Concrete Highway Bridges-External prestressing

The Design of Concrete HighwayBridges and Structures with

External and UnbondedPrestressing

Summary: This Advice Note provides guidance on the design of bridges with externaland unbonded prestressing.

This Advice Note provides advice on specification requirements for use inpublic purchasing contracts. It does not lay down legislative requirements forproducts and materials used in highway construction in the United Kingdom.

THE HIGHWAYS AGENCY BA 58/94

THE SCOTTISH OFFICE DEVELOPMENT DEPARTMENT

THE WELSH OFFICEY SWYDDFA GYMREIG

THE DEPARTMENT OFTHE ENVIRONMENT FOR NORTHERN IRELAND

Volume 1 Section 3Part 10 BA 58/94 Registration of Amendments

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DESIGN MANUAL FOR ROADS AND BRIDGES


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VOLUME 1 HIGHWAYSSTRUCTURES:APPROVALPROCEDURES ANDGENERAL DESIGN

SECTION 3 GENERAL DESIGN

PART 10

BA 58/94

THE DESIGN OF CONCRETEHIGHWAY BRIDGES ANDSTRUCTURES WITH EXTERNALAND UNBONDED PRESTRESSING

Contents

Chapter

1. Introduction

2. Conceptual Design

3. Detailed Design

4. Corrosion Protection

5. References

6. Enquiries

Volume 1 Section 3 Chapter 1Part 10 BA 58/94 Introduction

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1. INTRODUCTION

General

1.1 The Design of prestressed concrete structurewith external or unbonded prestressing is not coveredthe current Code of Practice for the design of concretbridges, BS 5400 Part 4. This coupled with durabilityproblems experienced by some structures with externprestressing has resulted in this type of constructionbeing seldom used in this country.

1.2 Tendon corrosion associated with the ingressde-icing salts through inadequately grouted ducts, habeen encountered in a number of structures of bondepost-tensioned construction. Recent sudden collapseof bridges with bonded tendons is a cause for concerespecially as there are no satisfactory means availabto monitor the condition of tendons. As a result, theOverseeing Organisations wish to encourage the useexternal and unbonded tendons as a form ofconstruction which can be easily inspected, monitoredand maintained.

1.3 This Advice Note should be used inconjunction with BD 57 (DMRB 1.3.7) and BA 57(DMRB 1.3.8) Design for Durability, particularly asregards access to enclosed spaces and the quality ofprotection from environmental effects (includingleakage) within enclosed spaces.

Implementation

1.4 This Advice Note may be used forthwith forschemes currently being prepared provided that, in thopinion of the Overseeing Organisation, this would noresult in significant additional expense or delayprogress. Design Organisations should confirm itsapplication to particular schemes with the OverseeingOrganisation.

1.5 Where this Document is applied for productswhich are procured through a contract incorporating tSpecification for Highway Works (MCHW 1), productsconforming to equivalent standards and specificationsof other states of the European Economic Area will beacceptable in accordance with the terms of the 104 a105 Series of Clauses of that Specification. Anycontract

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1.6 This Advice Note provides guidance on the u

of information on the design requirements set out inBD 58, the Design of Concrete Highway Bridges aStructures with External and Unbonded Prestressi

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or the procurement of products which do not includehese Clauses must contain a suitable clause of mutualecognition having the same effect regarding whichdvice should be sought.

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f unbonded prestressing and some background

efinition

.7 Unbonded prestressing is prestressing where, inhe finished structure, no continuous bond is providedetween the prestressing elements and the concreteection, either by the provision of grout or by any othereans. The term, external prestressing is applied to tha

lass of unbonded prestressed structures where some oll of the prestressing is unbonded and outside theoncrete section, and where the load is transferred to thoncrete through end anchorages and deviators. It is, inheory, possible to use unbonded prestressing elementsn ducts which lie within the concrete section. This isnbonded internal prestressing.

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Volume 1 Section 3 Chapter 2Part 10 BA 58/94 Conceptual Design

2. CONCEPTUAL DESIGN

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Factors affecting choice

2.1 The choice of whether to use external orunbonded prestressing should be made at the codesign stage as it is likely to affect a structure's foand dimensions. Although external prestressing unlikely to be economical in structures spanning lthan 40m, when whole life costs are considered imay be justified.

2.2 Advantages The following is a list of advantages associated wuse of unbonded prestressing:

i. Facilities for inspection, monitoring and restressing or replacing of tendons can be eincorporated.

ii. The lack of ducts within concrete when usexternal prestressing makes steel fixing eand facilitates the use of thinner webs, theresulting in a reduction in dead load.

iii. External tendons afford simpler cable layo

iv. Prestress loss due to both friction and crelower.

v. Flexural failures will always be preceded extensive cracking and excessive deflectithus giving prior warning of collapse.

vi. Since the opening and closing of cracks dnot affect the tendon stress, as in the casbonded construction, and as corrosionprotection to the tendons is not dependenthe encasing concrete, the serviceabilityrequirement for crack widths can be relaxexternal prestressing.

2.3 Disadvantages

The following is a list of disadvantages associatethe use of unbonded prestressing:

i. In external prestressing the maximum cabeccentricity is likely to be less than for bonpost-tensioning.

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Robustness

ith 2.6 The local failure of tendons in a bonded ptensioned structure would not normally have a

tendons rebond with the concrete. However, in thed of unbonded prestressing, failure in any position m

a tendon ineffective over its entire length. Hence

ii. The increase in cable force at ultimate is lessthan that with bonded tendons, and hence theultimate flexural capacity of a section is usuallyreduced. Slightly deeper structures may berequired to compensate for this.

iii. In external prestressing, the cables are notprotected by concrete and hence are morevulnerable to terrorist attack or fire.

iv. In the event of a tendon failure, tendons do nothave the capability to rebond.

v. The strength of the structure is dependent onthe anchorages.

Maintenance and Replacement

2.4 Concrete boxes housing external tendonsshould ideally have a reasonable working space andmeans of access; see Design for Durability, BD 57(DMRB 1.3.7) and BA 57. (DMRB 1.3.8). Thisrequirement would result in a practical minimum spanfor externally prestressed structures of about 40m. Where external prestressing is used on shorter spans,reduced headroom may be used with the agreement othe Overseeing Organisation provided that the fullrequirements of Health and Safety legislation are met.

2.5 Provision should be made for the replacementand re-stressing of tendons. The structure should becapable of accommodating a range of prestressing forcresulting from future tendon replacement, and galleriesshould be incorporated at abutments with sufficientroom to manoeuvre prestressing jacks. The provisionof runway beams, which may also be used to facilitateconstruction, would be desirable. For furtherinformation on the design of abutment galleries see ThDesign for Durability, BD 57 (DMRB 1.3.7) and BA57 (DMRB 1.3.8).

significant effect on its ultimate load capacity as the

structures with unbonded prestressing may be

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vulnerable to a disproportionate collapse. This isparticularly true for continuous bridge decks wherelocalised failure in one span could result in aprogressive collapse of adjacent spans.

2.8 The design criteria in BD 58 (DMRB 1.3)introduce a check to ensure that structures withunbonded prestressing will not collapse as a result of local tendon failure. This requirement may have asignificant influence on a structure's form. For instancthe position of anchorages in continuous structures maneed to be staggered or box girder decks may requiretransverse stiffening to prevent collapse following a losof tendons in an external web.

Demolition

2.9 In practice, when cut or broken, conventionalgrouted or pretensioned tendons will re-anchor andhence the energy stored in the tendon is not released.However, cutting unbonded tendons will release muchgreater amounts of energy. It should therefore beavoided in demolishing structures with unbonded pre-stressing.

2.10 As re-stressable tendons can be de-stressedindividually, demolition of structures with unbondedprestressing is usually quite straightforward. Howeverthe effect on adjacent spans of de-stressing multi-spancables should be carefully considered.

2.11 In short span bridges where the provision ofabutment galleries is not feasible, anchorages shouldnevertheless be detailed to facilitate future de-tensioning for demolition.


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Volume 1 Section 3 Chapter 3Part 10 BA 58/94 Detailed Design

3. DETAILED DESIGN

General

3.1 This Chapter deals with the detailed design o post-tensioned concrete with external and unbonded prestressing. It includes some information on the background to the design requirements contained in 58 (DMRB 1.3.9. The Design of Concrete Highway Bridges and Structures with External and Unbonded Prestressing).

3.2 References to Clause numbers in this chapte refer to BS 5400 as modified by BD 58 (DMRB 1.3.9

Flexure

3.3 As the ultimate strength of structures withunbonded prestressing is dependent on the prestresforce, allowance must be made for the actual prestreforce present being less than that which is assumeddesign. This may be, for instance, as a result of thejacking force being less than intended or the prestreloss being greater than calculated. A partial safetyfactor ( of 0.87 is therefore applied to the prestressFL

force. (See Clause 6.3.3.1(d)).

3.4 Since the strain in unbonded tendons at theultimate limit state is unlikely to be sufficient to causeyield, failure is likely to be through crushing of theconcrete. However, the overall behaviour of thestructure should remain ductile with extensive crackiand excessive deflections being apparent before yieIt is, therefore, not felt necessary to impose the 15%over strength requirement that is prescribed for overreinforced bonded prestressed sections.

3.5 In bonded prestressed construction planesections, including their prestressing tendons, areassumed to remain plane. This assumption dependthe tendons remaining bonded to the concrete and istherefore not valid with unbonded prestressing. Theincrease in steel strain in unbonded prestressing atfailure is less than for bonded tendons and usually nsufficient to reach yield. Clause 6.3.3.1(f) introducessimple but conservative rule to estimate the increasesteel strain, and hence stress at failure. As analternative a more rigorous non-linear analysis may used.

November 1994

Shear

f3.6 The design rules for shear in beams given inBS 5400 Part 4 are empirical and based on test resultson bonded tendons. They are therefore not considered

BDappropriate for unbonded prestressing. The approachadopted for determining shear resistance is to treatunbonded prestressing as reinforced sections with anexternally applied load (See Clause 6.3.4.1).

r3.7 The design for shear is based on a form of truss).analogy and thus implies a greater force in the tension

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cord than would be expected by simple bending theory. This has been reflected in BS 5400: Part 4: 1990. However, in the case of unbonded prestressing it isnecessary to check that the requisite tendon force ratherthan tendon strength, is available. This has beenaddressed in BD 58 (DMRB 1.3.9) by adding arequirement to extend tendons beyond the section atwhich they are required. (See Clause 6.3.3.1(g)).

Torsion

3.8 As the torsion rules for bonded tendons in BS5400 Part 4 make no allowance for the beneficial effectsof prestress, they do not require modification for the useof unbonded tendons.

Prestress Loss

3.9 The use of unducted systems or ducts withgreased or waxed strands means that prestressingsystems used in external and unbonded prestressinghave lower coefficients of friction than bonded systems. In addition, with external prestressing wobble losses areeliminated.

3.10 Although the lack of bond will not alter theaverage loss due to creep, as the creep losses areconstant over the length of a beam, losses at criticalsections are likely to be smaller than with bondedconstruction. This may be significant in shorter spanswhere the permanent compressive stresses for live loadsare high.

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Anchors and Deviators

3.11 The strength of structures with unbondedprestressing is dependent on the anchors and, wherethey are used, the deviators. This is particularlysignificant as the failure mode of anchors and deviatomay be brittle.

3.12 The design of members with unbondedprestressing for flexure and shear is based on a lowebound estimate of tendon force. However, anchors adeviators should be designed for the unfactored tendstrength. (See Clauses 6.7.5, 6.7.6).

3.13 If tendons are not adequately restrained withithe concrete section, the deformation of the concretebetween deviators can have a significant effect on thmoment applied by the tendon to the concrete sectioIn addition, inadequately restrained tendons may vibrexcessively and be susceptible to fatigue failure. Compliance with the requirements of Clause 6.8.8 wensure that tendons are adequately restrained.

Radius of Curvature

3.14 Clause 6.8.8 sets out the minimum radius ofcurvature of tendons in deviators. If a specific tendonand duct or shoe type is tested or investigated and foto be satisfactory for smaller radii, the resulting valuecan be used.

3.15 Smaller radii may be used in positions wherethere is no significant movement of the cable relativethe duct or shoe in the bend; for example loop anchoat the centre of cable length.

Serviceability

3.16 In the design of bridges with bondedprestressing to BS 5400 Part 4, for the serviceabilitylimit state, the force in the tendons due to deformatioresulting from the applied load is normally ignored. Athis is analogous to unbonded tendons, the BS 5400Part 4 design approach for serviceability can be adopfor unbonded prestressing.

3.17 As the tendons in externally prestressed,structures do not rely on the concrete for corrosionprotection, and as the problem of high stress fluctuat

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in tendon associated with cracks in bondedconstruction, are not relevant to unbonded prestressing,the serviceability cracking criteria have been relaxed inthe case of external prestressing (see Clause 4.2.2).

3.18 In the case of internal unbonded tendons, as thetendons are not easily inspected and as durabilityproblems associated with tendon corrosion are less easyto detect, the serviceability requirements have not beenrelaxed.

Untensioned Reinforcement

3.19 In order to control thermal and shrinkagecracking and facilitate construction, a requirement fornominal untensioned reinforcement has been introducedin Clause 6.8.4.

Segmental Construction

3.20 The "dry jointing" technique, where jointsbetween segments are constructed without the use of

several long span bridges, especially in the USA. However, with this method of jointing, a watertight sealcannot be achieved at the segment joints. This wouldtherefore reduce the corrosion protection of internalgrouted tendons. For this reason dry jointing may onlybe used in segmental construction in association withexternal tendons.

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Volume 1 Section 3 Chapter 4Part 10 BA 58/94 Corrosion Protection

4. CORROSION PROTECTION

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General

4.1 Corrosion protection systems for unbondedprestressing should be considered in relation to theoverall design of the structure. As tendon corrosionwould only take place in a damp environment, everyeffort should be made by good design, detailing andspecification of materials, to ensure that water isexcluded from ducts and concrete box enclosures. Sthe Design for Durability, BD 57 (DMRB 1.3.7) andBA 57 (DMRB 1.3.8).

External Prestress

4.2 For maintenance reasons it is advisable thaexternal tendons can be inspected easily and arereplaceable. External tendons enclosed within aconcrete box are subject to an environment in whichcorrosion rates are negligible. It is therefore considesufficient to provide corrosion protection by galvanistendons in accordance with BS 2763. Other forms ocorrosion protection may also be considered. Desigshould be aware of the possibility of hydrogenembrittlement and galvanic corrosion associated witgalvanising and take the necessary precautions to eagainst them.

4.3 Unducted galvanised tendons have theadvantage of being easily inspected throughout theirlength.

4.4 In detailing deviators, the use of open shoetype deviators may be preferable to the use of closeduct-type deviators since they allow more completeinspection of the condition of the tendons at deviatorpositions.

Unbonded Tendons

4.5 Ducts for unbonded tendons within theconcrete section should be made from High DensityPolyethylene (HDPE) with a minimum wall thicknessof 2mm, or other suitable materials. Ducts should bewaterproof with drainage holes at low points along thprofile to release any water that may be trapped andfacilitate inspection by means of an endoscope.

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4.6 Unbonded tendons within ducts may becorrosion protected with galvanising, painted in a lightcolour to aid inspection, or with a suitable system whichallows for tendon replacement and restressing.

Anchorages

4.7 As the prestressing force in unbondedprestressing is transferred to the concrete solely throughthe anchorages, care should be taken to exclude waterfrom the vicinity of anchorages. Anchorage endsshould be open and inspectable, and anchorage pocketshould be sealed by means of removable cover plates. Projecting tendons and anchor plates should beprotected with a suitable water excluding wrappingtape.

4.8 The position at which tendons are gripped atthe anchorage is a potential location of bi-metalliccorrosion. Anchorage plates and wedges shouldtherefore be of the same type of steel as the tendon.

4.9 Care should be taken to ensure that anchoragesystems and the detailing of tendons at the anchoragescan facilitate the de-tensioning and re-stressing oftendons.

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5. REFERENCES

1. Design Manual for Roads and Bridges

Volume 1: Section 3 General Design

BD 58 The Design of Concrete Highway Bridges andStructures with External and UnbondedPrestressing

(DMRB 1.3.9) BD 57 Design for Durability (DMRB 1.3.7) BA 57 Design for Durability (DMRB 1.3.8)

2. British Standards

BS 5400: Part 4: 1990: Code of Practice for the Designof Concrete Highway Structures.

BS 2763: 1982: Specification for Round Carbon SteelWire for Wire Rope.

Volume 1 Section 3 Chapter 6Part 10 BA 58/94 Enquiries


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6. ENQUIRIES

All technical enquiries or comments on this Advice Note should be sent in writing as appropriate to:

Head of Bridges Engineering DivisionThe Highways AgencySt Christopher HouseSouthwark Street A. J. PICKETTLondon SE1 0TE Head of Bridges Engineering Division

The Deputy Chief EngineerThe Scottish Office Industry DepartmentRoads DirectorateNew St Andrew's House J INNESEdinburgh EH1 3TG Deputy Chief Engineer

The Director of HighwaysWelsh OfficeY Swyddfa GymreigGovernment BuildingsTy Glas Road B H HAWKERLlanishen Head of Roads EngineeringCardiff CF4 5PL (Construction) Division

Chief Engineer - Roads ServiceDepartment of the Environment for Northern IrelandRoads Service HeadquartersClarence Court10-18 Adelaide Street D O'HAGANBelfast BT2 8GB Assistant Chief Engineer (Works)

Documents

Concrete Highway Bridges-External prestressing