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Acceptance of stay cable systems using prestressing steels

Recommendation prepared by

Task Group 9.2

January 2005

Subject to priorities defined by the Steering Committee and the Presidium, the results of fib’s work in Commissions and Task Groups are published in a continuously numbered series of technical publications called 'Bulletins'. The following categories are used:

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Any publication not having met the above requirements will be clearly identified as preliminary draft. This Bulletin N° 30 was approved as an fib recommendation in April 2004 by the fib Council.

This report was drafted by fib Task Group 9.2, Stay cable systems:Dieter Jungwirth (Convenor)György L. Balázs (Budapest University of Technology and Economics, Hungary), Pierre Boitel (Freyssinet International, France), Yves Bournand (VSL Technical Centre Europe, France), Pietro Brenni (BBL Systems Ltd., Switzerland), Alain Chabert (Laboratoire Central des Ponts et Chaussées, France), Gordon Clark (Gifford and Partners Ltd., United Kingdom), André Demonté (Trefileurope Fontainunion, Belgium), Hans Rudolf Ganz (VSL International, Switzerland), Christian Gläser (Technical University München, Germany), Philippe Jacquet (Bouygues Travaux Publics, France), Jean-Francois Klein (Tremblet SA, Switzerland), Jacob Koster (Ballast Neerdam, The Netherlands), Benoit Lecinq (SETRA, France), Manfred Miehlbradt (EPF Lausanne, Switzerland), Theodore L. Neff (Post Tensioning Institute, USA), Toshihiko Niki (Sumitomo Electric Industries, Japan), Oswald Nützel (DSI Int. GmbH, Germany), Amar Rahman (BBR Systems, Switzerland), Reiner Saul (Leonhardt, Andrä und Partner, Germany), S. Sengupta (Span Consultants Pvt. Ltd., India), Khaled Shawwaf (DSI, USA), J.H.A. Van Beurden (Nedri-Spanstaal B.V., The Netherlands), Yash Paul Virmani (Federal Highway Administration, USA)Full address details of Task Group members may be found in the fib Directory or through the online services on fib's website, www.fib-international.org .Cover pictures: Sunshine Skyway Bridge (Florida, USA), Olympic Tent (Munich, Germany), Rosario-Victoria Bridge (Argentina), Millau Bridge (France), Ibi Bridge (Japan), Yiling Yangtze River Bridge (People’s Republic of China)

© fédération internationale du béton (fib), 2005

Although the International Federation for Structural Concrete fib - féderation internationale du béton - created from CEB and FIP, does its best to ensure that any information given is accurate, no liability or responsibility of any kind (including liability for negligence) is accepted in this respect by the organisation, its members, servants or agents.All rights reserved. No part of this publication may be reproduced, modified, translated, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission.First published in 2005 by the International Federation for Structural Concrete (fib)Post address: Case Postale 88, CH-1015 Lausanne, SwitzerlandStreet address: Federal Institute of Technology Lausanne - EPFL, Section Génie CivilTel. +41 21 693 2747; Fax +41 21 693 6245; [email protected]; www.fib-international.org

ISSN 1562-3610ISBN 2-88394-070-3

Printed by Sprint-Digital-Druck Stuttgart

ForewordCable-stayed structures have become increasingly popular over the last 10 to 20 years. Many long span stay cable bridges have been built for highway traffic with spans up to and now even beyond 1000 m main span. A significant number of cable-stayed railway bridges has also been constructed. More recently many very elegant pedestrian cable-stayed structures have been built and have become landmark structures. But also cable-supported roofs and other building structures have found increasing interest.Surprisingly, there have been only a very limited number of specifications for the stay cable systems which form a key element of these structures. The best known are the recommendations published by the Post-Tensioning Institute in the USA, and more recently, the recommendations published by the French Highway Administration, SETRA.These recommendations are the first specifications published by fib for stay cable systems. They introduce a significant number of new developments and specifications which have not been available in previous documents: Corrosion protection philosophy with a multi-barrier approach for the prestressing steel used for the stay cables and a rational design approach for structural steel components. A leak tightness test is also specified to verify the connection details between the free length and the anchorage zone of the stay cable Design and testing of stay cables for the inevitable flexural effects which occur close to the anchorages and other deviation points of the stay cables. These recommendations specify testing that covers flexural effects up to a certain degree. Designers will not need to consider these flexural effects anymore in the design for stay cable systems tested in accordance with these recommendations Selected information on stay cable vibrations and special damping devices to control such vibrations Suitable details for lightning protection of cable-stayed structures Design considerations and testing procedures for stay cable saddles which are increasingly popular for cable-stayed structures with very slender pylons State-of-the-art specifications for the main materials and components for stay cable systems including quality control procedures Specific requirements for the common installation methods of stay cables are provided including the strand by strand installation and stressing methods A comprehensive list of references, relevant standards, and extended literature.

I express my sincere thanks to Professor Jungwirth, the convenor of Task Group TG 9.2, and the members of TG 9.2. Professor Jungwirth has taken up the work with great initiative and has been able to motivate the Task Group members to produce a comprehensive and most valuable document, which will become a standard reference for stay cable systems specifications. I also express my thanks to the several experts who have dedicated significant time to review and improve these recommendations, in particular Michel Virlogeux and David Goodyear. I also extend my thanks to Gordon Clark for his editing of the report for English grammar.

Hans Rudolf GANZ Chairman of Commission 9Reinforcing and Prestressing Materials and Systems

ContentsIntroduction1 Scope2 Definitions and symbols

2.1 Definitions2.2 Symbols

3 Design and detailing3.1 General

(3.1.1 Redundancy of cable-stayed structures – 3.1.2 Fire, impact, vandalism – 3.1.3 Replaceability of stay cables – 3.1.4 Transverse loads applied from stay cables to the structure – 3.1.5 Bending stresses in stay cables

3.2 Design / sizing of stay cables(3.2.1 Service conditions (SLS) – 3.2.2 Fatigue limit state (FLS) – 3.2.3 Ultimate limit state (ULS) – 3.2.4 Earthquakes – 3.2.5 Construction and cable replacement

3.3 Detailing and lightning protection(3.3.1 Detailing – 3.3.2 Lightning protection)

3.4 Saddles(3.4.1 General – 3.4.2 Transfer of differential stay cable forces – 3.4.3 Minimum radius of curvature of saddle pipe)

3.5 Execution aspects(3.5.1 Stage-by-stage analysis – 3.5.2 Length adjustment capability of stay cables - 3.5.3 Construction tolerances)

3.6 Cable vibrations(3.6.1 General – 3.6.2 Special damping devices – 3.6.3 Cross ties)

3.7 Inspection and maintenance

4 Functional requirements for stay cables4.1 Evolution of stay cable technology4.2 General requirements

(4.2.1 General – 4.2.2 Durability design, corrosion protection)

4.3 Requirements for the free length(4.3.1 Corrosion protection philosophy for tensile elements – 4.3.2 Protection philosophy for other materials – 4.3.3 Reference system for corrosion protection – 4.3.4 Equivalent systems for corrosion protection – 4.3.5 Systems with lower corrosion protection – 4.3.6 Additional requirements)

4.4 Requirements for the transition zones(4.4.1 Corrosion protection – 4.4.2 Stay pipe dilation – 4.4.3 Guide deviators – 4.4.4 Damping of stay cables – 4.4.5 Anti-vandalism pipes)

4.5 Requirements for anchorages(4.5.1 Types of stay cable anchorages – 4.5.2 Corrosion protection philosophy for mild steel anchorage components – 4.5.3 Additional requirements)

4.6 Requirements for saddles(4.6.1 General – 4.6.2 Corrosion protection – 4.6.3 Saddle performance)

5 Materials: properties, requirements, testing5.1 General5.2 High tensile steel for tensile elements (prestressing steel)

(5.2.1 General – 5.2.2 Hot dipped metallically coated prestressing steel)

5.3 Structural steel for anchorages, saddles, guide deviators and pipes5.4 Stainless steel5.5 Sheathing for prestressing strands5.6 Filling materials

(5.6.1 Soft filling materials – 5.6.2 Hardening filling materials)

5.7 Stay pipes and other pipes(5.7.1 General – 5.7.2 Thermoplastic stay pipes – 5.7.3 Steel stay pipes – 5.7.4 Other pipes)

5.8 Guide deviators5.9 Damping devices

6 Testing of stay cable systems6.1 General6.2 Initial approval testing (qualification testing)

(6.2.1 Anchorage fatigue and tensile testing – 6.2.2 Saddle fatigue and tensile testing – 6.2.3

Leak tightness testing)

6.3 Suitability testing6.4 Quality control testing

7 Installation7.1 General

(7.1.1 Quality management system – 7.1.2 Qualification of personnel – 7.1.3 Execution documents)

7.2 Shipment and storage of components7.3 Assembly and installation7.4 Stressing and adjustment7.5 Corrosion protection

8 Inspection and monitoring8.1 General8.2 Initial inspection8.3 Routine inspection8.4 Detailed inspection8.5 Exceptional inspection8.6 Monitoring

9 Maintenance, repair, replacement and strengthening10 References and literature

10.1 References10.2 Standards10.3 Extended literature

IntroductionCable-stayed bridges are structurally optimised systems employing light stiffening beams, continuously supported by the stays, for large span cantilevers with an efficient transfer of forces to the pylons. Spans of up to 500 m with concrete decks and up to 1000 m with steel stiffening beams (either composite or pure steel decks) are economically practicable.The most important elements in these aesthetic structures are the stay cables. More than 20 years ago stay cables consisted of spiral strands and fully locked cables, as originally used in steel construction. Today special quality tensile elements similar to those used in prestressed concrete construction are setting new standards in these fields of application. Stay cables can be manufactured with prestressing steels within HDPE pipes or steel pipes with fillers for corrosion protection or with individually protected strands, which are either prefabricated or fabricated on site. Modular concept of the systems allows design using very large stays, the largest today with up to 205 strands (ultimate strength of 54 MN) per cable.While most of these stay cables are used for bridge construction, similar cables are also widely used in extradosed structures and building construction.Over the past years some national Stay Cable Recommendations have been issued, e.g. by PTI (USA), [1], and SETRA/CIP (France), [2]. Also a European specification is in preparation, see [S1]. These recommendations and specifications typically cover locally available materials and construction practices.These fib recommendations have been formulated by an international working group comprising more than 20 experts from administrative authorities, universities, laboratories, owners, structural designers, suppliers of prestressing steels and stay cable suppliers. This text has been written to cover best construction practices around the world, and to provide material specifications which are considered to be the most advanced available at the time of preparing this text. For ease of use (for client, designer and cable supplier), the complex content has been arranged thematically according to the system components into chapters focusing on performance characteristics, requirements and acceptance criteria.References are provided with a separate section on standards. An extensive list of literature on the subject of stay cables and cable-stayed structures is also provided.

1 Scope

These recommendations are intended to give technical guidelines regarding design, testing, acceptance, installation, qualification, inspection and maintenance of stay cable systems using prestressing steels (strands, wires or bars) as tensile elements which can be applied internationally. These recommendations are meant to be applicable for cable-stayed bridges and other suspended structures such as roofs. They

may also be used for hangers in arch structures and as suspension cables, as appropriate.Requirements and comments have been specified for all parties involved in design and construction in order to aim for a uniform and high quality and durability. The interfaces to the structural Designer are highlighted. The essential subjects are:• Design and detailing of stay cables including saddles and damping devices• Durability requirements and corrosion protection systems• Requirements for the materials• Testing requirements for the stay cables• Installation, tolerances, qualification of companies and personnel• Inspection, maintenance and repair.

The main subject of these recommendations are stay cables with tensile elements consisting of prestressing steel. Generally, these are strands and wires. Bars may be of practical use for short, single bar stays and are normally not used for typical highway bridge stays. In particular for architectural applications, stainless steel bars have been used. However, this type of bar is not specifically considered in these recommendations although the general philosophy given applies.These recommendations do not cover the technology of stay cables whose tensile elements are ropes, locked-coil cables, etc. or which consist of composite materials. Nevertheless, in many cases the specified performance criteria may also be applicable to these systems, although numerical values given for the acceptance criteria may need to be adjusted. For these systems it has been difficult to provide multiple protective layers similar to those specified for stay cables made from prestressing steel and therefore, the quality of corrosion protection may not be equivalent.While extradosed cables have similarities with stay cables, generally agreed design and system acceptance criteria are not yet available and therefore, this type of cable is not covered here.

2 Definitions and symbols2.1 Definitions (see Fig. 2.1)Accessories: Auxiliary components such as anchorage caps, anti-vandalism pipe, sleeves, boots, etc.Anchorage: A mechanical device, usually comprising several components (anchor head or wedge plate, bearing plate, socket, ring nut, etc.), designed to retain the load in the stressed stay cable and to transmit the load to the cable-stayed structure. Anchorages can be as follows:

- Adjustable anchorage: Anchorage with a threaded nut or with shims, allowing an adjustment of the stay cable length without moving the prestressing steel relative to the anchorage

- Fixed anchorage: Anchorage which does not allow adjustment of the stay cable length. Anchorages may be further divided into:

- Stressing end anchorages which permit stressing of the stay cable- Dead end anchorages which are not provided for stressing of the stay cable.

Barrier / Corrosion barrier: Envelopment of the tensile element of the stay cable protecting the element or cable from environmental influences and their consequences, in particular corrosion. Barriers can be of two types:

- External barrier: A barrier which is exposed to the outside environment- Internal barrier: A barrier which is directly applied to the tensile element.

Bearing: See guide deviator.

Centralizer / Spacer: A non-load bearing device between or around the tensile elements to fix the position of the stay pipe relative to the tensile elements.

Cross tie: Element connecting the stay cables between each other and/or to the structure (bridge deck) to modify the period of vibration of the stay cable.

Damping device: A device to control cable vibrations.

Designer (consulting engineer): The engineer responsible for the design of the cable-stayed structure. His exact scope of works and role varies with local customs.

Fatigue load: Variable loads on the cable-stayed structure, in accordance with relevant standards for fatigue loading.

Filler / Filling material: An interface, filler, blocking agent or coating preventing the penetration of external contaminants to, or migration along, the tensile element.

Free length: The length of a stay cable beyond the cable anchorage or saddle and transition zones.

Guide deviator: A device (sometimes called elastic bearings) located at the end of the stay cable free length which provides two functions:

(1) laterally guiding the stay cable to protect the anchorage from transverse forces and bending stresses (Guide), and(2) deviating the tensile elements to form a compact bundle of parallel elements in the free length (Deviator).

These two functions may be combined into one single element or may be provided with two separate elements.

Guide pipe: A pipe used as recess former in a cable-stayed structure (deck and/or pylon) for the installation and possible removal of the stay cable anchorage zone. The guide pipe is sometimes called formwork tube or recess pipe.

Inspection: A primarily visual examination, often at close range, of a structure or its components with the objective of gathering information about their form, current condition, service environment and general circumstances. National requirements are often specified.

Lifetime / Service life / Design working life: The planned period of use of the structure, or parts of it, for its intended purpose with the anticipated maintenance but without major repair. It must be specified by the owner.

Maintenance: A usually periodic activity intended to either prevent or correct the effects of minor deterioration, degradation or mechanical wear of the structure or its components in order to keep their future functionality at the level anticipated by the Designer/owner.

Monitoring: To keep watch over, recording progress and changes with time; possibly also controlling the functioning or working of an entity or process. Structural monitoring typically involves gathering information by a range of possible techniques and procedures to aid the management of an individual structure or class of structures. It is often taken to involve the automatic recording of performance data for the structure and possibly also some degree of associated data processing.

Saddle: A device to deviate a stay cable continuous from the deck through the pylon back to the deck or anchorage to transfer loads from the stay cable to the pylon.

Sheathed strand: Prestressing strand encapsulated by a factory-extruded polyethylene (PE) or polypropylene (PP) sheathing filled with a corrosion-protective soft filler. It is sometimes also called monostrand.

Stay cable: Complete cable system comprising one or several tensile elements fitted with anchorages, including saddles, if applicable, and the relevant corrosion protection and accessories.

Stay pipe: An enclosure encapsulating a bundle of tensile elements forming a stay cable in the free length.

Transition zone: The length of the stay cable where the tensile elements are supported by guide deviators and/or deviated from their arrangement in the free length to their arrangement in the stay cable anchorage.

2.2 SymbolsAUTS Actual Ultimate Tensile Strength of steelGUTS Guaranteed Ultimate Tensile Strength of steelHDPE/PE Polyethylene (in the context of these recommendations, HDPE/PE stands

for high-quality, high-density polyethylene, now called PE 80/100 in Europe, and as specified in Chapter 5)

PP Polypropylene (in the context of these recommendations, PP stands for high- quality polypropylene, as specified in Chapter 5)

SLS Serviceability Limit StatesULS Ultimate Limit StatesFLS Fatigue Limit Statesσ StressΔσ Stress RangeMPa Mega-Pascal, 1 MPa = 1 N/mm2mrad Milliradian , 1mrad = 0.001 radian (1° = 17.4 mrad = 0.0174 radian)

Fig.2.1 Definition of stay cable length / segments

3 Design and detailing3.1 GeneralThis chapter addresses topics relevant for the stay cable system which must be considered and specified by the Designer.The owner specifies general requirements for the structure (building construction, bridge, etc.) including:• Function (see also Chapter 4)• Importance (e.g. high or low damage tolerance)• Type of utilization• Lifetime.For the design of cable-stayed structures the Designer should normally follow national codes. Design of the stay cables is not covered by these recommendations except as given below for some specific topics which are relevant for the sizing and detailing of stay cables, and for special protective features of the stay cable systems.

3.1.1 Redundancy of cable-stayed structuresThe failure of one single stay cable should not lead to immediate failure of the entire cable-stayed structure. The Designer should take into account in his design accidental breakage of any one stay cable in the structure including the dynamic effects caused by the breakage. Generally, redundant stay cable systems, i.e. systems consisting of multiple parallel tensile elements, are preferred to cables consisting of a single tensile element.

3.1.2 Fire, impact, vandalismContrary to tunnels, bridges are well ventilated, and therefore relatively little exposed to high temperature rises in the event of a fire. However, a tank truck carrying hydrocarbons catching fire has in the past caused significant rise in temperature on a bridge deck. On a cable-stayed bridge, a truck could burn near a cable stay. This actually happened on the Second Severn Crossing in 1999. A fire such as this would normally be unlikely to affect more than one stay cable at a time, except in the case of a set of closely grouped stay cables (e.g. back stays). Structural stability is therefore not generally a problem if the structure is designed to allow for the failure of one stay cable, as is recommended above. However, some bridges are located in special environments, e.g. near fuel depots or oil refineries, where they will be crossed by large numbers of tank trucks carrying hydrocarbons. In such cases, improved fire resistance of stay cables may be justified to avoid loss of main tensile elements in the event of fire. Care should also be taken to:• Facilitate removal of flammable materials from the deck (drainage)• Limit fuelling by flammable products on/in the structure (avoid the filling of stay

cables with hydrocarbon based products such as wax)

• Retard temperature rise in the main tensile elements for the time needed to extinguish the fire (usually not more than two hours). Special insulating materials may be added to the stay cable inside or around the guide pipe / anti-vandalism pipe.

The Designer should consider specifying the provision and performance of special protection pipes against impact, vandalism, fire, etc. near the anchorages, when required.

3.1.3 Replaceability of stay cablesParticularly in bridges, stay cable systems should be replaceable. The Designer should specify whether the stay cables of the particular structure shall be replaceable, either one or several at a time. He should also specify whether replacement is feasible under full, reduced or no traffic load. He should then design the structure accordingly. Typically, for highway bridges, stay cable replacement should be allowed for by the design, one at a time, with reduced traffic load (closure of the nearest traffic lane).

3.1.4 Transverse loads applied from stay cables to the structureGuide deviators installed near the stay cable anchorages laterally support the stay cable and limit the transverse displacements of stay cables at this location. As a consequence, they protect the stay cable anchorages from the effects of transverse loads. These transverse displacements and loads are mainly due to:

• Deformations of the structure and change of cable sag due to construction loads, wind and traffic loads, and due to temperature changes

• Cable vibrations.The transverse support provided by the guide deviator to the stay cable causes a kink in the stay cable geometry, see Fig. 3.1, and a corresponding transverse load applied from the cable to the guide deviator and to the structure supporting the guide deviator. The Designer has to design the structure supporting the guide deviator for the maximum transverse forces, F, applied from the stay cable. These transverse forces are the product of angular kink and axial stay cable force, usually the permanent stay force. As a guidance for preliminary design of the structure supporting the guide deviator, an angular kink of α=± 1.4 degrees (± 25mrad) is suggested as a reasonable assumption.

Fig. 3.1: Angular deviations of stay cable at guide deviator

3.1.5 Bending stresses in stay cables

Depending on the actual cable force and cable weight, and in the absence of other loads, the stay cable will take a specific catenary cable profile. The cable will be subjected to pure axial tension only if the anchorages at either cable end are placed in the tangent to that specific catenary cable profile. Any deviation of the actual cable profile from the above, due to:

• transverse displacement of the cable under loads applied to the stay cable or structure

• rotation of the anchorages relative to the tangent because of applied loads, temperature changes or installation tolerances of anchorages, bearing plates and guide pipes

• stay cable vibrationswill introduce local bending stresses into the stay cable, in the region where the transverse displacement or rotation is applied. Devices such as cable anchorages installed with an inevitable placement tolerance, guide deviators, saddles, and clamping devices along the cable length, e.g. for cross ties, introduce such deformations and corresponding bending stresses. The magnitude of these bending stresses can be calculated, see e.g. [2, 3], (3).When using stay cables designed and tested in accordance with these recommendations, rotations applied at the anchorage up to ± 0.6° (± 10 mrad) are covered by the system design and testing, and do not need to be considered in the design by the Designer, see Chapter 6. For structures with applied rotations which are larger, the Designer either has to consider the effects of the additional rotations beyond the above indicated value by design, or in extreme cases, he has to specify specific design and testing with the larger rotations. The Designer shall require from the stay cable supplier verification of the transition zones and stay cable anchorages for these rotations both for fatigue and strength.

3.2 Design / sizing of stay cables

Load and design standards constitute an inseparable unit. The design of stay cables must comply with the respective national regulations. However, should there be no such information available the following may be taken as basis for the design of stay cables.

3.2.1 Service conditions (SLS)The cross section of stay cables is typically sized such that the maximum axial stresses in a stay cable under service conditions at SLS do not exceed specified limits. In the past, typically the maximum axial stress was limited to 45% GUTS. One reason for limiting the axial stress to such a low level was that secondary effects such as local bending stresses were usually not considered. With these recommendations, stay cable

systems will be tested for axial and bending effects. Therefore, higher axial stresses of up to 50% GUTS are considered permissible. The permissible axial stresses are summarised in Table 3.1 and are based on and are compatible with the stay cable test performance specified in Chapter 6 for anchorages and saddles. This means that transverse bending due to an angular rotation of ± 0.6° (± 10 mrad) has already been taken into consideration. Only if these values are exceeded, the additional transverse bending must be considered separately by the Designer. But this is usually not the case.

Permissible service stresses for stay cable systems tested in accordance with Chapter 6 of these recommendations (axial fatigue test with bending effect)

0.50 × GUTS1)

Permissible service stresses for stay cable systems not tested in accordance with Chapter 6 (purely axial fatigue test without bending effect)

0.45 × GUTS1)

1) Reference to the yield stress is not applicable for the prestressing steels specified in Tables 5.1 and 5.2.

Table 3.1: Permissible tensile stresses in stay cables under SLS

3.2.2 Fatigue design (FLS)3.2.2.1 Design philosophyThe fatigue design of stay cables has to consider the relevant fatigue loads in accordance with national standards applied to the particular structure to determine actual stresses and fatigue relevant stress range in the stay cables (fatigue demand). In the simplest case, the relevant fatigue load is a specific truck (axle loads). Depending on the actual load definition used in the particular national standard, some allowance for span length, dynamic effects and others may have to be added.The actual axial stresses and fatigue stress range demand of the stay cables resulting from the above loads are then compared with the design strength of the fatigue stress range of the actual stay cable system. The design strength of the stress range is based on the actual performance of the stay cable system in fatigue tests (applied stress range and number of cycles), suitably reduced in accordance with the safety philosophy of the national standard, taking e.g. into account the material factor for stay cables and the statistical effects of size/length of stay cable and the limited number of test results, to establish the design strength of the fatigue stress range, e.g. the 5% fractile value of the design fatigue strength of the actual stay cable system.In the simplest procedure of fatigue verification, the above fatigue stress range demand is compared with the endurance limit of the design strength of the fatigue stress range (the endurance limit is typically specified as the design fatigue strength (stress range) at a number of load cycles between 2 × 106 and 100 × 106), see e.g. [2]. In a refined procedure, the above fatigue stress range demand can be modified with some factors to account for the actual span of the member, mix of traffic loads, actual traffic volumes, actual service life, multiple lanes, etc. to obtain a “damage equivalent stress range” which then is compared with the design strength of the fatigue stress range at e.g. 2 ×

106 load cycles. For special cases, the fatigue damage caused by different fatigue loads, each with a specific number of load cycles can be accumulated with the Palmgreen-Miner’s rule for a given S-N curve of a specific stay cable system.

3.2.2.2 Fatigue strength of stay cable systemPTI [1] has specified minimum fatigue test strengths/performance for stay cable systems depending on the type of prestressing steel used (wire = 200, strand = 160 and bar = 105 MPa) with 2 × 106 load cycles. These are historical values which have been chosen based on US domestic supply of strand and a 5% fractile strength limit design basis against computed design truck fatigue demand. These values have shown good results in the past. However, improved materials have become available since and more recent recommendations such as [2] and several project specifications have specified 200 MPa stress range for testing of strand stay cable systems. For these recommendations 200, 200, and 110 MPa minimum stay cable system test fatigue stress range performance have been specified for strand, wire, and bar stay cable systems, respectively. These minimum test performance requirements are applicable for 2 × 106 load cycles with an upper stress of 45% GUTS and in combination with an angular rotation of ± 0.6° (± 10 mrad) applied at the anchorage of the stay cable system, see Chapter 6.In an actual design situation, a fatigue verification may need to be done at another number of load cycles than 2 × 106. However, it would be most unpractical and expensive to perform stay cable system fatigue tests at various numbers of load cycles and stress ranges to establish “Wöhler-Curves” (S-N curves). Fortunately, a significant experience with fatigue tests is available both on tendons and individual anchored tensile elements which has demonstrated that the slope of the S-N curves (stress range – load cycle number) is reasonably well known for strand, wire and bar tendons. This knowledge of the slope of S-N curves provides sufficiently reliable “Wöhler-Curves” for stay cable systems which pass through the specified minimum test performance for 2 × 106 load cycles confirmed by tests. These “Wöhler-Curves” of the stay cable system performance are shown in Fig. 3.2 and are marked with the letter “C”. It should be noted that these curves do not represent actual performance of a stay cable system but minimum test performance requirements. Hence, the actual performance of an acceptable stay cable system must be above this Curve “C”, in general.If the material factor of the relevant national standard and the statistical effects as mentioned above are applied to the stay cable system test fatigue stress range performance, the stay cable design strength of the fatigue stress range at 2 × 106 load cycles is obtained. Maintaining the same slope of the S-N curves, similar to above, “Wöhler-Curves” for the design strength of the fatigue stress range of the stay cable system can be obtained. These curves are marked with the letter “D” and are shown indicatively in Fig. 3.2. Presently, the actual level of Curve “D” can only be chosen with due consideration of the actual fatigue load definition in the particular national standard.

3.2.2.3 Fatigue strength of tensile elementsFor stay cable systems to achieve the specified minimum test performance requirements, the stay cable anchorage systems need to be carefully designed and detailed. In addition, the prestressing steels need to satisfy special characteristics which go beyond the performance of traditional materials used for pretensioning and post-tensioning, in particular for fatigue performance. These characteristics are specified in Chapter 5. Using the minimum fatigue performance specified for the prestressing steels specified in Chapter 5, and using the well known slopes of the S-N curves, one can also establish “Wöhler-Curves” for the minimum test performance of individual prestressing steel elements assuming the use of laboratory anchorages which ensure that the failure of the element will be away from the anchorage. These curves are marked with the letter “A” in Fig. 3.2.If the same individual prestressing steel elements, as represented by Curves “A”, are combined with the actual anchorage details of the stay cable system, e.g. wedge anchorage, one may expect some reduction of the fatigue performance from Curve “A” to somewhere in between Curves “A” and “C” depending on the selected design and detailing of the actual anchorage. This performance of an individual prestressing element, anchored with the actual anchorage details of the stay cable system, is represented in Fig. 3.2 by the Curve “B”. Since the performance depends on the proprietary anchorage design, no specific values can be given for Curve “B”, in general. Curve “B” may be used as an indication of the approximate performance of the stay cable system. It may be verified by testing of a series of single prestressing steel elements with the actual stay cable anchorage details to different fatigue stress ranges, and thus determine the number of cycles to failure.The slopes and minimum stress ranges at 2 × 106 load cycles used to establish the curves in Fig. 3.2 are summarised in Table 3.2.

K1 K2Δσ (MPa) at 2 × 106 load cycles

upper stress of 0.45 GUTS

WIRE (Grade 1770 MPa)A =62) 8 370

C 4 6 200

STRAND (Grade 1860 MPa)A =62) 8 300

C 4 6 200

THREADBAR1) (Grade 1050 MPa)A =72) 8 180

C 5 6 1101) Smooth bar 20 % higher values2) Exact slope is given by stress range values at 105, 5 × 105 and 2 × 106 cycles

Table 3.2: Minimum performance requirements - S-N values for tensile elements and stay cable systems shown in Fig. 3.2

Fig.3.2. S-N diagrams for stay cable systems and individual tensile elements

3.2.3 Ultimate limit states (ULS)When verifying ULS, GUTS of the tensile elements can be considered as the characteristic tensile strength of the stay cable system. Safety factors in accordance with national standards shall then be applied to find the design strength. If such safety factors for stay cables are not provided in national codes, one may use a safety factor of γ=1.35 for stay cables tested with angular rotation as specified in Chapter 6, and of γ=1.50 for stay cables tested without angular rotation.

3.2.4 EarthquakesThis design situation is often checked at ULS. Special ULS conditions may need to be considered for earthquakes, e.g. to avoid plastic deformations in the stay cable taking into account flexural effects under large deformations of the structure.

3.2.5 Construction and cable replacementThese are design situations of relatively short duration with relatively little fatigue relevant loading. The main design objective is to avoid inelastic deformations in the stay cable system during construction or stay cable replacement. Therefore, verification of axial stresses against permissible stresses is often sufficient. The permissible axial stresses during construction and stay cable replacement under SLS load combinations are summarised in Table 3.3 and are based on and are compatible with the stay cable test performance specified in Chapter 6. This means that transverse bending due to an angular rotation of ± 0.6° (± 10 mrad) has already been taken into consideration. Only if these values are exceeded, the additional transverse bending must be considered separately by the Designer.

Maximum stresses during construction and stay cable replacement for stay cable systems tested in accordance with Chapter 6 of these recommendations (axial fatigue test with bending effect)

0.60 × GUTS

Maximum stresses during construction and stay cable replacement for stay cable systems not tested in accordance with Chapter 6 (purely axial fatigue test without bending effect)

0.55 × GUTS

Table 3.3: Maximum permissible tensile stresses in stay cables during construction and stay cable replacement

3.3 Detailing and lightning protection3.3.1 Detailing3.3.1.1 GeneralThe Designer has to design the structure such that:

• it is possible to inspect the stay cable at anchorages and saddles, and along the free length, e.g. by using cable cars

• it is also possible to fix clamps for lighting or to install vibration damping devices, if ever required.

3.3.1.2 Arrangement of stay cablesThe illustrations below show some important design and detailing features of cable-stayed bridges:

• Typical stay cable arrangements (Fig. 3.3): Fan, harp, and semi-fan• Typical cable arrangements at pylon heads and saddles for concrete and steel

pylons (Figs. 3.4a and 3.4b)• Cable attachments to the deck (Fig. 3.5).

Fig. 3.3: Typical stay cable arrangement

Fig. 3.4a: Concrete pylon heads

Fig. 3.4b: Steel pylon headsFig. 3.4: Types stay cables arrangement at pylon heads

Fig. 3.5 Types of connections of stay cables to bridge superestructures

3.3.1.3 Dimensions of stay cablesThe dimensions of cable anchorages are specified by the cable producer, as are also the space requirements for the assembly and for stressing jacks. The interested reader is referred to the specific literature and brochures of the cable suppliers. However, in case no such information is available, average values are presented in Fig. 3.6 for preliminary design.

Fig. 3.6 Dimensions of stay cables for preliminary design

3.3.2 Lightning protectionThe general concept of lightning protection consists of a collector, a transition line,and the earth (Fig. 3.7). The transition line connects the collector with the earth on the shortest distance possible. Lightning protection depends on the type of cable-stayed bridge, i.e. whether steel or concrete structure.

• Concrete cable-stayed structures:If the stay cables and / or the reinforcement of the pylon are not directly connected to the earth, a lightning strike into the pylon may cause significant spalling of

concrete up to pieces of several 100 kg. Lightning protection of concrete pylons and stay cables of cable-stayed structures should generally consist of the following:(1) Installation of collector lines from each stay cable anchorages in the pylon to the

transition line. Installation of a collector line from the reinforcement near the top of the pylon to the transition line. Collector lines should be made of copper and have a cross section of at least 50 mm².

(2) Installation of a transition line in the pylon, in direct contact with thereinforcement cage, from the pylon tip down to the foundation. The transition line should have a cross section of at least 200 mm² and may consist of specifically designated reinforcing steel bars properly welded together to ensure adequate electrical conductivity. The transition line should be connected to the foundation earth which typically consists of a horizontal closed loop of reinforcing steel bars (min. 200 mm² cross section) placed low in the foundation, inside the concrete.

The concrete deck does not need any specific protection, in general.In case electrically isolated bearings are used, they need to be electrically connected to the earth with cables (min. cross section of 50 mm², e.g. by copper bar ∅ 8 mm).Composite structures are suggested to be protected similarly to concrete structures.

• Steel cable-stayed structures:Pure steel structures need no specific lightning protection systems. The same comment for electrically isolated bearings applies as mentioned for concrete structures above.

Fig. 3.7 Lightning protection of concrete and composite cable-stayed structures

3.4 Saddles3.4.1 General

Saddles must be designed such as to ensure a safe transfer of vertical forces and of differential forces of stay cables from opposite sides of the pylon (main span and side span) in the erection and final state (friction, bond, shear keys, clamping) into the pylon structure. Load assumptions shall be in accordance with the relevant standards and the actual intended construction methods. If the stay cable has been specified to be removable, then the saddle needs to be designed such as to be removable also. This can e.g. be achieved with double steel-pipes, i.e. placing the saddle pipe, with a bundle of tensile elements grouted inside the saddle pipe, inside a guide pipe installed into the pylon structure. Transfer of differential forces from the saddle to the guide pipe may be achieved by shear keys or other mechanical connections. An alternative concept uses a battery of individual tubes, one for each tensile element, placed inside a guide pipe. The tensile element is not grouted inside the individual tube. However, the space between the individual tubes and the guide pipe is grouted. These two types of saddles are illustrated in Fig. 3.8.

Fig. 3.8: Saddles for replaceable stay cables

3.4.2 Transfer of differential stay cable forcesTransfer of differential forces from the stay cable or individual tensile element into the pylon may be ensured by either one of the following three methods:

• Transfer of differential forces is ensured by friction. The friction coefficient between the tensile element and the individual tube (or the stay cable to the saddle pipe before grouting) depends on the technology which is used, and may vary in the

range of 0.05 to 0.4. Whatever technology is used, the actual friction coefficient of the saddle system shall be confirmed by testing using realistic test set-up and surface conditions of all elements. The friction coefficient determined in the test shall then be reduced with a sufficient safety factor of not less than 1.5. This method allows the transfer of limited differential forces only

• Transfer of differential forces is ensured by bond (in fact, it is a combination of bond and Coulomb friction due to the curvature in the saddle). For this solution, the sheathing on the tensile elements needs to be removed, in general. Information on the actual bond strength of the uncoated tensile elements inside grout may be found in relevant standards. The actual surface conditions (cleanliness) shall be considered. However, typically the bond strength of clean uncoated tensile elements is not critical. If coated tensile elements are used and/or if no information is available, bond strength may need to be confirmed by testing. This method allows the transfer of significant differential forces

• Transfer of differential forces is provided with shear keys or other mechanical devices.

3.4.2 Minimum radius of curvature of saddle pipesThe performance of the stay cable in the saddle depends mainly on the effect of fretting fatigue (small relative movements due to fatigue loading between individual tensile elements combined with transverse pressure). The transverse pressure is a function of the stay cable force, the saddle radius, and the bundle effect (number of layers of individual tensile elements sitting on top of each other). As a starting point for the detailing of saddles by the Designer, the following minimum radii of curvature may be assumed:

• For individual tensile elements inside individual tubes (no bundle effect):Min R ≥ 400 ∅

(∅: diameter of wire / individual wire of strand)• For bundle of tensile elements inside saddle pipe (with bundle effect):

Min R ≥ 30 D(D: diameter of stay pipe).

The actual performance of the actual saddle details should be confirmed by testing in accordance with Chapter 6, in particular if radii of curvature in the order of the above minima are used.

3.5 Execution aspects

This section provides information on what the Designer should consider or provide for the erection of stay cables. Execution details relevant for installation on site are specified in Chapter 7.

3.5.1 Stage-by-stage analysisThe Designer has to perform a stage-by-stage analysis of the cable-stayed structure considering all stages including construction, service, and cable replacement. This analysis will provide amongst other results the force and elongation of each stay cable during each stage of erection of the cable-stayed structure. This will form the basis for the installation of the stay cables by the specialist contractor, see Chapter 7.For stay cables assembled on site during installation, the stiffness of the structure (pylonand deck) as assumed in the above analysis, has to be known. For these cases, the Designer has to provide these stiffness values of pylon and deck for each construction stage to the specialist contractor responsible for the erection. An early exchange of the data between the Designer and the specialist contractor is recommended.

3.5.2 Length adjustment capability of stay cablesStay cables need some capability for length adjustment e.g. because of design and construction tolerances and possible future increase of service loads. The Designer shall specify the minimum length adjustment capability to be provided at the anchorages of stay cables both for:

• Re-stressing (increase of stay cable force); and for• De-tensioning (reduction of stay cable force).

3.5.3 Construction tolerancesThe Designer should specify construction tolerances where relevant and where not already covered by relevant standards. In order to comply with the assumptions in these recommendations for flexural effects near anchorages, the Designer may need to specify a directional installation tolerance of the bearing plates and guide pipes of about ± 5 mrad (± 0.3 degrees) around the theoretical axis of the stay cable and a tolerance on the position of the reference point of the anchorage in x, y, z-direction of ± 10mm.

3.6 Cable vibrations3.6.1 GeneralStay cables, through their inherent geometrical and mechanical properties, have proven to be relatively insensitive to vibration. However, under certain conditions stay cable vibrations have been observed on some projects. Stay cable vibration is a complex topic which goes beyond the scope of these recommendations. The interested reader is referred to specialist literature such as [4, 5, 6] and (21-23, 27, 28, 42-51, 54-58, 60, 61, 85, 88-89).Typical stay cables have a relatively small internal damping ratio with a logarithmic decrement in the order of δ ≈ 1 % for individually protected tensile elements and parallel wire stay cables and down to δ ≈ 0.05-0.10 % for cement grouted stay cables, where δ = ln(fn/fn+1) = 2πξ (ξ = damping ratio, f = amplitude of stay cable vibration at cycles n and n+1, see Fig.3.9). The Designer has to verify whether the stay cables of the particular

structure are stable against the different forms of vibration with the expected internal damping ratio. This verification shall include, but may not be limited to, the following:

• Rain-wind induced vibration (wind forces create an equilibrium of the water rivulets moving on the cable which modify the cable aerodynamics): It is the most common cause of vibration observed on bridges. Large amplitudes may occur at moderate wind speeds in the order of 10-15 m/s when combined with light rain

• Parametric excitation:Due to periodic displacement of the anchorages caused by wind or traffic actions onthe deck or pylon. This action may be responsible for large amplitude vibration.Risk of parametric excitation shall be assessed at the design stage of the structure. A risk of large amplitudes exists when A=2B/k, where A=frequency of excitation, B=frequency of stay cable, k=positive integer. Experience seems to indicate that the first stay cable mode and k=1, 2 are particularly critical.

Other types of cable vibrations such as galloping etc. are often not critical if the above types of vibrations are adequately considered.If the stay cables are not stable with internal damping only, the Designer has to specify additional measures to control stay cable vibrations. The provision of texture on the stay cable surface and the provision of special damping devices are particularly effective against rain- wind induced vibration. The provision of cross ties is particularly well adapted to avoid parametric excitation. These different measures are presented in Clause 4.4.4. Some basic design information is given below for special damping devices.For further details on special damping devices and cross ties please refer to Clause 4.4.4 and to specific literature from cable suppliers.

Fig. 3.9: Stay cable vibration – Definitions and measuring of damping

3.6.2 Special damping devicesSpecial damping devices may include hydraulic, viscous and friction damping devices. These devices are typically installed near the stay cable anchorage(s), at a distance, ΔL, from the fixed point of the anchorage(s). All types of special damping devices have in common that the maximum theoretical damping ratio, δth, which one damping device installed near one stay cable anchorage can provide is:

δth = π (ΔL / L)(where L is the stay cable length).

If damping devices are provided near both stay cable anchorages, twice the above theoretical damping ratio can be achieved. The actual damping ratio of the special damping device installed in the structure is lower than the above theoretical value because of the actual efficiency of the damping device and because of the flexibility of the support of the damping device. However, with a known or estimated actual efficiency of a damping device and a sufficiently rigid support, the Designer may estimate an appropriate distance ΔL of the damping device from the stay cable anchorage. This may assist the Designer in good detailing of the anchorages either above or below deck level, and proper design of the supporting structure for the damping device.When specifying special damping devices, the following information should be provided by the Designer as a minimum:

• Effective logarithmic decrement δ which the special damping device has to provide after consideration of the actual efficiency of the damping device and support stiffness. In practice, effective logarithmic decrements of δ = 3-4% have often been found to be sufficient to control wind-rain induced stay cable vibrations

• Any required special texture of the stay pipe such as helical ribs or dimples• For certain projects, a maximum stay cable amplitude under service wind loads

has been specified. Amplitudes in the range of “stay cable length / 1700” for the first and second mode of vibration have been specified. These are quite severe limits and are provided mainly for aesthetical reasons.

The designer may choose to specify measures for the installation of such special damping devices on the stay cables at the time of construction for actual installation only at a later time during the design life of the structure if the need is in fact confirmed by actual vibrations.

3.6.3 Cross tiesThe Designer may choose to specify cross ties to control stay cable vibrations. Cross ties serve mainly to change the frequency of the stay cable and may be of interest if there is a risk of parametric excitation. Cross-ties are effective in the plane of the cables only. Strength, stiffness, prestressing forces, and connection points of the cross ties need to be specified by the Designer.

3.7 Inspection and maintenance

In the design of the structure, the Designer has to allow for the space to walk and climb inside the deck and pylon, if applicable, and the accessibility of components for the following operations on the stay cable:

• Inspection and maintenance of the anchorages and special damping devices• Replaceability of the stay cables• Installation of the stay cable stressing equipment• Installation of the saddles.

4 Functional requirements for stay cables

This chapter addresses mainly aspects which should be considered by the stay cable designer/supplier.

4.1 Evolution of stay cable technology

The stay cable technology as discussed in these recommendations has evolved gradually over the last about 25 years, from initial systems which were similar to normal post- tensioning tendons to today’s high performance stay cable systems, through the following main stages:

• Bare tensile elements (wire, strand and bar) encased inside cement grout (basic post- tensioning technology) in either steel or HDPE stay pipe

• Bare tensile elements (mainly wire) encased inside flexible tar epoxy grout in HDPE stay pipe

• Epoxy coated tensile elements (mainly 7-wire strands) encased inside cement grout in either steel or HDPE stay pipe

• Individually greased and sheathed monostrands encased inside cement grout in HDPE stay pipe

• Galvanised and individually waxed and sheathed monostrands without stay pipe and filler.

The most recent stay cable technology uses metallically coated tensile elements (mainly galvanised), individually protected with wax and PE sheathing, encased inside a HDPE stay pipe without filler. Alternatively to the individual protection with wax and sheathing, a general injection of the stay pipe with wax around the galvanised tensile elements has been used. These two recent systems will be referred to in the following as “Reference systems”, and used as reference in terms of corrosion protection.Parallel to the above evolution of the corrosion protection systems in the free length of the stay cables, anchorage systems have also evolved.

4.2 General requirements4.2.1 General

• Each system component (Fig. 2.1), from the stressing end anchorage through the free length to an eventual saddle providing continuity to the next anchorage, should have the same safety and durability under SLS, FLS and ULS (no weak member in the system/chain)

• The fatigue and ultimate capacity of the stay cable system must be verified by testing (see Chapter 6)

• Transverse loads on the anchorages and/or bending stresses in the stay cable must be kept low by using appropriate system details and possible use of guide deviators (Fig. 3.1)

• The design lifetime, specified by the owner and/or the Designer, must be satisfied for the specified exposure class (see Clause 4.2.2) for each component. A maintenance program for the stay cable system should be prepared by the stay cable supplier which ensures meeting the design lifetime

• In addition to load and execution aspects, the durability of stay cables mainly depends upon the provided corrosion protection. A clearly defined corrosion protection concept must be submitted and verified by testing, as applicable (see Clause 4.3, 4.5.2 and Chapter 6)

• Stay pipes are used to keep wind forces on the stay cable low, and to protect the cable against environmental influences while also serving aesthetic purposes. They must be able to sustain forces from clamps used to fix the lighting or damping devices

• In structures with long design lifetime and significant risk of accidental damage, e.g. highway bridges, the stay cables must be exchangeable, either as the whole cable or as single tensile elements

• High quality materials must be provided (see Chapter 5)• Installation of the stay cables must be by qualified companies with

experienced personnel (see Chapter 7), including suitable working instructions and adequate quality control

• Stressing of stay cables with single strand jacks or multistrand jacks shall be done to the required force (with control of elongation) or to the required elongation (with control of force). Adjustment of stay cables for fine tuning of superstructure geometry may be provided with small stroke jacks (see Clause 7.4)

• Allowance for inspection and maintenance of stay cables shall be included in the system design (see Chapters 8 and 9)

• Possibility for additional damping to the stay cable with guide deviators, surface texture of the stay pipe, special damping devices, cross ties, or similar shall be provided in the stay cable design

• Stay cable design should include measures against impact, vandalism, fire and lightning.

4.2.2 Durability design, corrosion protection4.2.2.1 Design working lifeThe owner specifies the working life during which the system is expected to be durable and to perform within the specified performance levels provided it is subjected to regular maintenance. The design lifetime must not be confused with the execution guarantee covering construction defects which are discovered between 2 to 10 years after completion of construction. Single components of stay cables may have a shorter lifetime, e.g. guide deviators and damping devices or corrosion protection systems such as coatings. Such components will need regular maintenance or replacement for the stay cable system to achieve the design lifetime. The design target for the actually provided protection should be to minimise the overall life cycle costing.The design lifetime of stay cables may be defined as temporary, e.g. for construction:

• temporary use (≤ 2 years)• semi-permanent use (2 to 15 years).

However, these temporary uses are not specifically considered in these recommendations. If there is no specific information available for a particular project the authors of these recommendations suggest 50 years and 100 years design lifetime for building and bridge structures, respectively. Adequate maintenance is a necessity to achieve these design lifetimes.

4.2.2.2 Aggressiveness of environment, other aspectsThe aggressiveness of the environment is grouped into Exposure Classes (see [S2, S3]). A possible classification of Exposure Classes in accordance with [S2] is:

•• "benign to low corrosion risk" (inside structures) C1, C2• “medium to high” corrosion risk (outside structures, humidity) C3, C4• "very high corrosion risk"

(e.g. bridges subject to de-icing salt or maritime climate) C5-1, C5-M

Further environmental influences which must be taken into consideration for durability of some materials such as plastics (HDPE/PP) are UV-radiation, temperature, rain and wind.In addition to the above environmental influences, classified as "benign" to "very highcorrosion risk", the following aspects have to be taken into consideration:

• Local exposure conditions of a particular component of the stay cable or the entire stay cable

• Accessibility for inspection: Access versus no access• Intervals of maintenance• Exchangeability of individual stay cable components.

Fatigue is another important consideration for the durability of the stay cables.

4.2.2.3 Application of corrosion protection concept

Any component of the stay cable system must be able to maintain its function during the specified design lifetime with the anticipated maintenance procedures. Below, the criteria for corrosion protection and durability are applied to stay cables for:

• The free length• The transition zone between free length and anchorage zone• The anchorages and the saddles.

These criteria are also adapted to the most commonly used types of materials including:

• Prestressing steel• Other materials• Mild steel.

4.3 Requirements for the free length

4.3.1 Corrosion protection philosophy for tensile elements

The free length of the stay cable consists mainly of the prestressing steels (strand, wire, bar) used as the main tensile elements of the stay cables and their protection layers. If not protected adequately, these types of steels may suffer from pitting corrosion and stress corrosion (see [7] to [9], (95)). There is presently no scientific model available to reliably predict these corrosion processes over time as a function of the exposure classes. Therefore, the design approach for these prestressing steels is to provide suitable permanent multi-layer corrosion protection which is adequate for the entire design life of the stay cable.4.3.2 Protection philosophy for other materials

This paragraph applies to other materials than those discussed above under Clause 4.3.1, and includes mainly non-metallic components such as HDPE/PP used for the stay pipe and sheathing.Based on present knowledge and experience, a 50 year design life may be justified based on accelerated testing for carefully selected virgin quality HDPE/PP materials such as specified in these recommendations in Chapter 5. Up to 100 year design life may be difficult to justify based on today’s knowledge. However, proper maintenance and, if necessary, replacement of the stay pipe, may achieve a 100 year design life of the system. This applies in particular for materials that contain a minimum of about 2 % well distributed carbon black. Presently, co-extruded or fully coloured HDPE is often used for the stay pipe. For these coloured stay pipes significant