Setra Cable Stays

Laboratoire Central des Pontset Chaussees

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Recommendations of French interministerial commission on Prestressing

~"~F~ REPUBLIOUE PIANCAISE

uin

~ ::L~(.,9 d..Oo~

2002

Issued by the Serviced'Etudes Techniques des Routes et Autoroutes Centre des Techniques des Ouvrages d'Art 46, avenueAristide Briand -BP 100 -92225 BAGNEUX CEDEX -France Tel. 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

Article

2.1 2.2

Operation Evolution

ofandcable-stay required

Article

3.1 3.2

Effects Inventory

Article

3.4 3.3

Choice Replaceabilityof

of

of mechanical cable-stay

technology qualities

of

cable

and ageingenvironmental factors

stays

factors

18

20 22

materials

24

Article Article

4.1 4.3 4.2 4.4

Specifications Physical Remedial Dynamic

phenomena par.ameters actions

to

prevent

of inducing cablecable-stay

stays vibration

25

vibration

2831 36

CHAPTER 5. STATIC BEHAVIOUR OE CABLE STA~ Article Article

5.1 5.3 5.2

Approximate Linear Introduction

model

effect of

a

cableof

cable-stay stay

39

selfweight.

39

42 Article

5.6 5.5 5.4

Catenary Model Modelling

of

inextensible model a real

cable

sagging stay

47

cable

51 53

Article

6.1 6.2

Taking Cable

stay account deviatedof the at flexural a saddle stiffness

of a cable

at its anchorage.

57 58

Article

6.4 6.3

Vibrations Cable bending in the and free durability length

of..""""'..."'.""'."""""""""""""".' a cable stay

63 63

7. CABLE-STAY MECHANICS DURING CONSTRUCTION Article 7.1 Preloading of cable stays

1115 ~ ~PTER

67 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

78

tensioning

CHAPTER 8. DYNAMIC BEHAVIOUR OF CABLE STAYS

81

--

Article

8.1 Taut-string

Article

8.2 Vibration

Article

8.3

Article

8.4 Parametric

Excitation

model.. modes

general

Article

PSC category: PWC category:

requirements

'

'

'

81 83 91

displacement

."..."'.."'...'

93

'

99

strand cable stays

103

parallel- wire cable stays

Article

9.4 MLS category:

Article

9.5

Article

9.6 Other kinds of main tensile

Collective

'...'.'

of an anchorage

by longitudinal

parallel

'

cable stay

displacement

excitation

Common

Article 9.3

of a sagging

by lateral

Article 9.1 9.2

'..."

multi-layer-strand

external

108

cable stays

,

barrier

111 "

'

'

element

115 119

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

125

--

of anchorages

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 Article

11.1

General

Article

11.2

Mechanical

Article

11.3

Qualification

TESTING OF A CABLE-STAY

SYSTEM

139 139

qualification

of cable stays

of cable-stay

CHAPTER 12. CABLE-STAY

140

watertightness

'

INSTAllATION

'

147

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

CIP recommendations

on cable stavs

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

182

of fatigue

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 Article 16.1

Glossary

Article

Notation

16.2

193 ,

,

193 197

~

CIP recommendations

on cable stays

Early in 1997, the French Interministerial Commission on Prestressing (Commission Interministerielle de la Precontrainte -CIP) set up a working group to study the technological problems involved in cable stays and to establish an approval procedure similar to that implemented for prestressing systems. The working group drafted these Recommendations, a state-of-the-art review advising on the design, qualification, and implementation of cable-stay systems. It calls on the experience acquired with cable-stayed bridges in France and elsewhere in the last thirty years or so. This experience includes large cable-stayed bridges such as the Brotonne Bridge and the Pont de Normandie Bridge in France, the Second Severn Crossing in the UK, and the Vasco de Gama Bridge in Portugal, but also involves a wide range of smaller bridges. The cable technology described in these Recommendations principally concerns cable-stayed bridges, the cables of which are characterized by large variations in tension, fatigue effects, and direct exposure to the elements. More generally, it is hoped the recommendations will be of use for all cables exposed to climatic aggression, particularly to the ties of bowstring bridges, extradosed or 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 these Recommendations which do not, therefore, deal with cabled spaceframe suspension-bridge technology. In addition, cable-stay saddles are addressed only in few recommendations on design, but using them is advised against, because of their durability of cable stays and because of maintenance and replacement difficulties.

the scope of structures or the form of a effect on the

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 choices recommended 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-stay systems 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.

FOREWORD 1.

7

CIP recommendations

on cable stavs

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)

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

-8 (GTM)

to 63: Freyssinet

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

8

were published.

CIP recommendations

on cable stavs

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 the case of prestressing tendons.

2.2.3 Environmental

aaaression

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

2.2.4 Corollary

aggression:

on the desian of cable stays

Cable stays, which are the key factor in the stability of cable-stayed bridges, must provide the best possible operational guarantees. Their durability must be analyzed very rigorously at the initial design 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 will remain some imponderables in the appreciation of their durability. Moreover, the possibility of damage due to a road accident cannot be excluded. For these reasons the design of cable stays must allow for their rapid replacement, without harmful consequences to the structure or serious disruption to traffic. All the protective arrangements must guarantee that inspection, adjustment, and maintenance can be carried out to attain the required lifetime or to determine the need for replacement.

2.2.5 Cateaories

of utilization

High-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, a second category of utilization has been defined (see Chapters 11 and 14).

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on cable stavs

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

6.4.3.2 Consequences

Photo 29: Crack at point of contact (wire in an inner layer of a cross-layed multi-layer-strand cable)

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 a higher tension or by plastification of contact areas, or by preferring linear contact to point contact; limitation of variations of curvature and of the maximum curvature, by increasing the radius of saddles 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 be maintained, or, on the contrary, by preventing any relative movement between wires and thus eliminating fretting fatigue and fretting corrosion phenomena. The experiments performed by Waterhouse, Patzak, and Siegert show that the 100 million cycle fatigue limit of multi-layer strands with bright or galvanized wires loaded to 50% of their breaking stress is about 100 MPa. For shorter lifetimes involving contact fatigue phenomena, the fatigue strength at 2 million cycles can attain 120 to 150 MPa. These values do not take account of the presence of an effective lubricant which might durably maintain the coefficient of inter-wire friction below 0.2 (value below which the fatigue strength increases).

6.4.4 Conclusion:

detailina

In practice the following detailing is recommended Abandon saddles and replace them by anchorages. If saddles are used all the same, ensure they provide a sufficiently large radius of curvature (see Article 14.7). Eliminate any unnecessary metal-on-metal contact between cables and parts of anchorages or saddles or deviators. Use flexible materials in zones of deviation: nylon, polychloroprene, zinc, aluminium alloy. Use a flexible guide to attenuate or even eliminate free bending of the cable where it leaves the anchorage, on both the bridge deck and the pylon. . Inject flexible lubricants to reduce the coefficient of inter-wire friction, and use cables made from 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 is flattened and partially extruded around the edges of the contact areas.

66

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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 deck were encastered into the pylon, when the falsework was removed there would be substantial sag of the main span, resulting in an unacceptable negative moment near the pylon (Figure 20). The solution for preventing this situation of course involves pretensioning the cable stays with a jack before 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 highly statically indeterminate, and pre-loading the cable stays by pre-tensioning is nothing other than introduction into the structure of a set of self-balanced forces. These forces-two equal and opposite forces at the two anchorages-induce no boundary forces overall, but are balanced by the distribution of forces in the structure (bending moments in the deck and pylon in the case of a cable-stayed bridge). This distribution of preloading forces enables the structure to take the effects of permanent loads with only very little bending. It is therefore natural to regard a cable stay as a preloaded2 structural element. Adjusting a cable stay then involves applying preloading of a given intensity.

7.1.2 Cable-stay

adiustment

from the desian point of view

During the design of a cable-stayed bridge, finding the right adjustment adjustments of cable-stay tension in order to achieve the following objectives:

involves

optimizing

.allowable stresses in the cable stays and in the structure, both during construction and after commissioning, 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 to facilitate mid-span jointing.

2 Or 'prestressed',

but in civil engineering

this term is often considered

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

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Photo 32: Uddevafla Bridge under construction

7.2.4 Deck saa Another restriction on using the initial tension for cable-stay adjustment comes to light in the case of flexible structures: the tension is determined by the pendulum rule (see § 7.1.2) and the vertical component is practically the same irrespective of cable length. This method results in irregular tension 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; it involves tensioning the cable stay until the correct deck sag is obtained at the end of the cable stay. This method cannot be used when the structure is rigid, especially not for adjusting the first cable stays of a cantilever end-fixed to a pylon. Nor is it very satisfactory to use both methods together, 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 introduces unfortunate confusion between geometrical adjustment and adjustment of cable-stay preloading.

7.2.5 Cable lenath under no tension (neutrallenath) The neutral length of cable 10is the length of cable measured between two anchorages when the cable is not tensioned and rests on a support which cancels out the effects of selfweight. Like cable mass, 10is an intrinsic quantity that is independent of the conditions to which the cable stay is subject in the works. 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 10of 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 accurately measuring cable-stay length 10 when they are made, and then tensioning them by appropriate means to tie the anchorages into the structure. This method is not affected by actual construction

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CIP recommendations

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.protective metal coating of zinc or standard zinc/aluminium alloy applied at coverage of between 190 al:ld 350 g/m2 (mean thickness of 26 to 40 11mapproximately); . .strength class fclass1770 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 capacities meeting 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 metal coating. The strand lengths commonly produced can have welds made on individual wires before drawing, but may not be welded during or after drawing.

Photo 35: Detail of strand The project specifications may lay down more stringent requirements, the protective metal coating, within the scope given in § 9.1.2.1.

9.2.2 Individually

sheathed

multi-strand

particularly with respect to

cable stavs

A sheathed, waxed/greased galvanized strand is a product made especially for cable-stay applications. The individual sheath is made by extruding high-density polyethylene (HOPE) directly onto the strand previously coated with an infilling material. The use of strands sheathed permanent cable stays. Experimental French standard following requirements. 9.2.2.1

Individual

by threading a preformed

sheath over an MTE is prohibited

NF XPA 35-037 (currently being drafted) contains

for

most of the

sheath

The individual sheath is an very characteristics are specific to each conditions:

important factor in cable-stay durability. Its functional cable-stay system. It must meet at least the following "

104

CIP recommendations

on cable stavs

9.2.2.3 Outer sheath (stay pipe) The individually sheathed strands can be enclosed in an outer sheath, or stay pipe-which mayor may not be watertight-whose purpose is fulfil supplementary functions, in addition to corrosion protection. It is therefore not necessarily a barrier, but the recommendations of Article 9.5 apply

nonetheless. The outer sheath may consist of a one-piece stay pipe through which the strands are threaded, or it may consist of two split shells clipped to each other around the cable stay once it has been tensioned. It improves the aerodynamic behaviour of the cable stay, and possibly also its watertightness and its CBsthetics. The surface of the outer casing may be textured or carry other relief; such as spiral ridges for example, to counter the effects of rain & wind instability.

Photo36: Individually sheathedstrandsinsidea staypipe 9.2.2.4 Diagram illustrating waxed/greased strands

the principle

of PSC stays

1 made with

individually

sheathed,

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 pipe For the free length of a cable stay, the external barrier of ducted multi-strand stays is generally one of 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

CIP recommendations

on cable stavs

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

Photos 37 and 38: MLS cables

Cables with round wires Oext of

bare cable (mm)

Resisting section

F GUTS

Locked-coil cables with round and Z-shaped wires

Lineic mass

(mm1

(kN)

20

219

316

1.8

30

530

766

40

942

50

1501

60

2125

3000

70

2936

80

Resisting section

F GUTS

Lineic mass

(mm1

(kN)

4.3

594

858

5.3

1362

7.6

1090

1580

9.8

2034

12.9

1801

2594

15.0

18.5

2502

3716

21.6

4100

25.3

3406

4946

30.5

3779

5327

31.9

4552

6604

40.2

(kg/m)

(kg/m)

90

4869

6625

40.8

5690

8454

49.0

100

5897

8179

50.9

7060

10316

61.2

110

7364

9854

60.3

8466

12441

72.8

120

8532

11844

72.8

9999

14497

85.8

130

12285

13819

86.3

11731

17004

101.5

140

11578

16405

97.6

13423

20170

115.1

150

13536

18850

115.8

15645

23314

133.9

Table 3: Common MLS cables

9.5.1 Watertiaht

outer orotection

For the free length of ducted PSG or PWG stays, the collective external barrier consists strong, watertight tube of regular shape throughout its entire length.

of a

The characteristics of this tube, especially its thickness and chemical composition, following requirements:

must meet the

.the

and MTEs;

materials

of which

it is made must not be aggressive

115

to the injection

materials

CIP recommendations

on cable stavs

If tube sections are to be welded together, the tube must be no less than 3 mm thick. Welds must comply with the terms of appropriate standards (e.g. NF P22-471, quality 1). The fatigue strength of welded joints must be substantiated. Steel stay pipes must have an external corrosion-protection system guaranteeing before rust index Ri 1 defined by the applicable standard is reached.

at least 6 years

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

9.5.2 Blockina comDound The filling material corrosion or fretting grout is prohibited. A flexible protective flow can be kept up

injected into the intermediate areas must not be a cause of wear (fretting fatigue) of the MTEs it is supposed to protect. It is for this reason that cement material is generally used to fill the inside of the duct. Alternatively, around the MTEs by means of a dehumidification system.

a dry air

Flexible protective products are generally pumpable petroleum products: .a

microcrystalline wax, i.e. a malleable crystallized solid consisting of saturated hydrocarbons which 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 insoluble thickener (such as complex metallic soap) in a lubricating fluid (mineral oil) to form a stabilized three-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 is made. 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 protect the steel despite the extreme thermal loading to which it might be subject throughout the lifetime of the project.

Photo 39: Pump for injecting cable stays

Photo 40: Check of injection

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CIP recommendations on cable stavs

10.1.4 Corrosion

protection

and watertiahtness

The design of anchorages must extend the two barriers defined in Chapter.9 without a break, to protect against corrosion and keep water out of the free length of the cable stay.

10.1.5 Removability Anchorage

design

ARTICLE

10.2

must

stated

same

of cables.

PROVISIONS

COMMON

TO ALL ANCHORAGE

TYPES

out anaular deviations

in Chapter

order

for renewal

GENERAL

10.2.1 Filterina As

allow

6, angular

of magnitude

deviation

as those

due

of cable to the

axial

stays

engenders

loads

and

can

stresses have

harmful

which

can

effects

be

of the

in terms

of

fatigue.

Appropriate deviations

systems should at the anchorage

therefore be provided to limit or eliminate head, i.e. to "filter out" changes in angle

the effect of cable-stay 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, and mechanically 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 certain distance from the anchorage. The effectiveness of such a guide system depends on its distance from the anchorage, as seen in § 6.2.4.1 The angular-deviation filtering system can also playa role in damping (absorption of vibrational energy by viscosity or friction). The design of a cable stay's guide system must take account of transverse resulting from cable deformations.

10.2.2 Directional Initial

directional

possible

by

anchorage by Such

the

adjustment

screw

a plastic

shaft,

suitable

hinge

stay

This

made or

and parts

with

the may

a spherical

shims,

a fork

or to a single

its

is achieved

connecting

anchorage

bicylindrical

is

the

to rotate.

connecting

surface, to

adjustment

inserting

between

adiustment

allowing

head

and flexural forces

parts structure. be

an

seating attached or double

etc.

In most cases, however, the systems for initial directional adjustment of stays cease to be very effective once the stay has been tensioned. They are useful above all during construction, and cannot be considered eliminate the effects of bending due cable-stay vibration.

I

to Photo 42: Bottom anchorage of a stay on the Pont de to Normandie Bridge, with hinged fork

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CIP recommendations on cable stavs

Abbreviation

Gripping

Category of cable concerned

c

Conical wedges (split-cone) gripped in the anchorage head *

PSC (sheathed or ducted)

5

Swaged sleeves bearing on the anchorage head *

PSC (sheathed or ducted)

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

B or B+R

Fanning

F+R

out + conical

socketing

action

PWC

byI

MLS

filling the socket casing with Resin, etc.

* 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 a cable 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 anchorage, the head of which can be moved axially.

respect

to the structure,

and adjustable

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 be changed during adjustment phases, unlike the other kinds of cable stays for which the unloaded length 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 system

Type C anchorages rely on wedging of each strand in a separate tapered hole in the anchorage head by means of jaws made up of two to four split-cone wedges. In some cable-stay systems the grip of the wedges is complemented by injecting the anchorage head with an appropriate rigid filling material such as resin with satisfactory fatigue performance. It would therefore be more appropriate to code these anchorages as type C + R.

Photo 45: Split-cone wedges

126

Photo Figure

CIP recommendations

on cable stavs

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

~14 FREE LENGTH

",:";'" TRANSITION ZONE

ZONE D'ANCRAGE

1 -Protection

cover

7 -Strand

deviation

2 -Sheathed

7 -wire strand

8 -Sealing

system

3 -Wedge

9 -Transition

4 -Anchorage

piece

10 -Deviator

block

5 -Fork 6 -Spacer

zones

tube

11 -Stay

pipe I transition

12 -Stay

pipe (where

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

piece joint

applicable)

anchorage on fork

The transition zone, which extends from the end of the free-length part of the cable stay to the anchorage 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 to deviate them. The transition zone contains one or more deviators which convert(s) a bundle of parallel strands into a cone of divergent strands. Steps must be taken to prevent fretting corrosion and fatigue at critical points: at each deviation of the bundle of strands, where the strands enter the anchorage head, etc. These measures must take account of axial overtension of the cable stay and permanent or transient angular deviations of the cables.

128

~ ~

CIP recommendations

on cable stavs

The commonly used means connecting socket casings are follows (see Figures 41 and 42):

of as

INTERNALLY THREADED SOCKET CASING WITH THREADED

EXTENSION

:g>

.socket casing with threaded bore behind the socket: this threading takes a threaded transfer rod anchored to the structure by means of an adjustment and locking nut,. .socket casing with external threading taking an adjustment and locking nut; .socket casing with fork and pin transferring the cable-stay force to a knuckle plate welded to the structure; .socket casing with lugs taking several high-strength threaded rods with adjustment and locking nut.

~

Adjustement

1 -Socket 2 -Internally

3- MLS cable stay threaded

4 -Externally threaded transfer rod

socket casing

EXTERNALLY THREADED SOCKET CASING WITH ADJUSTEMENT

NUT

Adjustement

~

1- Socket 2 -Externally

3 -MLS threaded

socket casing

cable stay

4 -Adjustement

and locking

nut

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

FORK-TYPE

SOCKET CASING @)

1 -Socket

4 -Knuckle pin

2 -Fork-type socket casing

5 -Knuckle plate

3 -MLS cable stay

SOCKET CASING WITH FIXING LUGS

..Photo

47: Top anchorage of a cable stay on

Seyssel Bridge

2 -Lug-type

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

3 -MLS cable stay

1 -Socket socket

casing

4 -High-strength

threaded rods

134

CIP recommendations

on cable stavs

Each specimen tested should reflect actual conditions of use, and have all the actual anchorage systems used with the cable stays, corrosion-protection accessories, and any products injected into the cable stays. Appropriate measures should be taken to reproduce the actual conditions in which the anchorages work in the actual structures. If the cable-stay system uses different live and dead-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 anchorage which allows the MTEs to fan out from the free length to the anchorage (see Chapter 10). MLS cable stays too are sometimes configured this way. On test specimens the deviator should be placed no further from the anchorage than the distance specified for the cable-stay system. In actual structures the deviator freedom of movement, as when damper. Since it is not reasonable structure for the qualification test, i.e. that with a totally free deviator

may be connected to the structure, either rigidly or with some it is connected to the structure by an elastic tube or viscous to attempt to reproduce exactly the particular conditions of each the most unfavourable transverse guide system should be used, 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'maxto stabilize the components of the system. These cycles are not counted in the two million test cycles.

Photo 48: LCPC fatigue test bed

142

1 2.

CIP recommendations

on cable stavs

anchorblock on side plate with transversejack

Figure 45: Watertightness

11.3.3 Watertiahtness

test rig

test procedure

11.3.3.1 Preparation of specimen The specimen tested is a cable-stay bottom anchorage under near-real conditions: unit with capacity of 7000 kN (i.e. 27 x 15 mm dia. strands in the case of a PSG stay) fitted with all its accessories. 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 and tensioned to 0.10 FOUTS.Moisture indicators of blotting paper are placed on each MTE and at any sensitive 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 anchorage specified for the system, and is left free to move laterally.

distance

from the

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 should reproduce the same seal conditions at both ends of the stay pipe of the specimen in order to check that temperature variation has no effect on the seals. However, it is not necessary to install the stay 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 jack on the top anchorage, at room temperature (20°C :t 5), for several hours. The cable any points with water anchorage

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

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CIP recommendations

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Appropriate measures should be taken to ensure that the wires are parallel throughout their length and 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 blocking compound 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. The second socket can be prepared on site. MLS cable stay prefabrication may also include extrusion of a sheath on the spun cable.

...Photo 52: Prefabricated sockets

MLS cable stay with anchorage

Photo 53: Preparation of anchorage sockets

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CIP recommendations

These requirements

on cable stavs

generally call for use of telescopic systems or sleeves.

12.3.4 Site delivery Description: If the cable stays are not made up at the site of the project, they should be delivered to the site, ready for use, on drums whose weight and dimensions will be defined by the supplier of the cable-stay system, in accordance 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 transport them 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 prefabricated cable stays should be adapted to: .prevent .preserve

imposed on the handling

of

any irreversible deformation of the MTEs and filling compound; 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 the outer diameter of the cable. For a greased/waxed and sheathed strand, the minimum coiling diameter is about 50 times the outer 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 absence of more specific information, the coiling diameter should be no less than 50 times the outer diameter of the HOPE stay pipe.

ARTICLE 12.4

12.4.1 Erection

ERECTION OF CABLE STAYS

of connectina

parts

Cable-stay connecting parts-formwork tubes in the case of concrete decks or bearing plates in the case of steel decks-are generally fitted and adjusted by the main contractor or structural steelwork contractor. Adjustment procedures guaranteeing accuracy of positioning consistent with the possibilities of the cable-stay system should be used. Unless stated otherwise in the design documents, the connecting parts should be installed to within directional accuracy of :t 5 milliradians (:t 0.29 degrees).

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CIP recommendations

on cable stavs

1. the strand is erected by being threaded up the stay pipe or hoisted with a telpher system. At the end of this phase the free length of the strand is approximately in its final position but its ends are not necessarily threaded through the anchorages. . 2. the ends of the strand are threaded through the appropriate holes in the two anchorage blocks and the split-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, guidance systems, and possibly saddles) while it is being threaded into place to avoid damaging the individual protection of strands. Appropriate measures should be taken to prevent the leading end of the strand damaging the stay pipe or the sheaths of the strands installed previously. Appropriate measures should be taken to ensure that the strands are parallel throughout their length and that they pass through matching holes in the two anchor blocks.

12.4.4 Corrosion

orotection

durina erection

Depending on the cable-stay system, not all the corrosion-protection systems for MTEs may have been put in place at the time the cable is installed on the structure. If the time to application of the definitive 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 verification

At the end of the erection operations, the cable stay may have been attached to its two anchorages 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 the cable stay. Depending on the sequencing of construction of the project, these operations may be carried out in one go or in a series of successive thresholds, in close collaboration with the designenginee Re-adjustments may also be necessary during the lifetime of the project (see Chapter 13).

158

CIP recommendations

on cable stavs

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 some or 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, maximum height of shims that can be placed between the anchorage head and its bearing plate on the structure, 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 threading behind the concentric nut or behind the nuts on the threaded transfer rods, or by introducing shims between the anchorage head and its bearing plate right from the start. This requirement must also be 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 system given by the SCSC.

should be specified in the technical data sheet of the cable-stay

This point must be examined with extra special care when adjustment on the structure in service is not 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 designed so 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 remain in operating condition throughout the lifetime of the structure: threads protected against any damage, knife-edge bearings protected against corrosion, etc.

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CIP recommendations

on cable stavs

Creep in concrete cable-stayed bridges generally results solely in shortening of the deck and pylons, which means the cable-stayed spans slump. This occurs practically without any variation in the tension of the cable stays. Special monitoring must therefore concentrate on deflection of the deck. Apart from this maintenance-related re-adjustment, unforeseen § 14.2.6 may also make cable-stay adjustment necessary.

13.3.2 Adiustment

factors such as those listed in

procedure

Re-adjustment generally involves displacing the anchorage head relative to its bearing point on the structure, using one or more high-capacity annular jacks capable of taking the force of the entire cable stay (see 12.5.2.1). Strand-by-strand retensioning of PSG stays anchored with split-cone wedges can be envisaged only if the cable-stay system is designed to allow this kind of operation (see 12.5.2.2), and under the 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 adjustment described in Article 12.5. The order of retensioning of the different cable stays, together with adjustment parameters Uack pressure, extension, etc.) must be closely studied in relation to the provisions 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 replacement depends on the following two conditions: 1. performance of the structure with one cable stay or a pair of cable stays removed, possibly with traffic restrictions; 2. technological possibility of removing the cable stay and installing a new one, as recommended in § 10.1. To meet the first condition, the design studies of modern cable-stayed bridges examine the specific load case of replacement of a stay, in accordance with the recommendations in § 14.2.7. If the above condition is not met, however, a PSG stay can be replaced strand by strand, or temporary stays can be erected for the duration of replacement operations.

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

168