable-stay durability. Its functionalcharacteristics are specific to each cable-stay system. It must meet at least the followingconditions: "

104


9.2.2.3 Outer sheath (stay pipe)

The individually sheathed strands can be enclosed in an outer sheath, or stay pipe-which mayormay not be watertight-whose purpose is fulfil supplementary functions, in addition to corrosionprotection. It is therefore not necessarily a barrier, but the recommendations of Article 9.5 applynonetheless.

The outer sheath may consist of aone-piece stay pipe through whichthe strands are threaded, or it mayconsist of two split shells clipped toeach other around the cable stayonce it has been tensioned. Itimproves the aerodynamicbehaviour of the cable stay, andpossibly also its watertightness andits CBsthetics. The surface of theouter casing may be textured orcarry other relief; such as spiralridges for example, to counter theeffects of rain & wind instability.

Photo 36: Individually sheathed strands inside a stay pipe 1

9.2.2.4 Diagram illustrating the principle of PSC stays made with individually sheathed,waxed/greased strands

Individual sheath

-Optional outer sheath

'""-A!L

Figure 31.. Diagram of PSG stay with individual sheaths

9.2.3 Ducted multi-strand cable stays

9.2.3.1 Stay pipeFor the free length of a cable stay, the external barrier of ducted multi-strand stays is generally oneof the following continuous stay pipes:

.plastics stay pipe made of rigid or semi-rigid tubes (high-density polyethylene (HOPE) or

similar);.steel stay pipe made from pipe sections welded together; they are either protected against

corrosion or are made of stainless steel.

106


The thickness of the sheath should be taken into account to determine the total volume of the MLScable. Refer to the technical data sheets on the cable-stay system for more details.

Photos 37 and 38: MLS cables

Cables with round wires Locked-coil cables with round andZ-shaped wires

Oext of

bare cable(mm)

Resistingsection

(mm1

Resistingsection

(mm1

Lineicmass

(kg/m)

F GUTS

(kN)F GUTS

(kN)

20

30

40

50

60

70

80

90

100

110

120

130

140

150

219

530

942

1501

2125

2936

3779

4869

5897

7364

8532

12285

11578

13536

316

766

1362

2034

3000

4100

5327

6625

8179

9854

11844

13819

16405

18850

Lineicmass

(kg/m)

1.8

4.3

7.612.9

18.5

25.3

31.9

40.8

50.9

60.3

72.8

86.3

97.6

115.8

594

1090

1801

2502

3406

4552

5690

7060

8466

9999

11731

13423

15645

858

1580

2594

3716

4946

6604

8454

10316

12441

14497

17004

20170

23314

5.3

9.815.0

21.6

30.5

40.2

49.0

61.2

72.8

85.8

101.5

115.1

133.9

Table 3: Common MLS cables

9.5.1 Watertiaht outer orotectionFor the free length of ducted PSG or PWG stays, the collective external barrier consists of astrong, watertight tube of regular shape throughout its entire length.

The characteristics of this tube, especially its thickness and chemical composition, must meet the

following requirements:

.the materials of which it is made must not be aggressive to the injection materials and MTEs;

115


If tube sections are to be welded together, the tube must be no less than 3 mm thick. Welds mustcomply with the terms of appropriate standards (e.g. NF P22-471, quality 1). The fatigue strengthof welded joints must be substantiated.

Steel stay pipes must have an external corrosion-protection system guaranteeing at least 6 yearsbefore rust index Ri 1 defined by the applicable standard is reached.

At the time of publication of these Recommendations, the applicable standard is French standardNF T30-071, "Degradation des surfaces peintes". This level of guarantee of corrosion protectioncan be achieved by cleaning the surface to level OS 2.5 and painting the bare steel. Regularmaintenance is required thereafter (every ten to fifteen years approximately). Alternatively, paintedgalvanized or stainless-steel tubes can be used.

9.5.2 Blockina comDoundThe filling material injected into the intermediate areas must not be a cause of wear (frettingcorrosion or fretting fatigue) of the MTEs it is supposed to protect. It is for this reason that cementgrout is prohibited.A flexible protective material is generally used to fill the inside of the duct. Alternatively, a dry airflow can be kept up around the MTEs by means of a dehumidification system.

Flexible protective products are generally pumpable petroleum products:

.a microcrystalline wax, i.e. a malleable crystallized solid consisting of saturated hydrocarbonswhich are injected in a liquid state (temperature between 80 and 120°C) [6]; or

.a mineral-oil-based grease, i.e. a plastic lubricant obtained by dispersion of an insolublethickener (such as complex metallic soap) in a lubricating fluid (mineral oil) to form a stabilizedthree-dimensional network; or

.a resin or flexible polymer injected at an appropriate temperature.

The filling material must not be aggressive to the MTEs or the material of which the stay pipe ismade. The absence of aggressivity is determined by physical testing or by reference to previous

projects.The filling material must retain its protective properties without interruption, and continue to protectthe steel despite the extreme thermal loading to which it might be subject throughout the lifetime ofthe project.

Photo 39: Pump for injecting cable stays Photo 40: Check of injection

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10.1.4 Corrosion protection and watertiahtnessThe design of anchorages must extend the two barriers defined in Chapter.9 without a break, toprotect against corrosion and keep water out of the free length of the cable stay.

10.1.5 RemovabilityAnchorage design must allow for renewal of cables.

ARTICLE 10.2 GENERAL PROVISIONS COMMON TO ALL ANCHORAGE TYPES

10.2.1 Filterina out anaular deviationsAs stated in Chapter 6, angular deviation of cable stays engenders stresses which can be of the

same order of magnitude as those due to the axial loads and can have harmful effects in terms of

fatigue.

Appropriate systems should therefore be provided to limit or eliminate the effect of cable-staydeviations at the anchorage head, i.e. to "filter out" changes in angle between the cable and

anchorage.

There are two main techniques for this, the effects of which are quite similar:.stiffening the anchorage zone: this involves increasing the flexural stiffness of the cable stay.

One means of achieving this is to attach a steel tube around the cable, near the anchorage, andmechanically fix the end of the tube to the anchorage head or to the structure.

.guiding the cable: this involves partially or totally preventing transverse cable-stay movement,i.e. the movement of each of its component parts, using a guide system placed a certaindistance from the anchorage. The effectiveness of such a guide system depends on its distancefrom the anchorage, as seen in § 6.2.4.1

The angular-deviation filtering system can also playa role in damping (absorption of vibrationalenergy by viscosity or friction).

The design of a cable stay's guide system must take account of transverse and flexural forcesresulting from cable deformations.

10.2.2 Directional adiustmentInitial directional adjustment is made

possible by allowing the stay or its

anchorage head to rotate. This is achieved

by inserting suitable connecting parts

between the anchorage and the structure.

Such connecting parts may be an

adjustment screw with a spherical seating

surface, bicylindrical shims, a fork attached

to a plastic hinge or to a single or double

shaft, etc.

In most cases, however, the systems forinitial directional adjustment of stays ceaseto be very effective once the stay has beentensioned. They are useful above all during I

construction, and cannot be considered to Photo 42: Bottom anchorage of a stay on the Pont deeliminate the effects of bending due to Normandie Bridge, with hinged forkcable-stay vibration.

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Abbreviation Gripping Category of cable concerned

Conical wedges (split-cone) gripped in theanchorage head *c PSC (sheathed or ducted)

Swaged sleeves bearing on theanchorage head *5 PSC (sheathed or ducted)

Buttonheads bearing against a plate+ possible conical socketing action byfilling with Resin, etc.--

Bor B+R PWC

Fanning out + conical socketing action byI

filling the socket casing with Resin, etc.F+R MLS

* May be complemented with a suitable rigid filling material to improve fatigue strength

Further distinctions can be made between the different kinds of anchorage:

.live (or stressing) anchorage, where the cable is tensioned, and dead-end (or fixed)anchorage where no tensioning is performed;

.bottom anchorage, on the deck, and top anchorage in the pylon. The bottom anchorage of acable stay is particularly exposed to water running down the stay, and therefore requires special

preventative measures,..static anchorage, the head of which is static with respect to the structure, and adjustable

anchorage, the head of which can be moved axially.

There is an important difference between cable stays using type C anchorages and all the others:the unloaded length of a cable made up of parallel strands anchored by split-cone wedges can bechanged during adjustment phases, unlike the other kinds of cable stays for which the unloadedlength of the tensioned wire, strand, or cable is irreversibly fixed before jacking takes place.

ARTICLE 10.4 TYPE C ANCHORAGES FOR PARALLEL-STRAND CABLE STAYS

Type C anchorages are used for sheathed or ducted multi-strand cable stays.

10.4.1 Principle of the systemType C anchorages rely on wedging of eachstrand in a separate tapered hole in theanchorage head by means of jaws made upof two to four split-cone wedges.

In some cable-stay systems the grip of thewedges is complemented by injecting theanchorage head with an appropriate rigidfilling material such as resin with satisfactoryfatigue performance. It would therefore bemore appropriate to code these anchoragesas type C + R.

Photo 45: Split-cone wedges

126


Photo

46: Anchorage of a cable stay for the Stade de France stadium

~14 ",:";'"FREE LENGTH TRANSITION ZONE ZONE D'ANCRAGE

7 -Strand deviation zones

8 -Sealing system

9 -Transition piece

10 -Deviator

11 -Stay pipe I transition piece joint

12 -Stay pipe (where applicable)

1 -Protection cover

2 -Sheathed 7 -wire strand

3 -Wedge

4 -Anchorage block

5 -Fork

6 -Spacer tube

Figure

38: Principle of type-C anchorages for sheathed strands -Static anchorage on fork

The transition zone, which extends from the end of the free-length part of the cable stay to theanchorage proper is where the strands fan out from the free length to the anchorage head.

The length of the transition zone depends on the number of strands and the technologies used todeviate them. The transition zone contains one or more deviators which convert(s) a bundle ofparallel strands into a cone of divergent strands.

Steps must be taken to prevent fretting corrosion and fatigue at critical points: at each deviation ofthe bundle of strands, where the strands enter the anchorage head, etc. These measures musttake account of axial overtension of the cable stay and permanent or transient angular deviationsof the cables.

128


ofas

The commonly used meansconnecting socket casings arefollows (see Figures 41 and 42):

INTERNALLY THREADED SOCKET CASING WITH THREADED EXTENSION

:g>

~~~

.socket casing with threaded borebehind the socket: this threadingtakes a threaded transfer rodanchored to the structure by meansof an adjustment and locking nut,.

.socket casing with external threadingtaking an adjustment and locking nut;

.socket casing with fork and pintransferring the cable-stay force to aknuckle plate welded to the structure;

.socket casing with lugs takingseveral high-strength threaded rodswith adjustment and locking nut.

Adjustement~

1 -Socket

2 -Internally threaded socket casing

3- MLS cable stay

4 -Externally threaded transfer rod

EXTERNALLY THREADED SOCKET CASING WITH ADJUSTEMENT NUT

Adjustement~

1- Socket 3 -MLS cable stay

2 -Externally threaded socket casing 4 -Adjustement and locking nut

Figure 41.. Different kinds of adjustable type F+R socketcasings for MLS cable stays

FORK-TYPE SOCKET CASING

@)

4 -Knuckle pin

5 -Knuckle plate1 -Socket

2 -Fork-type socket casing

3 -MLS cable stay

SOCKET CASING WITH FIXING LUGS

..Photo 47: Top anchorage of a cable stay on

Seyssel Bridge

Figure 42: Different kinds of adjustable typeF+R socket casings for MLS cable stays (contd)

3 -MLS cable stay

4 -High-strength threaded rods

1 -Socket

2 -Lug-type socket casing

134


Each specimen tested should reflect actual conditions of use, and have all the actual anchoragesystems used with the cable stays, corrosion-protection accessories, and any products injectedinto the cable stays. Appropriate measures should be taken to reproduce the actual conditions inwhich the anchorages work in the actual structures. If the cable-stay system uses different live anddead-end anchorages (dead-end anchorage with swaged sleeves and live anchorage with jaws,

for instance), both anchorages should be tested simultaneously.

PWC and PSC cable-stay systems generally use a deviator a certain distance from the anchoragewhich allows the MTEs to fan out from the free length to the anchorage (see Chapter 10). MLScable stays too are sometimes configured this way. On test specimens the deviator should beplaced no further from the anchorage than the distance specified for the cable-stay system.

In actual structures the deviator may be connected to the structure, either rigidly or with somefreedom of movement, as when it is connected to the structure by an elastic tube or viscousdamper. Since it is not reasonable to attempt to reproduce exactly the particular conditions of eachstructure for the qualification test, the most unfavourable transverse guide system should be used,i.e. that with a totally free deviator without any damping.

11.2.2.2 Fatigue test procedure

Once the specimen has been set up on the test bed, 5 to 10 cycles (possibly more, depending on

the requirements of the party requesting the test) are carried out between O'max/2 and O'max tostabilize the components of the system. These cycles are not counted in the two million test cycles.

Photo 48: LCPC fatigue test bed

142


anchor block on sideplate with transverse jack

Figure 45: Watertightness test rig

11.3.3 Watertiahtness test procedure

11.3.3.1 Preparation of specimenThe specimen tested is a cable-stay bottom anchorage under near-real conditions: unit withcapacity of 7000 kN (i.e. 27 x 15 mm dia. strands in the case of a PSG stay) fitted with all itsaccessories. The top end is a live anchorage for tensioning the cable stay and connecting its stay

pipe (where applicable).

The top anchorage is placed on the tube centreline and the cable is threaded into the test rig andtensioned to 0.10 FOUTS. Moisture indicators of blotting paper are placed on each MTE and at anysensitive point in the local casing where water should not penetrate.

All deviation and sealing systems are installed (caps, stay pipe and couplers, ring seals, etc.

In the case of PWG or PSG stays, the deviator is placed at the minimum distance from theanchorage specified for the system, and is left free to move laterally.

The stay pipes of ducted PWG and PSG stays have watertight seals with both anchorages,creating a single watertight compartment over the full length of the cable. The test shouldreproduce the same seal conditions at both ends of the stay pipe of the specimen in order to checkthat temperature variation has no effect on the seals. However, it is not necessary to install thestay pipe of individually sheathed PSG stays if it is not intended to be a watertight barrier.

11.3.3.2 Mechanical and thermal ageing

The following ageing sequence is applied for a period of about 6 weeks (see Figure 46):

10 loading and unloading cycles between 0.2 and 0.5 FGUTS, using a multistrand or annular jackon the top anchorage, at room temperature (20°C :t 5), for several hours.

1

The cable is stressed to 0.30 FOUTS and held at this stress for the rest of the test. After sealingany points of possible leakage between the anchorage and the bearing plate, the tube is filledwith water to 100 mm below the top bearing plate (representing water pressure on the bottomanchorage of about 0.2 bar). Mains water (no salt) coloured red with a suitable dye is used.

2.

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Appropriate measures should be taken to ensure that the wires are parallel throughout their lengthand that they are anchored in matching holes in the two anchorage heads. .

Photo 51: Preparation of a PWC stay

12.3.1.3 Preparation of MLS cable stays

Description:The MTEs of MLS cable stays are assembled in the workshop at the time of stranding, when a blockingcompound is also applied to fill the wire interstices. .At least one of the two anchorage sockets of MLS cable stays is generally prepared at the same time. Thesecond socket can be prepared on site.MLS cable stay prefabrication may also include extrusion of a sheath on the spun cable.

...Photo 52: Prefabricated MLS cable stay with anchoragesockets

Photo 53: Preparation of anchorage sockets

..

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These requirements generally call for use of telescopic systems or sleeves.

12.3.4 Site deliveryDescription:If the cable stays are not made up at the site of the project, they should be delivered to the site, ready foruse, on drums whose weight and dimensions will be defined by the supplier of the cable-stay system, inaccordance with transport and handling conditions and the characteristics of the cable stay.When the cable stays are not suitable for coiling, special handling resources have to be used to transportthem from the workshop to the construction site.

Photo 55: Prefabricated cable stay en route to site

The minimum radii of transport drums and the radii of curvature imposed on the handling ofprefabricated cable stays should be adapted to:

.prevent any irreversible deformation of the MTEs and filling compound;.preserve the integrity of the stay pipe (where applicable).

For the usual kinds of multi-layer-strand cables, the minimum coiling diameter is about 30 times theouter diameter of the cable.For a greased/waxed and sheathed strand, the minimum coiling diameter is about 50 times theouter diameter of the strands, i.e. 900 mm.For ducted PWG or PSG stays, the minimum coiling diameter depends on the outer diameter,thickness, and temperature of the stay pipe, and on the time they will remain coiled. In the absenceof more specific information, the coiling diameter should be no less than 50 times the outerdiameter of the HOPE stay pipe.

ARTICLE 12.4 ERECTION OF CABLE STAYS

12.4.1 Erection of connectina partsCable-stay connecting parts-formwork tubes in the case of concrete decks or bearing plates inthe case of steel decks-are generally fitted and adjusted by the main contractor or structuralsteelwork contractor.

Adjustment procedures guaranteeing accuracy of positioning consistent with the possibilities of thecable-stay system should be used. Unless stated otherwise in the design documents, theconnecting parts should be installed to within directional accuracy of :t 5 milliradians(:t 0.29 degrees).

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1. the strand is erected by being threaded up the stay pipe or hoisted with a telpher system. At the end ofthis phase the free length of the strand is approximately in its final position but its ends are notnecessarily threaded through the anchorages. .

2. the ends of the strand are threaded through the appropriate holes in the two anchorage blocks and thesplit-cone wedges are fitted.

3. the strand is tensioned with a monostrand jack.stay tension is adjusted in accordance with the instructions of the design engineer.

Photo 58: Cable-stay strands being threaded into the stay pipe one by one (Vasco de Gama Bridge)

Close attention should be paid to special features of the cable stay (anchorages, guidancesystems, and possibly saddles) while it is being threaded into place to avoid damaging theindividual protection of strands.

Appropriate measures should be taken to prevent the leading end of the strand damaging the staypipe or the sheaths of the strands installed previously.

Appropriate measures should be taken to ensure that the strands are parallel throughout theirlength and that they pass through matching holes in the two anchor blocks.

12.4.4 Corrosion orotection durina erectionDepending on the cable-stay system, not all the corrosion-protection systems for MTEs may havebeen put in place at the time the cable is installed on the structure. If the time to application of thedefinitive corrosion-protection system exceeds a few months, the SCSC should have an

appropriate temporary corrosion-protection system applied.

ARTICLE 12.5 TENSIONING AND ADJUSTMENT

12.5.1 Oraanization of adiustment and verificationAt the end of the erection operations, the cable stay may have been attached to its twoanchorages temporarily (in the case of certain prefabricated cable-stay systems) or permanently,and tensioned to a given stress.

Tensioning and adjustment introduce the level of preloading specified by the designers into thecable stay. Depending on the sequencing of construction of the project, these operations may becarried out in one go or in a series of successive thresholds, in close collaboration with the designengineer.

Re-adjustments may also be necessary during the lifetime of the project (seeChapter 13).

158


Photo 59: Annular jack Photo 60: Erection of MLS cable stays on Seyssel

Bridge -tensioning system

The design of the anchorage head must allow an amplitude of adjustment taking account of someor all of the following quantities, depending on the cable-stay technology used:.uncertainty on the unloaded position of the anchorages;.uncertainty on the loading of the structure at the moment of tensioning phases, and on the

stiffness of the structure;.uncertainty on the unloaded length, tension, and temperature of the cable stay;.extension of the cable stay to attain the required preloading;.factors outlined in § 14.2.6;.safety factor.

The amplitude of adjustment is defined once and for all after manufacture of the anchorage parts(length of peripheral threading on the anchorage head, length of threaded transfer rods, maximumheight of shims that can be placed between the anchorage head and its bearing plate on thestructure, etc.). It must therefore be correctly predicted.

It is possible to make allowance for de tensioning of the cable stay by leaving a length of threadingbehind the concentric nut or behind the nuts on the threaded transfer rods, or by introducing shimsbetween the anchorage head and its bearing plate right from the start. This requirement must alsobe correctly predicted.

The design of the area where the anchorage is attached to the structure must take account of:.sufficient clearance around and behind the anchorage, for installation of jacks and other

systems necessary for tensioning or adjustment;.means of access and handling systems suitable for heavy equipment.

These geometrical conditions should be specified in the technical data sheet of the cable-staysystem given by the SCSC.

This point must be examined with extra special care when adjustment on the structure in service isnot done in the same way as the initial tensioning and adjustment.

If clamps are used to attach the adjustment systems directly to the cable, they must be designedso as not to damage the corrosion protection of the cable stay.

It must be ascertained that the adjustment systems (concentric nuts, transfer rods, etc.) will remainin operating condition throughout the lifetime of the structure: threads protected against anydamage, knife-edge bearings protected against corrosion, etc.

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Creep in concrete cable-stayed bridges generally results solely in shortening of the deck andpylons, which means the cable-stayed spans slump. This occurs practically without any variation inthe tension of the cable stays. Special monitoring must therefore concentrate on deflection of thedeck.

Apart from this maintenance-related re-adjustment, unforeseen factors such as those listed in§ 14.2.6 may also make cable-stay adjustment necessary.

13.3.2 Adiustment procedureRe-adjustment generally involves displacing the anchorage head relative to its bearing point on thestructure, using one or more high-capacity annular jacks capable of taking the force of the entirecable stay (see 12.5.2.1).

Strand-by-strand retensioning of PSG stays anchored with split-cone wedges can be envisagedonly if the cable-stay system is designed to allow this kind of operation (see 12.5.2.2), and underthe following conditions:.there must be sufficient excess strand length;.there must be a flexible injection compound in the anchorage zone;.any deviators and dampers must be compatible with this sort of operation.In practice this method is seldom used for re-adjustment.

The technical modalities for re-adjustment are similar to those for initial tensioning and adjustmentdescribed in Article 12.5. The order of retensioning of the different cable stays, together withadjustment parameters Uack pressure, extension, etc.) must be closely studied in relation to theprovisions of the initial design or of the repair project (see example of Brotonne Bridge in § 7.1.3).

ARTICLE 13.4 CABLE-STAY REPLACEMENT.

The possibility of cable-stay replacementdepends on the following two conditions:1. performance of the structure with one

cable stay or a pair of cable staysremoved, possibly with trafficrestrictions;

2. technological possibility of removingthe cable stay and installing a newone, as recommended in § 10.1.

To meet the first condition, the designstudies of modern cable-stayed bridgesexamine the specific load case ofreplacement of a stay, in accordance withthe recommendations in § 14.2.7.

If the above condition is not met, however,a PSG stay can be replaced strand bystrand, or temporary stays can be erectedfor the duration of replacement operations.

Photo 61: Temporary stays used duringcable-stay replacement on Penang'Bridge

168

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