Post Tension: External Tendon

E X T E R N A L P O S T - T E N S I O N I N G

ContentsPreface.................................................................................................. 1

1. Introduction ............................................................................................ 21.1. Historical developments ....................................................................... 21.2. Areas of application today ..................................................................... 41.3. Types and components of external tendons ........................................ 5

General ................................................................................... 5Prestressing steel ..................................................................... 5Tendon anchorages .................................................................. 5Corrosion protection systems ................................................... 6Saddles at points of deviation .................................................. 7

1.4. Future developments .................................................................................. 7

2. Design Considerations for Bridges with External Tendons ................... 92.1. General.................................................................................................. 92.2. Serviceability and ultimate limit states................................................... 10

Serviceability limit state ........................................................... 10Ultimate limit state..................................................................... 10

2.3. Particular aspects ................................................................................. 11Saddles......................................................................................11Minimum tendon radii .............................................................. 11Prestress losses due to friction................................................ 11

3. VSL External Tendons .......................................................................... 123.1. Introduction.............................................................................................123.2. Types of VSL External Tendons and Technical Data ........................... 12

General .................................................................................... 12Selection criteria ...................................................................... 12Strands .................................................................................... 12Characteristic breaking loads of VSL External Tendons ......... 13Tubing ...................................................................................... 13Anchorages ............................................................................. 13Grouting compounds ............................................................... 13

3.3. Experimental evidence ........................................................................ 15

4. Application of the VSL External Tendons.............................................. 164.1. Manufacture and installation ................................................................ 16

Prefabrication .......................................................................... 16Fabrication in the final position ............................................... 16

4.2. Stressing .............................................................................................. 164.3. Grouting................................................................................................ 164.4. Completion work................................................................................... 16

5. VSL Service Range ...............................................................................175.1. General.................................................................................................. 175.2. Tender Preparation ............................................................................... 17

7. Bibliography and Reference ................................................................. 30

Copyright 1992 by VSL INTERNATIONAL LTD., Switzerland - All rights reserved - Printed in Hong Kong.

6. Examples from Practice ....................................................................... 186.1. Introduction ........................................................................................... 186.2 Bridges originally designed with external tendons ............................... 186.3. Other structures originally designed with external tendons.................. 256.4. Bridges with subsequently added external tendons.............................. 266.5. Other structures with subsequently added external tendons ................ 28


PrefaceThe purpose of this report is to discuss the principles and applications of external

post-tensioning and to present the VSL External Tendons. It should assist engineers in

making decisions regarding both design and construction. This document does not

represent a complete manual for detailed design and practical construction of structures

with external tendons. In this respect the reader is referred to the relevant technical

literature (see bibliography in Chapter 7). Furthermore, it must be mentioned that the

emphasis is clearly on the applications for bridges. Where appropriate, however,

reference is also made to other applications such as in buildings and circular structures.

There are many similarities between external tendons, stay tables and permanent

prestressed ground anchors. In fact, regarding many aspects there is hardly any

difference. Reference is therefore made to the report on VSL Stay Cables for Cable-

Stayed Bridges [1] and the VSL documentation on ground anchors (e.g. [2]).

The VSL Organizations will be pleased to assist and advise you on questions relating

to the use of external post-tensioning. The authors hope that the present report will help

in stimulating new and creative ideas. The VSL Representative in your country or VSL

INTERNATIONAL LTD., Berne, Switzerland, will be glad to provide you with further

information on the subject.

Authors

H.U. Aeberhard, Civil Engineer ETH

P. Buergi, Mechanical Engineer HTL

H.R. Ganz, Dr. sc. techn., Civil Engineer ETH

P. Marti, Dr. sc. techn., Civil Engineer ETH

P. Matt, Civil Engineer ETH

T. Sieber, Civil Engineer HTL.

1


1.1. Historical developments

The idea of actively compressingstructural elements with a high tensilematerial such as steel is very old.Everyone is familiar with timber barrelsand timber wheels stressed together bysteel hoops. In ancient Egypt, the sametechnique was used for shipbuilding.

In the history of modern engineering,Farber may first be mentioned. He wasgranted German patent DRP 557,331 in1927. In essence, this patent describes aprestressing system in which bond withthe surrounding concrete structure isprevented by covering the prestressingsteel with a bond-breaker such asparaffin. It is not known whether Farbersidea was actually applied in practice [3].

1. Introduction

2

In 1934, Dischinger was granted hispatent DRP 727,429 (Fig. 1). It containsthe innovative idea of post-tensioningreinforced concrete girders with externaltendons. For the determination of themagnitude of prestressing, he proposedthe concept of concordant prestressing,which later became known as the load-balancing method. Dischingers mainconcern was the long-term deformationdue to the time-dependent, visco-elasticbehaviour of the concrete. He was awareof the pioneering work of Freyssinet andhis classical experiments, carried out inthe years 1926 to 1929. While Freyssinetclearly recognized the nature of concretewith regard to creep and shrinkage [4], itwas Dischinger who first proposed a validmathematical model in 1939 [5]. Thus, inthe absence of a sound theory in the mid-thirties, it was quite logical for Dischingerto opt for external post-tensioning. Hewished to retain the possibility ofrestressing the tendons should undesir-able deflections occur. Furthermore,Dischinger specifically mentioned in hispublications the longer life of suchtendons resulting from the reducedinfluence of fatigue loadings and thesystem-inherent possibility of replacingtendons, even under traffic, should this berequired.

In the years 1936 and 1937, theseideas were applied in practice for the nowquite well known bridge crossing thevalley basin and the railway lines at Aue,Saxony, now the German DemocraticRepublic (Fig. 2, 3). For the main spans

(25.20 - 69.00 - 23.40 m), externaltendons consisting of smooth bars with ayield strength of 520 N/mm and a diame-ter of 70 mm were used [6]. Due to WorldWar II and its aftermath, the originallyplanned restressing operations were notperformed until 1962, together with otherrepair work [7]. In 1983, the original bartendons were again stressed [8]. Today,the bridge has been in service for morethan 50 years. Some years ago theGerman Democratic Republic listed thisremarkable structure as one of itstechnical monuments.

In the late thirties and early forties,Dischinger designed other road andrailway bridges with spans of up to 150 m.The construction of the Warthe Bridge inPosen (today Posnan, Poland) with spansof 55.35 - 80.50 - 55.35 m was stoppedbecause of the war. The external ten-dons, consisting of steel ropes 65 mm,were already on site. They were, how-ever, more urgently needed as externaltendons for post-tensioning the heavyreinforced concrete trusses carrying trav-eling cranes in a large steel mill [9].Dischinger also conceived compositebridges with external post-tensioning [10].

Based on Freyssinets ideas, Wayss &Freytag AG designed and constructed in1938 the bridge over the Dortmund-Hannover Autobahn in Oelde, FRG,where for the first time high tensileprestressing steel arranged inside theconcrete section was used for 4 simplysupported girders of 33 m span. The pre-tensioning method was applied and the

Figure 1: Dischingers patent DRP727,429

Figure 2: Bridge at Aue; externalprestressed bars of the drop-in span

Figure 3: First prestressed concrete bridge: Bridge over valley basin and railway at Aue,German Democratic Republic (designed by Dischinger)


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prestressing steel was therefore bondedto the structure [ll]. This was actually thefirst bridge in what is now called conven-tional prestressed concrete.

In the same year, Finsterwalderdeveloped his concept of the self-stressing concrete beam, which wasput to the actual test for the bridge overthe same Autobahn at Rheda-Wieden-brck, FRG (a simply supported girder of34.50 m span). The external bar tendons 65 mm with a yield strength of 520 N/mm were stressed by the self-weight ofthe superstructure using a hinge at mid-span and a precamber of 272.5 mm. Thebar tendons were later encased inconcrete [12].

In the years 1938 to 1943, Haggbohmdesigned and built the KlockestrandBridge (Fig. 5), near Stockholm, Sweden[13]. For the main spans (40.50 - 71.50 -40.50 m) the Dischinger concept was ap-plied. The main span superstructure wasprestressed with a total of 48 bars of 30mm having a yield strength of 520 N/mm .

It is worth mentioning that these fourbridges, of which three utilize externalpost-tensioning, are still in service todayafter more than 50 years of use.Why was external post-tensioningvirtually discarded in the succeedingyears?

Under the influence of Freyssinet andother prominent engineers, the advanta-geous characteristics of structures withbonded tendons were emphasized. Thesecharacteristics include the higher utiliza-tion of the prestressing steel underultimate bending moment (both withregard to achievable tendon eccentricities

and stress increase up to yield strength)and the free-of-charge corrosionprotection by the surrounding concrete.In 1949, Dischinger also was convertedand became an advocate of the bondedconcept. Despite this pronounced trend,external post-tensioning did not disappearcompletely. Several externally post-ten-sioned bridges were constructed inFrance [14], Belgium [15], Great Britainand a few other countries. Not all of theseprojects were successful. In some casesthe corrosion protection system chosendid not fulfill the required purpose andtendons had to be replaced.

Also during this period, the firstapplications of external tendons for thestrengthening of existing structures canbe found. An early example is the two-span steel truss bridge (48-48 m) over theRiver Aare at Aarwangen, Switzerland(Fig 4). This bridge was built in 1889 andwas no longer capable of supportingmodern traffic foads. In 1967 the bridgewas strengthened with two locked-coilstrands 63 mm having an ultimatestrength of 1,370 N/mm [16]. A rebirth of external post-tensioningcan be observed from the mid-seventiesonward. Freeman Fox and Partners

Figure 4: Bridge over River Aare at Aarwangen, Switzerland

Figure 5: Klockestrand Bridge near Stockholm, Sweden


designed the Exe and Exminster Viaductsin England (see para. 6.2.1.) usingexternal tendons each consisting of abundle of 19 greased and plastic-sheathed strands 13 mm. The primeobjective was to minimize the weight ofthe superstructure to overcome difficultsoil conditions.

The main developments certainly camefrom French engineers. In 1978/79 Mullerintroduced external post-tensioning in theUnited States, for the Key Bridges inFlorida [17]. His main goals were speed ofconstruction and economy. Many otherimportant structures followed [18]. Since1980, many bridges have been designedand built in France under the auspices ofVirlogeux of SETRA (State design officeof highway authority) using either externaltendons or a combination of internal andexternal tendons [19]. At present severalsizeable bridges using external post-tensioning are in planning and underconstruction in Switzerland and in theFederal Republic of Germany (see para.6.2.9.).

It is true that external post-tensioning isprimarily applied in bridges. There are,however, applications for other types ofstructures such as large-span roofs [20]and for the strengthening of buildings,silos and reservoirs [21].

1.2. Areas of applicationtoday

External post-tensioning can be usedfor new structures as well as for existingstructures needing strengthening. As Will

4

be shown, the application is by no meansrestricted to concrete structures. Anymaterial with reasonable compressioncharacteristics can be combined withexternal tendons. Thus, applications instructural steel, composite steel-concrete,timber and masonry structures are known.As mentioned earlier, the technique hasbeen used for various types of structuressuch as:

- Bridge superstructures

- Girders in buildings (see para. 6.5.2.)

- Roof structures (Fig. 6)

- Circular structures such as silos, reser-voirs and large masonry chimneys (seepara. 6.3.1. and 6.5.1. and [21], [22])

- Buildings with masonry walls (Fig. 7).

In the following chapters, the explana-tions are limited to the application ofexternal post-tensioning to bridges, whichat this stage represent the main field ofactivity.

In designing a new bridge superstruc-ture, a designer may opt for internal orexternal tendons or a combination ofboth. Whereas for many years internaltendons were selected almost exclusively,there are a number of good reasons fordeciding on external tendons.

It is interesting to note that some of thearguments used previously to promoteinternal, bonded tendons are nowweighed differently. There is no doubt thatmost bridges with internal, cementgrouted tendons behave very well. Insome cases, however, substandardconcrete (exhibiting high porosity,excessive carbonation etc.), missing ordeteriorated bridge deck insulation(allowing free attack by de-icingchemicals), badly cracked concrete(resulting from inadequate design,insufficient provision of minimum

reinforcement etc.) and incomplete fillingof the tendon ducts by cement grout etc.have contributed to the rapid degradationof the prestressing steel by corrosionattack. Since no reliable, non-destructiveinspection methods for internal tendonsyet exist, it is very difficult to properlyassess the degree of deterioration.Furthermore, internal tendons can neitherbe detensioned nor removed.

On the other hand, external tendonsprovide desirable features, such as thepossibility of controlling and adjusting thetendon forces, inspecting the corrosionprotection and replacing tendons, shouldthis become necessary. This is, however,possible only if the tendon systemtogether with its anchorages and saddlesis designed accordingly.

Other advantages of external post-tensioning include:

- The absence of tendons inside a webmeans that pouring of concrete is madeeasier; there is no weakening of thecompression area due to ducts. In thisway a minimum web thickness isachievable.

- A polygonal tendon layout allowsangular deviations to be concentratedat carefully designed saddle locations,thus eliminating the influence ofunintentional angular changes (wobbleeffect).

The difference in mechanical behaviourbetween internal, bonded and externaltendons is discussed in Chapter 2.

With regard to material quantities, itshould be mentioned that with externaltendons the concrete dimensions cannormally be reduced. However, due toreasons such as the reduction of theavailable tendon eccentricity, the amountof prestressing steel generally needs tobe slightly increased (Fig. 8).

Figure 7: Strengthening of a villadamaged by the Friuli earthquake in Italy,1987 [23]

Figure 6: Hangar at Belgrade International Airport, Yugoslavia; roof with external tendons


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1.3. Types and componentsof external tendons

1.3.1. GeneralA great variety of tendon types has

been used and is described in thetechnical literature. It is outside the scopeof this report to discuss all possibilities atlength.

Essentially, an external post-tensioningtendon consists of the following elements:

- prestressing steel as tensile members,

- mechanical end anchorage devices,

- corrosion protection systems.

In the case of deflected tendons:

- saddles at points of deviation are alsorequired.

1.3.2. Prestressing steelAt present, most material standards for

prestressing steel distinguish betweensmooth and ribbed bars, wires andstrands. No official worldwide statistics ofthe market shares are available. Unofficialfigures, however, suggest that today thetotal built-in tonnage of steel consists of75% strands, 15% wires and 10% bars.Whereas strands and wires can beapplied more or less universally, bars arenormally limited to short, preferablystraight tendons of up to 20 m length.Table I compares the variouscharacteristics of the most commonlyused prestressing steels (based on official approval documents from the FederalRepublic of Germany [24]). The figuresare self-explanatory. Furthermore, theexcellent groutability and the favourableton per force price of the strands may beemphasized. It therefore becomes clearwhy strands have taken such a largeshare. This trend is continuing. Usuallyseven-wire strands of 13 mm (0.5) or 15 mm (0.6) with low relaxationproperties (stabilized material) are used.

The question arises whether one dayprestressing steel will be replaced byother materials, such as glass, aramide orcarbon fibres. Despite the fact that inrecent years engineers and researchershave made serious efforts [25], [26], theauthors believe that for many years tocome the application of such materials willbe limited to demonstration researchprojects and applications with specialconditions. One of the major reasonspreventing a rapid transition to other

materials is the much higher costscompared with prestressing steel.

1.3.3. Tendon anchoragesUntil very recently, external tendons

were anchored with the same mechanicaldevices as those used for ordinaryinternal, bonded post-tensioning. Underthe prevailing economic circumstances,

under which the suitability of a structure isjudged primarily on the basis of initialconstruction costs, this seems to be thenormal choice.

From a technical point of view, itshould be remembered that the anchor-ages for external tendons must withstandthe tendon force plus any potentialsubsequent force increase during the

Figure 8: Comparison of box girder geometry with internal and external tendons

Bar 36 mmhot-rolled,

cold-workedand tempered,

ribbed

Wire 7mmcold-drawnstabilized

Strand 13 mm

cold-drawnstabilized

Ultimate tensile strength N/mm2

Yield strength N/mm2

Min. elongation at rupture %

Relaxation from 0.7 after1,000 at 20C %

Modulus of elasticity N/mm2

Fatigue amplitude (N/mm2) :2 . 106 load cyclesat max.upper stress of 0.9 min.

Min. diameter of curvature atmax. allowable stress of m

Friction coefficient

1,230

1,080

6

3.3

2.05 . 105

210210

6.83

0.50

1,670

1,470

6

2

2.05 . 105

430265

0.98

0.17

1,770

1,570

6

2

1.95 . 105

250205

0.85

0.19

Table I : Characteristics of prestressing steels (according to German approval documents)

P

fpkt

fpy

fpy


lifetime of a structure. For anchoragesmeeting the requirements of approvalregulations such as those published byFIP [27], the effectiveness is normallygiven.

As mentioned earlier, external tendonscan provide additional features such asthe possibilities of monitoring, adjustment,replacement etc. These are increasinglyattracting the interest of maintenance-conscious bridge authorities. Suchoperations are not possible with typical

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post-tensioning anchorages of bondedsystems. Specially designed devices aretherefore required and have beendeveloped accordingly (see Chapter 3).

1.3.4. Corrosion protectionsystems

It is known that prestressing steelneeds careful protection against thevarious types of corrosion attack. Forinternal, bonded prestressing thisprotection is provided by the alkalineenvironment of the cement grout and thesurrounding concrete. Experience hasshown, however, that there are severalaspects to which attention must be paid,in both design and construction, to makethe protection really effective.

In [28] a corrosion protecting strategyis proposed, which is summarized inTable II. It is in line with more recentrecommendations in various nationalstandards. In addition to the given design

measures, adequate materials and goodworkmanship are needed. From compari-sons with the practice of the past andexperience gained with existing struc-tures, it has been recognized for sometime that improvements are necessarywith regard to concrete quality, detailingand the amount of reinforcement.

For external tendons, other means of

protection are required. However, as forinternal tendons, it seems advisable toapply a corrosion protection strategywhich is based primarily on environmentalconditions and also safety considerations(e.g. with regard to fire, strand failure,etc.).

Many different solutions have beenadopted in the past [14]:

a) Z inc coating: Its corrosion resistancedepends upon the type of galvanizationand the applied thickness. Z inc coatedprestressing steel has been used inFrance on several occasions. There isdoubt, however, as to whether zinccoating provides a permanent corrosionprotection. It seems to be durable onlyunder very favourable environmentalconditions. As reported in [14], coatingshave been damaged during handlingand installation. Another problem arosewhen zinc accumulated in the stressinganchorage inside the wedges.

b) Polymer coating: This technology, inwhich polymers are bonded to the steelby fusion, has been developed in theUnited States primarily for the protec-tion of reinforcing steel. Polymer coated

strands have also been available for some time [29] and a number of applications are reported (see para. 6.5.2.). It remains to be seen whether this

Table II: Corrosion protection strategy for internai, bonded post-tensioning tendons, pre-tensioning and reinforcing steel(as proposed in [28])

Evironmental Environmentalclass conditions

1 Modest Structural elements alwaysdry or under water

2 Moderate Structural elements sub-ject to moist conditions

3 Severe Structural elements sub-ject to permanent humidconditions and / orchanging wetting anddrying conditions

4 Aggressive Structural elements sub-ject to aggressive condi-tions

Environ-mentalclass (seeabove)

Prestressing steel intension zone under

sustained loads

Post-tensioning

Pre-tensioning

1

Yes

Yes Yes

Yes

Yes

4

3

2

No

NoNo

No

No

Special protectionmeasures necessary

No

No

Post-tensioning

Pre-tensioning

Post-tensioning

Post-tensioningSheathing

Pre-tensioning

Pre-tensioning

Steel

Allowable design crack width (mm)under sustained loads

Prestressed concrete

Yes Yes

0.2

0.2

0.2

* *)

0.2* * * )

* *)

* * )

* *)

* * )

0.1

0.1* * * ) 0.25

0.25

0.4

0.40.1

0.1

Reinforcedconcrete

* *)

40*)

50*)

50

60 65

45

35

35

45

25

55

55

Concrete cover (nominal values in mm)

Prestressed concrete Reinforcedconcrete

*) Corrosion protection not relevant for cover of sheathing * * ) Not relevant for corrosion protection * * *) Under rare load combinations


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system will prove to be a viable solutionfor prestressing steel. Problems couldoccur due to the fact that only the outerstrand surface is protected, the kingwire and inner surfaces of the sixsurrounding wires having no coating. Atthe anchorages, the coating is locallyinterrupted by the indentations of thewedge teeth. It is also possible that, aswith zinc coated strands, problems mayoccur in the anchorages. Special caremust be taken to prevent damage tothe coating during handling andinstallation.

c) Protective sheathing: The protectivesheathing represents an envelopearound the prestressing steel. Suitablematerials are steel or plastic tubes(polypropylene [PP] or polyethylene[PE]). In order to achieve an effectiveprotection system, proper solutions arerequired for coupling these tubes witheach other, with the anchorages andwith the saddles.

Injection of the remaining voids insidethe sheathing with cernent grout hasproven to be economical and reliable. Inthe case of restressable anchorages,cernent grout must be replaced at leastfocally by grease or similar soft plasticmaterial. Particularly in France, greaseand wax products have been applied,instead of cement grout, on the entiretendon length [14], [30]. Besides beingrather expensive, these products aredifficult to inject (e.g. preheating up to100 C required) and special measuresare needed to prevent leakages (see alsopara. 6.2.5.).

In this category, individually greasedand plastic-sheathed monostrands offermany advantages. They are manufac-tured under factory conditions. Theprestressing steel is therefore effectivelyprotected against corrosion duringtransportation, storage on site andinstallation, provided that proper care istaken not to damage the sheathing.Monostrands can be used either indivi-dually or in bundles as multistrandtendons. In the latter configuration theyare usually placed inside a plastic or steeltube. The remaining voids are filled withcernent grout.

1.3.5. Saddles at points ofdeviation

When designing saddles it is importantto consider the following:

- Saddle arrangements: Various solutionshave been used in practice (see Fig. 9).In most cases saddles consist of a pre-bent steel tube cast into the surround-ing concrete or attached to a steelstructure by stiffening plates. Theconnection between the free tendonlength and the saddle must be carefullydetailed in order not to harm theprestressing steel by sharp angulardeviations during stressing and inservice; also, the protective sheathingmust be jointed properly.

If tendon replacement is a designrequirement, the saddle arrangementmust be chosen accordingly (e.g.double sheathing; see Alt. 3, Fig. 9).

- Minimum radii: L imits must be respect-ed because otherwise either theprestressing steel or the protectivesheathing could suffer. Although some tests exist indicating reasonable values,which may be used for preliminarydesigns, more research work isrequired in this respect. It is thereforeadvisable to verify the feasibility of aparticular practical solution by tests.

1.4. Future developments

The revival of external post-tensio-ning has been a stimulus for engineers.

Further innovation may be expected orindeed can already be seen on thehorizon. This will include progress inmaterials (e.g. corrosion protectionsystems), in design proedures, instructural concepts and in constructiontechnology.

The following few examples Willhighlight what could be expected:- Bridge superstructures with underlying

external tendons: It is not an entirelynovel idea to arrange external tendonsunderneath the bridge girder. Forexample, this concept has been usedfor steel bridges, such as the NeckarValley Bridge at Weitingen, Federal Re-public of Germany [31], (Fig. 10), andfor the Bridge Obere Argen, FederalRepublic of Germany [32], [33]. In bothcases, the extremely difficult local soilconditions led to such a design. In [34]Wittfoht proposes underlying externaltendons as a standard solution for boxgirder bridges for road or rail traffic.Menn describes a similar system forslab bridges, by which the feasiblespan range can be extended up toabout 40 m [35], (Fig. 11). A compre-hensive test programme for determiningthe structural behaviour has beencarried out at the Swiss FederalInstitute of Technology (ETH), Zurich,Switzerland [36] (Fig. 12).

Figure 9: Various saddle arrangements


New corrosion protection systems: Inrelated fields of application, newconcepts have already been imple-mented. It is known that corrosion doesnot occur in a dry environment (relativehumidity 40%). This fact has beenutilized in steel construction. Thedesigners of the suspension bridgeover the L ittle Belt (1970) and later onof the Far Bridges (1985), both inDenmark, introduced a dehumidificationand ventilation system for the interior ofthe large steel box sections, thusprotecting the inside surfaces againstcorrosion [37]. In Sweden, the SwedishState Power Board used a speciallydesigned dehumidification and ventila-tion system for all containment tendonsof Forsmark 1-3 Nuclear Power Sta-tions. It is obvious that the conditions ina well-attended power station are morefavourable than in an ordinary bridgestructure. In the future, however, furtherdevelopments in this direction may beexpected.

Monitoring of tendons: Externaltendons make possible monitoring ofthe tendon force and of the soundnessof the tendons. Refined techniques formonitoring the integrity of the corrosionprotection systems and for inspectingthe tendons are being developed.

8

Figure 11: Details of slab bridge with underlying external tendons Figure 12: Scale model

figure 10: Neckar Valley Bridge at Weitingen, Federal Republic of Germany

d


9

- Girder with unbonded tendons:Because relative longitudinal displace-ments between concrete and steel arenot prevented by bond, the tendonforce increases only due to deformationof the entire structural system. Similarto slabs with unbonded tendons [38],the tendon force increase dependsprimarily on the geometry and theoverall deformation of the structure aswell as the tendon profile.For long tendons and slender struc-tures this increase will be relativelysmall, even for large overall deforma-tions of the system. Therefore in Fig.13, the tendon force increase has beenneglected and hence the decompres-sion moment is equal to the ultimatemoment. Of course, friction at deviationpoints would somewhat improve the be-haviour, but unless intermediate an-chorages or partial bond at sufficientlyclosely spaced locations along thetendons and/or additional bondedreinforcement are provided, it shoutd berecognized that decompressionessentially means ultimate.At any rate, the strength of an exter-nally post-tensioned girder at one par-ticular section depends on the behavi-our of the entire structural system, or atleast parts of the system if intermediateanchorages are used.

Good crack distribution can only beobtained if the flexural resistance at asection exceeds the cracking moment.This principle is well known from minimumreinforcement requirements. For case a)of Fig. 13 this principle is only barely metand hence deformations may take placein just one or a few cracked sections.This may lead to an undesirable strainlocalization with a subsequent prematurefailure.

As a consequence of the describedbehaviour, externally post-tensionedstructures are inherently more sensitive tosecondary effects since, unlike bondedsystems, they do not have the capabilityto adapt to local overloads by localyielding. Hence, while a realistic assess-ment of secondary effects is not ofprimary importance for bonded systems,this is quite different for externally post-tensioned systems.

In practice, continuous bondedreinforcement and partial bond of externaltendons, at tendon deviations due tofriction or cernent grout will contribute toincreasing the ratio of flexural resistanceto cracking moment and thus result in amore forgiving behaviour of the structure.

Finally, it should be mentioned thatprior to grouting of the tendons a structurewith bonded tendons behaves similarly to

Figure 13: Moment-curvature curves for a typical bridge cross-section with bonded andwith unbonded prestressing

2. Design Considerations for Bridgeswith External Tendons

2.1.General

The purpose of this chapter is tohighlight some special aspects thatshould be considered in the design ofexternally post-tensioned bridge super-structures.

As for any design, it is normally at theconceptual stage that the fate of astructure with regard to economy anddurability is determined. A straightforwardstructural system, good detailing, and theearly integration of the construction proc-ess are the major elements of a success-ful design. In this respect, bridges withexternal tendons are no exception.

To obtain a satisfactory behaviour of astructure both under serviceability andultimate limit state conditions, it isessential to recognize the peculiarities ofgirders with external tendons. Fig. 13shows moment-curvature curves for atypical bridge cross-section with eitherbonded or unbonded prestressing. Forcomparison, a curve for non-prestressedbonded reinforcement is also shown. Thecross-sections of the reinforcement andprestressing were chosen such that allthree sections would reach the sameultimate moment. As can be seen fromFig. 13, there is no fundamental differ-ence between girders with bonded or withunbonded tendons below decompressionmoment. The section with unbondedtendons has a larger initial prestressingforce and, therefore, a higher decompres-sion moment than the section withbonded tendons. With regard to thefatigue behaviour, Dischinger [10] alreadymentioned the advantage that forunbonded tendons only negligible stressfluctuations occur in the prestressingsteel under live load.

A closer look at the behaviour of thesections is required following decompres-sion:- Girder with bonded tendons: After

decompression, the tendon forceincreases up to the yield strength. Thetendon force increase and the associ-ated increase of the internal lever armof the section provide a yield strengthconsiderably larger than the decom-pression moment of the section. Owingto the bond between concrete andsteef, the flexural behaviour at a sectionis more or less independent of adjacentgirder zones.


an externally post-tensioned system. Inone case a bridge failed during construc-tion [39] primarily because the specialconditions of the construction stage wereoverlooked. This failure clearly exhibitedthe effects of strain localization and pointsto the need for a careful evaluation of allpossible effects of loads and imposed de-formations when designing externallypost-tensioned systems.

2.2. Serviceability andultimate limit states

2.2.1. Serviceability limit stateUsually, the amount of prestressing is

selected at a relatively early stage in aproject. This selection is influenced byconsiderations regarding serviceabilityand economy of the structure:

- Under dead load only, the structureshall remain substantially uncracked orexisting cracks shall be closed. On theother hand, the requirement of anuncracked structure for dead and liveload inctuding secondary effects mightlead to undesirable long-term hoggingdeflections under dead load only.However, under such loading condi-tions cracked sections can normally beaccepted if the stresses in the rein-forcement are limited such that thecracks close again after removal of theload.

- From an economic point of view, oneshould ideally provide just enoughnon-prestressed reinforcement and re-stressing steel as necessary to obtainthe required resistance.

10

A reasonable prestressing force maybe estimated using the load balancingmethod. If a substantial part of the deadload is balanced by the prestressing, asatisfactory behaviour of the structuremay be expected regarding both deflec-tions and cracking.

As soon as the tendons and theirprofiles are selected, the tendon forcediagram can be determined. As externaltendons are generally arranged in apolygonal shape, the force diagram willhave steps at the deviation points. Fig. 14shows the forces applied to a deviationpoint.

Long-term losses due to relaxation ofthe prestressing steel, as well as creepand shrinkage of the concrete, cause adecrease in the tendon force. As long asno relative displacements betweentendon and concrete occur at the devia-tion points, either because of the pres-ence of high friction coefficients orbecause of partial bond, these lossesmay be estimated section by section asfor bonded tendons. However, for lowfriction coefficients there will be someslippage between tendon and concreteand the losses may be estimated frommean axial deformations due to creepand shrinkage of the entire structure.

As mentioned in Section 2.1. structureswith external tendons may be sensitive tosecondary effects. Therefore, it isessential to assess tendon forces andsecondary effects due to temperature,creep, shrinkage and other effects asrealistically as possible when performingchecks at serviceability limit states. Theeffects of prestressing may be consideredeither by the primary and secondarymoment method or by the load balancingmethod. The first method is generallyused for the final design of a structurebecause it allows for an easy considera-tion of friction losses. On the other hand,the second method is prlmarily suited forpreliminary designs if friction losses areneglected.

2.2.2. Ultimate Iimit stateAs mentioned in Section 2.1. the

tendon force increase in externally post-tensioned structures will generally berather small unless intermediate anchor-ages or partial bond at sufficiently closelyspaced locations are provided. Hence, forultimate timit state considerations onemay opt to neglect any possible tendonforce increase and use the tendon force

after all losses in order to get an estimateof the ultimate resistance. Alternatively, arigorous analysis can be performed by in-tegrating the strain increments in thestructure along the tendon axis (Fig. 15a),or an estimate of the tendon elongationcan be obtained from the consideration ofa rigid body mechanism (Fig. 15b).

Integration of the strain incrementsalong the tendon axis requires an iterativenon-linear analysis [14], [40]. For a givenload increment and an assumed tendonforce the strain increments at eachsection and the associated tendon forceincrease can be computed. Repeating thecalculations with the new value of thetendon force will result in an improvedestimate and after a few iterations areasonable approximation will be ob-tained. Similar computations can then bemade for the next load increment and soon.

Table III: Recommended minimum tendonradiiFigure 14: Forces at deviation points

Figure 15: Tendon elongation +L

Tendon size(VSL tendon unit)

up to 5-19 or 6-12up to 5-31 or 6-19up to 5-55 or 6-37

Minimum radius(m)

2.503.005.00


11

ment of secondary effects is usuallyessentiat for externatly post-tensionedsystems because of their inherentsensitivity to such effects (see Section2.1.). Hence, while a liberal attitude to-wards secondary effects due to imposeddeformations of any sort may be assumedwhen designing a bonded structure, amore cautious approach is necessary forexternally post-tensioned structures.

2.3. Particular aspects

2.3.1. SaddlesThe design and detailing of saddles at

points of tendon deviations is a delicatetask. An early coordination between thedesigner and the tendon supplier isadvisable. It is of utmost importance thatthe forces transferred at the saddlelocations are carefully evaluated. It isrecommended to use higher safetyfactors as the proper functioning of theseelements is essential for the entirestructure. Typical examples of saddlesare shown in Fig. 9 and 18.

2.3.2. Minimum tendon radiiMinimum tendon radii as recom-

mended in Table Ill must be respected inorder to avoid damage of the prestressingsteel and the plastic sheathings as well asthe outer tubing. It is also known thatfriction problems may occur if the tendonradii are too small.

2.3.3. Prestress losses due tofriction

Similarly to conventional prestressing,the force -friction retationship can bedescribed with the following formula:

P(X) = P 0 e -( +kx)

where

P(X) = Post-tensioning force at a distance xfrom the stressing anchorage

P 0= Post-tensioning force at the stressinganchorage

e = Base of Napierian logarithms

= Coefficient of friction

= Sum of angular deviations (in radians)of the tendon in ail planes over thedistance x

k = Wobble factor (inaccuracies inplacing) per unit length

However, the wobble factor k cannormally be neglected since the tendonsare straight between the points ofdeviation.

Based on test results, site experienceand the technical literature, the frictioncoefficient varies as follows:

- bare, dry strands over steelsaddle 0.25-0.30

- bare, greased strands oversteel saddle 0.20-0.25

- bare strands inside plastictube running over saddle 0.12-0.15

- greased and plastic-sheathedmonostrands inside plastictube over saddle 0.05-0.07

Figure 16: Influence of girder deflectionon tendon eccentricity

Figure 17: Ultimate resistance

Rigid body mechanism considerationshave been successfully applied indesigning post-tensioned slabs withunbonded tendons [41], [42], [43], [44].Typically, a nominal failure characterizedby a maximum deftection of about twopercent of the stab span is assumed andthe resulting elongation of the tendon isdetermined from geometry. Knowing theelongation and the stress-strain relation-ship of the tendon, the tendon stress increase and the tendon force can bedetermined. While this procedure is wellestabtished for slabs, some caution isrecommended for the application tobridge girders until more informationon maximum deflection values is avai-lable.

If the tendon force increase is takeninto account, second order effects asexemplified by Fig. 16 have to beconsidered as well. However, if anappropriate number of deviation points isprovided, the influence of such secondorder effects may be kept small.

Knowing the tendon force, the ultimateresistance of an externally post-tensionedstructure can be determined usingconventional methods. Similar to bondedstructures, an external tendon can eitherbe treated as part of the integral loadresisting system (Fig. 17a) or it may beconsidered to be separated from theconcrete and its action can be modelledby applying the equivalent anchorage,deviation and friction forces onto theconcrete (Fig 17b). However, in contrastto bonded structures, a realistic assess-

Figure 18: Examples of deviation points

P

P

PP


3.1. Introduction

The VSL External Tendons are inessence an adaptation of the well-provenVSL Post-Tensioning System [45] to therequirements for external tendons. Thus,they do not represent a completely newtechnology, but simply a further develop-ment of a technology relying on manyyears of practical experience.The main characteristic of VSLExternal Tendons is the use of strands forthe tensile members. As shown in para.

12

1.3.2., strands have a relatively highbreaking strength, which results in areduced consumption of steel. Anothermain advantage of VSL External Tendonsis the modular principle, which enablesany desired tendon size and tendonanchorage to be made up from standardunits. Thus, the construction principle isalways the same. On the other hand, thesystem is sufficiently flexible to allow foradaptation to any requirement. Thismeans, therefore, that the informationpresented in Chapters 3 and 4 is merelyrepresentative and does not constituteany limitation to the possible range.

3.2. Types of VSL ExternalTendons and Technical Data

3.2.1. GeneralThe VSL External Tendons consist ofthe following main elements (Fig. 19):

- a bundle of prestressing strands (eitherbare or individually greased and plastic-sheathed) as the tensile member,

- a plastic or steel tubing for the strandbundle,

- end and intermediate anchorages, andcouplers,

- a grouting compound.

VSL offers two main types of externaltendons (Fig. 20) which can be character-ized as follows:

Type 1: Bundle of strands inside asteel or plastic tube; the grouting com-pound normally consists of cement grout.

Type 2: Bundle of greased and plastic-sheathed monostrands inside a steel orplastic tube; the grouting compoundconsists of cement grout, except in theanchorage zones, where especially suitedcorrosion preventive compounds areused.

3.2.2. Selection criteriaThe main technical criteria for selecting

the tendon type are:

- Environmental conditions and tendonexposure: as for internal, bondedprestressing, for external tendons itseems logical to select the degree ofcorrosion protection according to theenvironmental conditions and theexposure of the tendons. Based on theclasses given in Table II (p. 6), it isrecommended to Use Type 1 forclasses 1, 2 and 3 and Type 2 for class4.

- Need for tendon force adjustmentduring lifetime of the structure: in thiscase Type 2 is recommended.

- Tendon friction during stressing opera-tion: as shown in para. 2.3.3., thefriction with tendon Type 2 is muchsmaller than with Type 1. In the case oflong tendons running over severalspans with sizeable angular changes,Type 2 offers technical and economicaladvantages.

Table IV summarizes the selectioncriteria. It should be mentioned that otherfactors may influence the decision suchas price, availability of materials, localpractice, etc.

3.2.3. StrandsFor VSL External Tendons, cold-drawn

7-wire prestressing strands of 13 mm(0.5) and 15 mm (0.6) normally of lowrelaxation quality are used. Thegeometrical and mechanical propertiesare given in Table V.

Whereas for tendon Type 1 barestrands are envisaged, greased andplastic-sheathed strands (monostrands)are used for Type 2. The grease hasfavourable characteristics with regard to

Figure 19: Composition of the VSL External Tendon (schematical)

Figure 20: Cross-sections of VSL External Tendons Type 1 and Type 2

3. VSL External Tendons


13

3.2.6. AnchoragesThe anchorage principle of VSL

Externat Tendons corresponds inessence to the VSL Post-TensioningSystem. Figures 21 to 29 represent avariety of possible anchorages. Differentparameters, such as required adjustabil-ity, replaceability, load monitoring,installation procedure, access to the endanchorages (e.g. for the strengthening ofstructures), static considerations andenvironmental conditions as per para.3.2.2., influence the selection of theparticular anchorage type.

The exposed surfaces of the anchor-ages are properly coated for corrosionprotection.

3.2.7. Grouting compoundsThe tubing around the strand bundle

constitutes its primary corrosion protec-

Table V: Strand types

its long-term stability and its suitability inproviding corrosion protection of theprestressing steel. The sheathing is ofpolyethylene (or alternatively polypro-pylene) and has a minimum thickness of1 mm for straight tables and 1.5 to 2 mmfor curved tables.

3.2.4. Characteristic breaking loadsof VSL External Tendons

Table VI gives the nominal breakingloads for the VSL External Tendonsaccording to the four strand types asdetailed in Table V. The characteristics ofthe strand may, however, slightly deviatefrom these values, depending on themanufacturer and applicable standard.

3.2.5 TubingThe strand bundle (consisting of either

bare or greased and plastic-coatedstrands) is usually encased in a plastictube. Alternatively, steel tubes may beused. In certain areas, such as deviationsaddles or parts of the tendon embeddedin concrete, regular corrugated steel ductas normally used for post-tensioningtables may be chosen. The latter,however, is only applicable when thetendon does not need to be replaceable(see also para. 3.2.6.), and tendons Type1 are used.

In general, the plastic material ispolyethylene and meets the requirementsof appropriate standards such as DIN8074 and 8075, ASTM D 1248 and 3035or equivalent. Alternatively, polypropylenemay be used. The ratio of internaldiameter to wall thickness is approxi-mately 16:1. In general carbon black isadded as ultraviolet stabilizer. Thismaterial is chemically inert againstpractically any foreseeable agent (seee.g. DIN 16934). It has shown excellentdurability behaviour in structural applica-tions.

In the case of steel tubes, a higherinternal diameter/wall thickness ratio canbe used (approx. 30:1 to 50:1). Thedimensions used are primarily dictated bythe availability of standardized tubes. Theouter surface of the tubing is normallyprovided with a paint giving sufficientcorrosion protection.

The plastic or steel tubing representsthe prime barrier against corrosive attack.It is connected to the anchorages and thesaddles, thus providing an effective andcontinuous envelope around theprestressing steel.

Table VI: Characteristic breaking loads

Strand type 13 mm (0.5)

(A)Euronorm

138-79Super

15 mm (0.6)

(B)ASTM

A 416-85Grade 270

(C)Euronorm

138-79Super

(D)ASTM

A 416-85Grade 270

Nominal diameter (mm)

Nominal steel area (mm2)

Nominal mass per m (kg)

Yield strength (N/mm2)

Ultimate strength (N/mm2)

Min. breaking load (kN)

12.9

100

0.785

1,580

1,860

186.0

12.7

98.7

0.775

1,670

1,860

183.7

15.7 15.2

150

1.18

1,500

1,770

265.0

140

1.10

1,670

1,860

260.7

13 mm (0.5) Strand

Cabletype

15 mm (0.6) Strand

Cabletype

Max.number of

strands

Strandtype C

1) 0.1 % proof load method 2) 1 % extension method

Environmental con-ditions (see Table II):class 1class 2class 3class 4

Need for tendon forceadjustement:noyes

Tendon frictionshorter tendon andsmall longer tendon andhigh

Type 1 Type 2

Table IV: Main technical criteria forselection of tendon type

Max.number of

strands

Strandtype A

Strandtype B

5-35-45-65-75-125-195-225-315-375-435-55

3467

12192231374355

558744

1,1161,3022,2323,5344,0925,7666,8827,998

10,230

Breaking load (kN)

Strandtype D

Breaking load (kN)

6-36-46-66-76-126-196-226-316-376-436-55

3467

12192231374355

7951,0601,5901,8553,1805,0355,8308,2159,805

11,39514,575

7821,0431,5641,8253,1284,9535,7358,0829,646

11,21014,339

551735

1,1021,2862,2043,4904,0415,6956,7977,899

10,104


tion. In addition, the tendon is completelyfilled with a grouting compound.

grout is used.

Asmentioned under 3.2.1.. normally cement

14

and fulfills the same requirementsas the one used in traditional post-tensioning. With its alkaline properties,it provides active corrosion protection.The grout completely fills the intersticesbetween the strand bundle and the outer

Due to the fact that the envelopereduces or eliminates the diffusion ofgases and liquids, carbonation of thecement grout is inhibited.

Figure 21: Anchorage Type Ed

The grout is made from Portland cement

Notes: *Bursting steel not shown for clarity.*Figures 26 to 29 have been omitted.

Features : Replaceable stressing or dead endanchorage where no adjustabilityand load monitoring is required.Available for 0.6" diameter barestrand tendons (Tendon Type 1).

tubing.

Features : Large-sized guide pipe enablingpush-through for trumpet/anchorhead assembly. Fully adjustable,detensionable and replaceableanchorage (Tendon Type 2).Ring nut can be used instead ofsplit shims. Can also be used withbare strands, if only load monitoring,small adjustements or replaceabilityis required.

Figure 22: Anchorage Type A

Figure 23: Anchorage Type CSd

Features : Replaceable stressing or dead endanchorage where no adjustabilityand load monitoring is required (Tendon Type 1)


15

by VSL (Fig.31). The first test aimed atdetermining the groutability of a bundle ofmonostrands, especially in the saddlearea. The second test involved stressingthe tendon in stages up to 70% of thebreaking load. To simulate actualconditions, a relative displacement of600 mm was applied. In the third test, the

3.3. Experimental evidence Several tests have been conducted

during the development of the VSLExtemal Tendons. These tests haveprovided valuable data for materialselection and procedures, anchoragedesign, and friction losses in saddles. Forthe Bois de Rosset Viaduct project(para.6.2.9.), four tests were performed

Figure 30: Intermediate tendon supports

Figure 31: Test installation

tendon was slightly stressed from bothanchorages prior to grouting. The tendonwas then stressed to 70% of the breakingload, again applying a 600 mm relativedisplacement. The fourth test was similarto the third, but incorporated the improve-ments obtained through the earlier tests.

Composition: Anchor block with wedges, retainer plate on passive side to securewedges, steel case. Tube for strand overlenght if detensionability or ad-justability required.

Features: For tendons with insuficient access for stressing at end anchorages(e.g. strenghtening of structure) or for circular tendons.

Figure 25: Centre-stressing anchorage Type Z

Figure 24: AnchorageType ECd

Feature: Replaceable stressing or dead endanchorage where no adjustabilityand load monitoring is required(Tendon Type 1)


4.1. Manufacture andinstallation

Basically there are two differentmethods used for the installation of VSLExternal Tendons

- Installation of completely prefabricatedtendons.

- Installation of the empty tube in the finalposition followed by insertion of thestrands.

4.1.1. PrefabricationThe method of complete tendon

prefabrication is usually applied for short,light tendons where easy access on siteallows the placing of the entire tendon.

Prefabrication may be carried outeither in a factory or in a prefabricationarea on site depending on the means oftransport, the time between manufactureand installation, and the availability ofadequate space on site. The standardlengths of tube are connected toachieve the required total length. PEtubes are connected by butt-welding,steel tubes by welding or with couplers.

16

The strands are inserted by pushing thetube over the prepared strand bundle orby pushing individual strands through thetube.

The bearing plates or anchoragebodies and the supports at the deviationpoints are fixed to the structure. Theprefabricated tendon is then placed intoits final position either manually or bymechanical means using hoists orwinches. Intermediate temporary fixingsalong the straight lengths are provided tokeep the tendon in its correct position.

4.1.2. Fabrication in the finalposition

Besides the fixing of bearing plates anddeviation points, it is necessary to providetemporary intermediate tendon supportsalong the straight length of the tendonprior to the placing of the tubes (Fig. 30).

The tube (steel or PE) is prepared insuitable sections and placed in its finalposition by attaching it to the previouslyfixed supports. The tube sections areconnected by welding or by usingcouplers. At the ends the tube is tightlyconnected to the anchorages.

When the tube is securely fixed in thefinal position it is ready to receive thestrands. The strands are inserted bypulling the prepared strand bundle (asone unit or in groups) through the tube bya winch. If the tendon consists of barestrands, the VSL push-through machinecan be used by taking the strands directlyfrom the coil and pushing them throughthe tube one by one (Fig. 32).

4.2. Stressing

The VSL External Tendons arestressed with the appropriate VSLmultistrand jack. All the strands arestressed simultaneously but individuallylocked off (Fig. 33). The stressingoperation normally follows the proceduresestablished by the specifications, by localcodes of practice or by the FIP recom-mendations.

Type 1 Tendons (bare strands) arestressed at a steady rate in one or severalincrements until the required stressingforce is reached. Grouting is carried outafter completion of the stressing opera-tion.

Type 2 Tendons (greased and plastic-coated monostrands) are stressed in twostages. In the first stage, an initial force isapplied which removes the slack in thetendon. Then the tendon is grouted. Afterthe grout has attained the requiredstrength, the stressing operation iscontinued. In the second stage, thestressing force is raised in uniform fashionto its final value.

Depending upon the anchorage typechosen, the tendon force can be checked,adjusted or released using the samemulttstrand stressing jack.

4.3. Grouting

The VSL anchorages incorporate agrout connection which can be used asinlet or as outlet. Further grout connec-tions are provided at the deviation points.

Grouting commences at the lower endof the tendon and proceeds at a steadyrate until grout of the same consistency isejected at the deviation points and finalyat the other end of the tendon. For longtendons, the grout is injected at subse-quent inlets along the tendon. Whenusing greased and plastic-sheathedstrands inside a steel or PE tube (tendonType 2), the tendon is grouted after initialtensioning by injecting cement grout intothe tubing only. The anchorage zones arefilled with a non-hardening corrosionpreventive compound.

4.4. Completion Work

The exposed surfaces of the anchor-ages are properly coated for corrosionprotection.

As a result of the simplicity of theconstruction principle, the high quality ofthe materials used and the excellentcorrosion protection, VSL ExternalTendons are virtually maintenance-free.

The use of anchorages type Am,(anchor head with thread) enables theattachment of a VSL Load Cell (Fig. 34),allowing the monitoring, checking andsmall adjustments of the tendon forceduring the whole life of the structure.Anchorages type As, and Ar, provide foradjusting and detensioning of the tendonforce and, if required, replacing of theentire tendon.

Figure 32: Pushing strand

Figure 33: Stressing a VSL tendon

4. Application of the VSL External Tendons


17

- Pamphlet Life Extension and Strength-ening of Structures

- Various Job Reports

- VSL Newsletters.

5.2. Tender Preparation

The basic requirements for a tender forone or more of the above services, in sofar as they concern the carrying out ofdetailed design, supply, installation andexecution, are detailed drawings andspecification documents. This appliesboth for structures which are about to beconstructed and also for Clients propos-als which require further technicaldevelopment, or to which alternativeproposals are to be prepared.

A VSL tender for external post-tensioning may consist of:

- Supply of material plus manufactureand complete installation of the cables,including provision of personnel andequipment, or

- Supply of material, provision of supervi-sory personnel and provision ofequipment.

5.1. General

The VSL Organizations provide acomprehensive range of services inconnection with externally post-tensionedstructures, including:

- Consulting services to owners, engi-neers and contractors

- Preliminary design studies

- Assistance with the design of externallypost-tensioned structures

- Detaifed design of external tendons

- Supply and installation of externaltendons

- Supply of materials, equipment, andsupervisory personnel

- The use of other VSL Systems, such asslipforming, soil and rock anchors,heavy lifting, bearings, expansion jointsetc.

The extent of VSLs services willusually be clarified in discussionsbetween the owner, engineer, contracter,and the VSL Organization.

In many cases the application ofseveral VSL Systems is possible on asingle project. This enables the use oflabour and material to be rationalizedwith consequent cost savings.

At this point, reference may be made toother VSL publications which are ofimportance in the construction of exter-nally post-tensioned structures:

- Pamphlet VSL Post-tensioning [45]

- Technical report Concrete StorageStructures [21]

- Technical report The IncrementalLaunching Method in PrestressedConcrete Bridge Construction

- Technical report The Free Cantilever-ing Method in Prestressed ConcreteBridge Construction.

In addition, the following VSL publica-tions are available that may also be ofinterest in connection with externally post-tensioned structures:

- Pamphlet VSL Slipforming

- Pamphlet VSL Soil and RockAnchors [2]

- Pamphlet VSL Heavy Lifting

- Pamphlet VSL Messtechnik/MeasuringTechnique/Technique de mesure

Figure 34 : Stressing of an external tendon equipped with a VSL Load Cell

The first solution, in most cases, willprove to be the better one and thereforeshould be selected as a rule. Theforemost reason is quality assurance. Thedurability over the lifetime of a structuremainly depends on quality of materialsand on quality of workmanship. Theexperience available with VSL, whosepersonnel is engaged exclusively on post-tensioning, is the most suitable for theeffective manufacture and installation oftendons. Another reason is economy. Aspecialist worker can achieve a better rateof progress both by his experience and bythe advanced type of equipment he has athis disposal. He will require less time tosolve unforeseen problems on site.

In addition, considerable savings arepossible if a Main Contractor investigatesjointly with VSL how best to use theavailable material, plant, equipment andmethods for a specific project. VSL has,for this purpose, built up its own designengineering staff. The combination ofengineering skill with detailed knowledgeof the possibilities and special features ofthe VSL Systems has proved to be anattractive service to Main Contractors foroptimizing their construction work.

5. VSL Service Range


6.1. Introduction

The purpose of this chapter is to givethe reader information about a number ofstructures in which VSL External Tendonshave been used, and at the same time toillustrate in what structures externaltendons are applicable. The descriptionsinclude concrete and steel bridges,buildings, silos and other structures. Thechapter is split into a first part presentingstructures that have been designed forthe use of external tendons from thebeginning, while the second part com-prises structures which had to bestrengthened by means of externaltendons.

The job reports show that externalpost-tensioning is applied in variouscountries; a certain predominance ofFrance and the USA cari,, however, beobserved because these countries are atthe forefront of the development of thistechnique. Wherever available, detailsregarding the design of the structures andthe reasons for the selection of externalpost-tensioning are also presented.In both parts, the projects are listedchronologically. The designs of the olderstructures may differ from what is outlinedin the previous chapters, but at the sametime they may make evident the improve-ments in the state of the art achieved overthe past years.

18

6.2. Bridges originallydesigned with externaltendons

6.2.1. Exe and Exminster Viaductsnear Exeter, Great BritainOwner Department of Transport,

South Western RoadConstruction Unit

Engineer Freeman Fox and Partners,London

Contractor Cementation ConstructionLtd., Croydon

Post- Losinger Systems Ltd.,tensioning ThameConstructionPeriod 1974-1976

These two viaducts are part of themotorway M5 linking Birmingham in theMidlands to Plymouth in the south. Not farfrom Exeter the two structures, which are

separated from each other by an embank-ment approx. 380 m long, cross the ExeValley. The Exe Viaduct spans the riverExe and the Exeter Canal, while theExminster Viaduc$ carries the motorwayacross a double-track railway line (Fig.35).

The Exe Viaduct has a length of 692 mand comprises eleven spans (53.50 - 9 x65.00 - 53.50 m). The Exminster is 302 mlong and has five spans (53.50 - 3 x 65.00- 53.50 m). The superstructures of bothbridges consist of two parallel, twin-cellbox girders of 7.00 m width and 2.80 mdepth, having 5.00 m cantilever wings oneach side. Diaphragms are provided in theboxes at 7.50 m and 10.00 m distances.

The viaducts were designed with theobjective of obtaining the lightest possiblestructure, as soil conditions were found tobe poor. Therefore, the dimensions of thesuperstructure were minimized and thepost-tensioning tables placed inside the

Figure 36: Construction procedure

6. Examples from practice

Figure 37: Tendon layout scheme

Figure 35: Exe and Exminster Viaducts

Stage 3

Stage 1

Stage 2

Span 1 Span 3Span 2


19

6.2.2. Seven Mile Bridge, Florida,USAOwner Florida Department of

Transportation, Tallahassee,Florida

Engineer Figg and Muller Engineers,Inc., Tallahassee, Florida

Contractor Misener Marine Construc-tion, St. Petersburg Beach,Florida

Erection ofsuperstruc-ture andpost- VSL Corporation,tensioning Los Gatos, CaliforniaConstructionPeriod 1979-1982

The Seven Mile Bridge, which leadsfrom south of Marathon to Little DuckKey, is the longest in the chain of roadstructures connecting the mainland ofFlorida to Key West. With a total length of10,931 m (35,863) it is also the longestconcrete box girder bridge in the world. Itconsists of 266 spans, most of themhaving the standard 41 .15 m (135) spanlength. The superstructure with its single-cell box section has a total width of11.89 m (39) and a constant depth of2.13 m (7).

The design especially aimed at speedof construction, in addition to economy.Thus some structural details are quiteunusual, even somewhat bold, withregard to concept and durability. Thesedetails are:

- No gluing material to bond or seal thejoints of the match-cast segments.Multiple keys only were provided totransfer shear forces.

- Segments stressed together withexternal tendons running inside the box

and connected to pier diaphragms anddeflector blocks.

- No overlay or wearing surface on thesegments; the traffic runs directly onthe precast concrete.

The segments of the superstructurewere cast in a yard in Tampa, Florida,approx. 400 km (250 miles) north of thebridge site. The five sets of forms allowedfor an average production rate of threespans (i.e. 24 segments) per week. Afterproper curing, the segments were bargedto site. There, the segments of a spanwere placed aboard a shuttle barge, thenwinched together, aligned to the requiredalignment and connected with four

boxes. The construction procedure waschosen accordingly. Construction of thesuperstructure was split into two phases.First the boxes were constructed onfalsework, while the cantilever slabs wereadded later, using movable shuttering.

The technique applied in the construc-tion of the boxes was as follows (Fig. 36):- Stage 1: Construction of span 1 plus

115th of span 2 by successivelyconcreting diaphragms, base slab andwebs, soffit slab.

- Stage 2: Installation and stressing ofthe first half of the span 1 tables, con-creting of the remainder of span 2.

- Stage 3: Moving of scaffolding andfrom span 1 to span 3 formwork,installation and stressing of tendonscovering spans 1 and 2, concreting ofspan 3.

- repetition of cycle.

The tendons were made up frommonostrands and are not encased in anyadditional sheathing between the dia-phragms. In each span, there are 16 VSLtendons type 6-19 dyform (breaking loadapprox. 5,700 kN each) having normalVSL anchorages type E at both ends. Thetendons caver two spans and overlap atthe piers. Thus the maximum tendonlength is approx. 170 m (Fig. 37). Profilingwas achieved by means of saddles castinto the diaphragms. The saddlesconsist of mild steel tubes of 110 mmoutside diameter welded into a steel box(Fig. 38).

The tendons were prefabricated in theworkshop and introduced into the boxthrough a hole in the deck slab. Tendonsnear the top of the box or inclinedtendons showed a considerable sagbetween diaphragms, due to their deadload. As this would have created prob-lems during stressing, intermediate propswere temporarily placed betweenneighbouring diaphragms.

Originally, stressing at both tendonends was required to compensate for thefriction losses, as in the design a frictioncoefficient u = 0.30 was adopted. Testson site, however, showed an effective ofbetween 0.05 and 0.10. A revisedcalculation with these values proved thatunilateral stressing was therefore accept-able for obtaining the required forces.Post-tensioning work started in June1975, and was completed in Cctober1976. The total quantity of strand incorpo-rated in both viaducts is 1,100 tonnes.

Figure 38: Saddle detail

temporary prestressing strands. The piersegment, accompanying the assembledspart, was also transferred to the shuttlebarge, which then moved beneath theerection truss (Fig. 39).

The truss was a very sophisticatedgantry combining innovative engineeringwith proven techniques and practicalexperience. VSL proposed to use such atruss as a variant to the constructionscheme given in the contract documents.The following are the main factors that ledVSL to alter that scheme:

- The segment alignment is taken off thecritical path. thus increasing speed ofconstruction and adding flexibility to theoperation.

- A cleaner division is achieved betweenthe work performed by the generalcontracter and by VSL.

Figure 39: Construction phase


- VSL can control all its operationsoverhead.

The truss, designed and operated byVSL, in a typical operation lifted the piersegment into place, then cantilevereditself forward to position its centre on thatnewly placed pier segment and finallyraised the lifting frame bearing thepreassembled span from the barge.Concrete blocks were then inserted intothe gap and the post-tensioning tendonsstressed to 15% of the ultimate force.After the closure concrete had reached astrength of 17.5 N/ mm (2,500 psi)overnight, the tendons were fullystressed.

VSL started erection of the first span ofthe bridge on May 30, 1980. On average,three spans were installed per week, witha maximum of six spans in a six daysingle shift period. The structure wascompleted in May 1982.Each span contains 4 VSL tendons5-27 (breaking load approx. 4,960 kNeach) and 2 of the unit 5-19 (Fig. 40). Thetendons are anchored in the pier dia-phragms by VSL stressing anchoragestype EC. In the five central segments of aspan, the tendons are deflected indeviation saddles. In the pier segment,semi-rigid duct was embedded to bringthe tendon to the anchorage, while ashort piece of galvanized pipe guides thetendons through the deviation saddles.Between these, the tendons are encasedin plastic pipes. Plastic pipe and duct orpipe are connected with rubber boots andhose clamps (Fig. 41). All tendons wereprefabricated and pulled in by hydraulicwinch; they were cement-grouted forcorrosion protection.

Transverse post-tensioning wasalso applied, in the deck slab of the piersegments only. The tables used areinternal, consist of four strands 13 mm(0.5) and are provided with VSL anch-orages.

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Additional details about the Seven MileBridge and its construction can be foundin [ 46] .

6.2.3. Chtelet Viaduct, Charleroi,BelgiumOwner I.A.C. (Intercommunale

Autoroute Charleroi),Charleroi

Engineer Office J. Rondas, BrusselsContractor Joint Venture Socol S.A.,

Brussels/ Ateliers de Con-struction Jambes-Namur;later Ateliers de Construc-tion Jambes-Namur/ SocitPieux Franki S.A., Lige

Post- Civielco B.V., Leiden,tensioning NetherlandsConstructionPeriod 1981-1982In order to take the transit traffic off thecity centre and to connect the industryzones at the periphery, an outer ring roadwas built around Charleroi, an industrialcentre in southern Belgium. In the suburbof Chtelet the ring road crosses thevalley of the river Sambre, which isdensely built-up and through which therailway line Paris-Cologne also passes.To cross the valley a 1,097 m longviaduct had to be constructed.The design adopted was put forward inthe tender stage as an alternative to thetender design. It comprises two independ-ent superstructures, each made up of two3.00 m deep steel girders carrying a16.00 m wide light-weight concrete deck.Most of the 25 spans, the lengths ofwhich vary between 34.98 and 58.60 m(except the main span which measures68.40 m), are simply supported beams.In view of the bad soil conditions, thelightest possible structure was sought. Forthis reason the steel girders are post-tensioned with external tables. At thebase of each girder, up to 6 VSL tendons

type EE 5-12 are installed, providingforces from 600 to 1,200 kN. The tendonsare encased in PE tubes and weregrouted after stressing (Fig. 42).As the Belgian standard in force at thattime did not contain any prescriptions forthis type of construction, full size modeltests were performed with tendons of theabove-mentioned unit in order to assessthe fatigue behaviour of the tendon itselfand especially of the anchorages. In thisway an anchorage design fulfilling the re-quirements was found. Another detailchecked was the tightness of the tendonsystem at every point, in particular at theanchorages and the saddles.The tables being practically straight,saddles were required only near theanchorages, for reasons of space. Thesesaddles consist of thick-walled steel tubeswelded to the web of the steel girder. ThePE tubes of the tendons are fitted into thesaddles and the joints tightly sealed.

Figure 40: Tendon layout

Figure 41: Detail of deviation saddle

Figure 42: Cross-section and detail withtendons


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designed for twin-track operation, is2.13 m (7) deep and has a deck width of9.22 m (30 -3).

Both structures are longitudinally post-tensioned with external VSL tendons 5-12to 5-27 which run in the interior of thebox. These are between 21.34 and 43.59m (70 and 143) long and have EC an-chorages at both ends. The strands wereplaced in polyethylene ducts, which weregrouted with cernent mortar after stress-ing.

Deviator blocks are provided in every3.05 m segment. In these and in the endblocks, the strands lie in steel pipes whichare connected to the polyethylene pipesby means of rubber hoses clamped onthe pipes (Fig. 45).Transversely, the deck was preten-sioned in the casting bed.

6.2.5. Loir Bridge, La Flche, FranceOwner Direction Dpartementale

de IEquipement de laSarthe, Le Mans

Engineer SETRA, Bagne/ BureaudEtudes Dragages etTravaux Publics, Paris

Contractor Dragages et TravauxPublics, Tours

Post- VSL France s.a.r.l.,tensioning Boulogne-BillancourtConstructionPeriod 1982-1983The construction of a by-pass road inthe town of La Flche made a new bridgeover the river Loir necessary. In view ofthe large areas of land liable to floodingand the bad soil conditions, a very lowroad level was adopted in order to avoidembankments. This fact, of course, hadalso a decisive influence on the design ofthe bridge.A three-span bridge with the smallestpossible depth of the superstructureobviously was the only solution corre-sponding to the given conditions. Thelength of the centre span was fixed at64 m, while the lateral spans were chosenat 26 m each. In order to equalize themasses between the lateral span and halfthe centre span, lightweight concrete wasselected for 59 m of the centre span whileadditional concrete was required in thelast 10 m of the side spans.For hydraulic reasons, the depth of thesingle-cell box girder superstructure hadto be limited to 2.80 m at piers; at mid-

6.2.4. MARTA Bridges, Atlanta,Ga., USAOwner Metropolitan Atlanta Rapid

Transit Authority (MARTA),Atlanta, Ga.

Engineer Figg and Muller Engineers,Inc., Tallahassee, Fla.

Contractor J. Rich Steers Inc.,New York, N.Y.

Post- VSL Corporation,tensioning Atlanta, Ga.ConstructionPeriod 1982-1983

The two bridges described below, onedesignated CS-360, the other one CN-480, are the first precast segmentalconcrete box girder railway bridges built inthe USA (Fig. 43,44). Originally the struc-tures should have been constructed of in-situ concrete; the successful contracter,however, took advantage of a value-engineering clause and had a redesignprepared which resulted in the leastamount of expenditure and saved time.Thus up to four spans were completedper week. Construction of both bridgesfasted for 64 weeks.

CS-360 is 1,594-10 m (5,230) long andhas spans of 21.34 to 30.48 m (70 to100), while CN-480 has a length of579.12m (1,900) and spans between22.86 and 43.59 m (75 and 143). Thesingle-cell box girder superstructure,

Figure 44: Prefabrication yard

span a depth of 1.75 m was adopted. Thedeck slab width is 10.75 m.Each half superstructure was con-structed parallel to the river with two 9 mlong form-works, which were advancedlike travellers of a free cantilever bridge.Thus segments of about 5 m length wereobtained. After completion, each half wasrotated to the final alignment (Fig. 46).Construction lasted from March 1982 toFebruary 1983.Post-tensioning consists of two familiesof tendons. For the quasi free cantilever-ing stages, internal tendons VSL type EC/EC 6-12 were chosen, one tendon wasanchored at the top of the web at eachsegment end. In the bottom slab of thecentre span 4 VSL tendons EC/ EC 6-12had to be placed, within the concretesection. In addition, 8 external tendons

Figure 46: One bridge half nearly completely rotated

Figure 43: General view of one bridgeunder construction

Figure 45: Detail of pipe connection at deviator block


VSL 6-19 run near each web. These areprovided with standard VSL anchoragestype EC (Fig. 47). The strands wereinstalled by means of the VSL Push-through Method. The sheathing of the ex-ternal tendons consists of steel tubes. Toallow for possible force monitoring or thereplacement of an external tendon, thesewere grouted with grease. This, however,proved to be a very expensive methoddue to the cost of the grease and of thesteel tubes.

The external tendons are deviated indiaphragms and cross-beams provided atvarious sections of the superstructure.Deviation is obtained by means of acurved piece of rigid steel tube which isconnected to the tendon sheathing by aconnecting sleeve welded to both tubes.

6.2.6. Bridge O.A. 33, Marseille,FranceOwner Direction Dpartementale

de IEquipement desBouches-du-Rhne,Marseille

Engineer Bureau dEtudes Dragageset Travaux Publics, Paris

Contractor Dragages et TravauxPublics, Marseille

Post- VSL France s.a r.l.,tensioning Boulogne-BillancourtConstructionPeriod 1983-1985

In Marseille, the motorway A55 leavesthe centre of the City northwards. Soon itcrosses an industrial and railway area.

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This is the location of the bridge desig-nated O.A. 33. The bridge consists of twosuperstructures each having three trafficlanes. The two structures have spans of23-29-43-2x40-43-38-20 m and 31.69-2x40.01 -38.61-2x43.02-33.01 -27.01 mrespectively. They are curved both in thehorizontal and the vertical plane. Thesingle-cell box girders have a depth of2.85 m, the width of each deck slab being14.42 m.

Both superstructures were built usingthe incremental launching method (Fig.

48) as this variation offered the lowestprice. Fabrication was carried out behindthe Marseille abutment which is thelowest point of the structure. The pierdiaphragms and internal diaphragms forthe external tables were constructed atthe same time as the respective incre-ments.In the tender a post-tensioning layout,partly or entirely external, could beproposed. For execution the followingthree groups of tables were selected (Fig.49):

Figure 49: Tendon scheme

Figure 48: Bridge O.A.33 under construction

Figure 47: Longitudinal section with external tendons


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Engineer Figg and Muller Engineers,Inc., Tallahassee, Fla.

Contractor Paschen Contractors, Inc.,Chicago, Ill.

Post- VSL Western,tensioning Campbell, Cal.ConstructionPeriod 1983-1987

The Sunshine Skyway Bridge replacesa section of the bridge structure leadingacross Tampa Bay between St. Peters-burg and Sarasota. The replacementbecame necessary, as in May 1980 atanker veered out of the navigationchannel and rammed one of the mainpiers, thus sending 400 m (1,300) ofbridge into the water. In October 1982,the owner opened bids for a replacementbridge.

The winning design for the high-leveland main approach spans consisted of aconcrete box girder structure with a365.76 m (1,200) table-stayed mainspan. It should be noted that all stays areVSL Stay Cables System 200 with up to82 strands of 15 mm (0.6). The rebuiltcrossing was opened to traffic in April1987 (Fig. 51).

The table-stayed part with spans of164.59-365.76-164.59 m (540 -1,200 -540) is adjoined on each side by threespans each of 73.15 m (240) and onespan of 42.67 m (140). The superstruc-ture of this main part consists of a 4.47 m(14 -8) deep single-cell box girder withsteeply inclined webs. The deck slab is28.78 m (94 -5) wide. Along the centralaxis it is supported in the box at intervals

- Permanent tendons which werestressed before a new increment wasjacked forwards. These tables arepolygonal and parabolic and are withinthe concrete section.

- Temporary tendons that were stressedbefore jacking and afterwards weredestressed and removed. Aftercompletion of the first superstructure,these temporary tendons were reusedin the second one. All of these tendonswere external. Some were straight,while others followed a polygonalprofile to give, in conjunction with thefirst group, a central prestress.

- Permanent tendons which werestressed after the increments werejacked forwards. They are all externaland either straight or polygonal inlayout.All tendons are of the VSL unit 6-12

(breaking force 3,024 kN each) and havestressing anchorages type EC at bothends. The temporary tendons and thefinal external tendons were placed inpolyethylene tubes, while steel pipeswere used for the final internal tendons(Fig. 50). All final internal tendons weregrouted with cement grout.

The external tendons are deviated inconcrete frames provided in the boxes,into which curved steel tubes are placed.A short piece of PE tube is placed on theends of the steel tubes and fixed in theconcrete. To this the PE tube of thetendon is joined by means of a jointwelded on both ends of the PE tubes.

6.2.7. Sunshine Skyway Bridgeacross Tampa Bay, Florida, USAOwner Florida Department of

Transportation, Tallahassee,Fla.

Figure 51: Sunshine Skyway Bridge

of 3.66 m (12) by inclined struts ending atthe intersection point between web andbottom slab. The main structure isfollowed on each side by 18 spans of41 .15 m (135) standard length twinsingle-cell box girders (Fig. 52).

All box-sectional parts were made upfrom precast segments joined togetherwith an adhesive, some with in-situconcrete. In the main structure, thesegments were hoisted from barges andplaced at alternate ends in the freecantilevering manner. In the 41.15 m(135) spans, however, the erection trussfrom the Seven Mile Bridge (see para.6.2.2.) was reused after adaptation andcomplete spans were placed by thecontracter, who had bought the truss fromVSL.

The superstructure is post-tensionedlongitudinally and partially transversely.The latter tables consist of VSL tendonsSO/ SO 6-4 placed in the concrete sectionin flat corrugated ducts of high-densitypolyethylene. Longitudinal post-tensioningin the free cantilevered part consists oftendons within the concrete section

Figure 52: Cross-section of twin, singlebox girder structure

Figure 50: Tendons inside the box


installed during free cantilevering whilethe continuity cables run inside the box.These are VSL units 5-17 and 5-27, up to238 m (781) long. In the 41.15 m (135)spans, VSL tendons 5-19 to 5-27 arearranged inside the box similarly to thecables of Seven Mile Bridge (see para.6.2.2.). Anchorages used were standardEC type. After stressing (double endstressing for longer tendons) the cableswere cement-grouted.

6.2.8. High Bridge, St. Paul, Mn.,USA

Owner State of Minnesota,St. Paul, Mn.

Engineer Strgar-Roscoe, Inc.,Wayzata, Mn./T.Y. Lin International,San Francisco, Cal.

Contractor Lunda Construction, blackRiver Falls, Wi.

Post- VSL Corporation,tensioning Burnsville, Mn.ConstructionPeriod 1985-1987

The new High Bridge, which replaces astructure that was built in 1889, is the firstbridge in the USA to use a combinationof table and steel tension-tie design for adeck-tied arch bridge. It is 839.72 m(2,755) in length with a width varyingfrom 20.02 to 27.13 m (65 -8 to 89). It

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has two river piers. The height above theriver varies from 24.38 m (80) at the northend to 58.22 m (191) at the south end,making it one of the worlds

Post Tension: External Tendon

Documents

types of vsl external

components of external

p o s t t e n s i o

e x t e r n

n gcontentspreface

serviceability limit

ultimate limit states

tendon anchorages