2. Design Considerations for Bridges with External Tendons

E XTERNAL P OST-TENSIONING

Contents Preface...... 1 1. Introduction ...... 2 1.1. Historical developments ...... 2 1.2. Areas of application today ...... 4 1.3. Types and components of external tendons ...... 5 General ...... 5 Prestressing steel ...... 5 Tendon anchorages ...... 5 Corrosion protection systems ...... 6 Saddles at points of deviation ...... 7 1.4. Future developments ...... 7

2. Design Considerations for Bridges with External Tendons ...... 9 2.1. General...... 9 2.2. Serviceability and ultimate limit states...... 10 Serviceability limit state ...... 10 Ultimate limit state...... 10 2.3. Particular aspects ...... 11 Saddles...... 11 Minimum tendon radii ...... 11 Prestress losses due to friction...... 11

3. VSL External Tendons ...... 12 3.1. Introduction...... 12 3.2. Types of VSL External Tendons and Technical Data ...... 12 General ...... 12 Selection criteria ...... 12 Strands ...... 12 Characteristic breaking loads of VSL External Tendons ...... 13 Tubing ...... 13 Anchorages ...... 13 Grouting compounds ...... 13 3.3. Experimental evidence ...... 15

4. Application of the VSL External Tendons...... 16 4.1. Manufacture and installation ...... 16 Prefabrication ...... 16 Fabrication in the final position ...... 16 4.2. Stressing ...... 16 4.3. Grouting...... 16 4.4. Completion work...... 16

5. VSL Service Range ...... 17 5.1. General...... 17 5.2. Tender Preparation ...... 17

6. Examples from Practice ...... 18 6.1. Introduction ...... 18 6.2 Bridges originally designed with external tendons ...... 18 6.3. Other structures originally designed with external tendons...... 25 6.4. Bridges with subsequently added external tendons...... 26 6.5. Other structures with subsequently added external tendons ...... 28

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

Preface

The 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 E XTERNAL P OST-TENSIONING

1. Introduction

1.1. Historical developments In 1934, Dischinger was granted his (25.20 - 69.00 - 23.40 m), external patent DRP 727,429 (Fig. 1). It contains tendons consisting of smooth bars with a The idea of actively compressing the innovative idea of post-tensioning yield strength of 520 N/mm² and a diame- structural elements with a high tensile reinforced concrete girders with external ter of 70 mm were used [6]. Due to World material such as steel is very old. tendons. For the determination of the War II and its aftermath, the originally Everyone is familiar with timber barrels magnitude of prestressing, he proposed planned restressing operations were not and timber wheels stressed together by the concept of concordant prestressing, performed until 1962, together with other steel hoops. In ancient Egypt, the same which later became known as the “load- repair work [7]. In 1983, the original bar technique was used for shipbuilding. balancing” method. Dischinger’s main tendons were again stressed [8]. Today, In the history of modern engineering, concern was the long-term deformation the bridge has been in service for more Farber may first be mentioned. He was due to the time-dependent, visco-elastic than 50 years. Some years ago the granted German patent DRP 557,331 in behaviour of the concrete. He was aware German Democratic Republic listed this 1927. In essence, this patent describes a of the pioneering work of Freyssinet and remarkable structure as one of its prestressing system in which bond with his classical experiments, carried out in technical monuments. the surrounding concrete structure is the years 1926 to 1929. While Freyssinet In the late thirties and early forties, prevented by covering the prestressing clearly recognized the nature of concrete Dischinger designed other road and steel with a bond-breaker such as with regard to creep and shrinkage [4], it railway bridges with spans of up to 150 m. paraffin. It is not known whether Farber’s was Dischinger who first proposed a valid The construction of the Warthe Bridge in idea was actually applied in practice [3]. mathematical model in 1939 [5]. Thus, in Posen (today Posnan, Poland) with spans the absence of a sound theory in the mid- of 55.35 - 80.50 - 55.35 m was stopped thirties, it was quite logical for Dischinger because of the war. The external ten- to opt for external post-tensioning. He dons, consisting of steel ropes  65 mm, wished to retain the possibility of were already on site. They were, how- restressing the tendons should undesir- ever, more urgently needed as external able deflections occur. Furthermore, tendons for post-tensioning the heavy Dischinger specifically mentioned in his reinforced concrete trusses carrying trav- publications the longer life of such eling cranes in a large steel mill [9]. tendons resulting from the reduced Dischinger also conceived composite influence of fatigue loadings and the bridges with external post-tensioning [10]. system-inherent possibility of replacing Based on Freyssinet’s ideas, Wayss & tendons, even under traffic, should this be Freytag AG designed and constructed in required. 1938 the bridge over the Dortmund- In the years 1936 and 1937, these Hannover Autobahn in Oelde, FRG, ideas were applied in practice for the now where for the first time high tensile quite well known bridge crossing the prestressing steel arranged inside the valley basin and the railway lines at Aue, concrete section was used for 4 simply Saxony, now the German Democratic supported girders of 33 m span. The pre- Republic (Fig. 2, 3). For the main spans tensioning method was applied and the

Figure 1: Dischinger’s patent DRP 727,429

Figure 2: Bridge at Aue; external Figure 3: First prestressed concrete bridge: Bridge over valley basin and railway at Aue, prestressed bars of the drop-in span German Democratic Republic (designed by Dischinger)

2 E XTERNAL P OST-TENSIONING

Figure 4: Bridge over River Aare at Aarwangen, Switzerland

prestressing steel was therefore bonded and stress increase up to yield strength) Also during this period, the first to the structure [ll]. This was actually the and the “free-of-charge ” corrosion applications of external tendons for the first bridge in what is now called conven- protection by the surrounding concrete. strengthening of existing structures can tional prestressed concrete. In 1949, Dischinger also was “converted” be found. An early example is the two- In the same year, Finsterwalder and became an advocate of the bonded span steel truss bridge (48-48 m) over the developed his concept of the “self- concept. Despite this pronounced trend, River Aare at Aarwangen, Switzerland stressing ” concrete beam, which was external post-tensioning did not disappear (Fig 4). This bridge was built in 1889 and put to the actual test for the bridge over completely. Several externally post-ten- was no longer capable of supporting the same Autobahn at Rheda-Wieden- sioned bridges were constructed in modern traffic foads. In 1967 the bridge brück, FRG (a simply supported girder of France [14], Belgium [15], Great Britain was strengthened with two locked-coil 34.50 m span). The external bar tendons and a few other countries. Not all of these strands  63 mm having an ultimate  65 mm with a yield strength of 520 N/ projects were successful. In some cases strength of 1,370 N/mm ² [16]. mm² were stressed by the self-weight of the corrosion protection system chosen A rebirth of external post-tensioning the superstructure using a hinge at mid- did not fulfill the required purpose and can be observed from the mid-seventies span and a precamber of 272.5 mm. The tendons had to be replaced. onward. Freeman Fox and Partners bar tendons were later encased in concrete [12]. In the years 1938 to 1943, Haggbohm designed and built the Klockestrand Bridge (Fig. 5), near Stockholm, Sweden [13]. For the main spans (40.50 - 71.50 - 40.50 m) the Dischinger concept was applied. The main span superstructure was prestressed with a total of 48 bars of  30 mm having a yield strength of 520 N/mm ² .

It is worth mentioning that these four bridges, of which three utilize external post-tensioning, are still in service today after more than 50 years of use. Why was external post-tensioning virtually discarded in the succeeding years? Under the influence of Freyssinet and other prominent engineers, the advanta- geous characteristics of structures with bonded tendons were emphasized. These characteristics include the higher utiliza- tion of the prestressing steel under ultimate bending moment (both with regard to achievable tendon eccentricities Figure 5: Klockestrand Bridge near Stockholm, Sweden

3 E XTERNAL P OST-TENSIONING

designed the Exe and Exminster Viaducts in England (see para. 6.2.1.) using external tendons each consisting of a bundle of 19 greased and plastic- sheathed strands  13 mm. The prime objective was to minimize the weight of the superstructure to overcome difficult soil conditions. The main developments certainly came from French engineers. In 1978/79 Muller introduced external post-tensioning in the United States, for the Key Bridges in Florida [17]. His main goals were speed of construction and economy. Many other important structures followed [18]. Since Figure 6: Hangar at Belgrade International Airport, Yugoslavia; roof with external tendons 1980, many bridges have been designed and built in France under the auspices of be shown, the application is by no means reinforcement etc.) and incomplete filling Virlogeux of SETRA (State design office restricted to concrete structures. Any of the tendon ducts by cement grout etc. of highway authority) using either external material with reasonable compression have contributed to the rapid degradation tendons or a combination of internal and characteristics can be combined with of the prestressing steel by corrosion external tendons [19]. At present several external tendons. Thus, applications in attack. Since no reliable, non-destructive sizeable bridges using external post- structural steel, composite steel-concrete, inspection methods for internal tendons tensioning are in planning and under timber and masonry structures are known. yet exist, it is very difficult to properly construction in Switzerland and in the As mentioned earlier, the technique has assess the degree of deterioration. Federal Republic of Germany (see para. been used for various types of structures Furthermore, internal tendons can neither 6.2.9.). such as: be detensioned nor removed. It is true that external post-tensioning is On the other hand, external tendons primarily applied in bridges. There are, - Bridge superstructures provide desirable features, such as the however, applications for other types of - Girders in buildings (see para. 6.5.2.) possibility of controlling and adjusting the structures such as large-span roofs [20] tendon forces, inspecting the corrosion and for the strengthening of buildings, - Roof structures (Fig. 6) protection and replacing tendons, should silos and reservoirs [21]. - Circular structures such as silos, reser- this become necessary. This is, however, possible only if the tendon system 1.2. Areas of application voirs and large masonry chimneys (see para. 6.3.1. and 6.5.1. and [21], [22]) together with its anchorages and saddles today is designed accordingly. - Buildings with masonry walls (Fig. 7). Other advantages of external post- External post-tensioning can be used tensioning include: for new structures as well as for existing In the following chapters, the explana- structures needing strengthening. As Will tions are limited to the application of - The absence of tendons inside a web external post-tensioning to bridges, which means that pouring of concrete is made at this stage represent the main field of easier; there is no weakening of the activity. compression area due to ducts. In this In designing a new bridge superstruc- way a minimum web thickness is ture, a designer may opt for internal or achievable. external tendons or a combination of both. Whereas for many years internal - A polygonal tendon layout allows tendons were selected almost exclusively, angular deviations to be concentrated there are a number of good reasons for at carefully designed saddle locations, deciding on external tendons. thus eliminating the influence of It is interesting to note that some of the unintentional angular changes (wobble arguments used previously to promote effect). internal, bonded tendons are now The difference in mechanical behaviour weighed differently. There is no doubt that between internal, bonded and external most bridges with internal, cement tendons is discussed in Chapter 2. grouted tendons behave very well. In With regard to material quantities, it some cases, however, substandard should be mentioned that with external concrete (exhibiting high porosity, tendons the concrete dimensions can excessive carbonation etc.), missing or Figure 7: Strengthening of a villa normally be reduced. However, due to deteriorated bridge deck insulation reasons such as the reduction of the damaged by the Friuli earthquake in Italy, (allowing free attack by de-icing 1987 [23] available tendon eccentricity, the amount chemicals), badly cracked concrete of prestressing steel generally needs to (resulting from inadequate design, be slightly increased (Fig. 8). insufficient provision of minimum

4 E XTERNAL P OST-TENSIONING

1.3. Types and components materials is the much higher costs under which the suitability of a structure is of external tendons compared with prestressing steel. judged primarily on the basis of initial construction costs, this seems to be the 1.3.3. Tendon anchorages normal choice. 1.3.1. General Until very recently, external tendons From a technical point of view, it A great variety of tendon types has were anchored with the same mechanical should be remembered that the anchor- been used and is described in the devices as those used for ordinary ages for external tendons must withstand technical literature. It is outside the scope internal, bonded post-tensioning. Under the tendon force plus any potential of this report to discuss all possibilities at the prevailing economic circumstances, subsequent force increase during the length. Essentially, an external post-tensioning tendon 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 also required.

1.3.2. Prestressing steel At present, most material standards for prestressing steel distinguish between smooth and ribbed bars, wires and strands. No official worldwide statistics of the market shares are available. Unofficial figures, however, suggest that today the total built-in tonnage of steel consists of 75% strands, 15% wires and 10% bars. Whereas strands and wires can be applied more or less universally, bars are Figure 8: Comparison of box girder geometry with internal and external tendons normally limited to short, preferably Bar Ø 36 mm Wire Ø 7mm Strand straight tendons of up to 20 m length. hot-rolled, cold-drawn Ø 13 mm Table I compares the various cold-worked stabilized cold-drawn characteristics of the most commonly and tempered, stabilized used prestressing steels (based on official ribbed approval documents from the Federal Republic of Germany [24]). The figures Ultimate tensile strength N/mm2 1,230 1,670 1,770 are self-explanatory. Furthermore, the excellent groutability and the favourable Yield strength N/mm2 1,080 1,470 1,570 ton per force price of the strands may be emphasized. It therefore becomes clear Min. elongation at rupture % 6 6 6 why strands have taken such a large share. This trend is continuing. Usually Relaxation from 0.7fpkt after 3.3 2 2 seven-wire strands of  13 mm (0.5”) or 1,000 at 20°C %  15 mm (0.6”) with low relaxation properties (stabilized material) are used. Modulus of elasticity N/mm2 2.05 . 105 2.05 . 105 1.95 . 105 The question arises whether one day prestressing steel will be replaced by Fatigue amplitude (N/mm2) : other materials, such as glass, aramide or 2 . 106 load cyclesat max. 210 430 250 carbon fibres. Despite the fact that in upper stress of 0.9fpy min. 210 265 205 recent years engineers and researchers have made serious efforts [25], [26], the Min. diameter of curvature at authors believe that for many years to max. allowable stress of f py m 6.83 0.98 0.85 come the application of such materials will be limited to demonstration research Friction coefficient  0.50 0.17 0.19 projects and applications with special conditions. One of the major reasons Table I : Characteristics of prestressing steels (according to German approval documents) preventing a rapid transition to other

5 E XTERNAL P OST-TENSIONING

lifetime of a structure. For anchorages post-tensioning anchorages of bonded protection are required. However, as for meeting the requirements of approval systems. Specially designed devices are internal tendons, it seems advisable to regulations such as those published by therefore required and have been apply a corrosion protection strategy FIP [27], the effectiveness is normally developed accordingly (see Chapter 3). which is based primarily on environmental given. conditions and also safety considerations As mentioned earlier, external tendons 1.3.4. Corrosion protection (e.g. with regard to fire, strand failure, can provide additional features such as systems etc.). the possibilities of monitoring, adjustment, It is known that prestressing steel Many different solutions have been replacement etc. These are increasingly needs careful protection against the adopted in the past [14]: attracting the interest of maintenance- various types of corrosion attack. For conscious bridge authorities. Such internal, bonded prestressing this a) Z inc coating: Its corrosion resistance operations are not possible with typical protection is provided by the alkaline depends upon the type of galvanization environment of the cement grout and the and the applied thickness. Z inc coated surrounding concrete. Experience has prestressing steel has been used in Evironmental Environmental shown, however, that there are several France on several occasions. There is class conditions aspects to which attention must be paid, doubt, however, as to whether zinc in both design and construction, to make coating provides a permanent corrosion the protection really effective. protection. It seems to be durable only 1 Modest Structural elements always In [28] a corrosion protecting strategy under very favourable environmental dry or under water is proposed, which is summarized in conditions. As reported in [14], coatings Table II. It is in line with more recent have been damaged during handling 2 Moderate Structural elements sub- recommendations in various national and installation. Another problem arose ject to moist conditions standards. In addition to the given design when zinc accumulated in the stressing anchorage inside the wedges. 3 Severe Structural elements sub- measures, adequate materials and good ject to permanent humid workmanship are needed. From compari- b) Polymer coating: This technology, in conditions and / or sons with the practice of the past and which polymers are bonded to the steel changing wetting and experience gained with existing struc- by fusion, has been developed in the drying conditions tures, it has been recognized for some United States primarily for the protec- time that improvements are necessary tion of reinforcing steel. Polymer coated 4 Aggressive Structural elements sub- with regard to concrete quality, detailing strands have also been available for ject to aggressive condi- and the amount of reinforcement. some time [29] and a number of appli- tions For external tendons, other means of cations are reported (see para. 6.5.2.). It remains to be seen whether this

Environ- Prestressing steel in Special protection Allowable design crack width (mm) Concrete cover (nominal values in mm) mental tension zone under measures necessary under sustained loads class (see sustained loads above) Reinforced Prestressed concrete Reinforced Prestressed concrete concrete concrete Post- Pre- tensioning tensioning Post- Pre- Post- Pre- Post- Pre- Sheathing Steel tensioning tensioning tensioning tensioning tensioning tensioning

1 Yes No 0.2 0.1 0.4 40*) 35 25

2 Yes No 0.2 0.1 0.4 50*) 45 35

Yes Yes **) **) 3 Yes No 0.2 0.25 50 45 No No 0.1 55

Yes Yes ) ) **) **) ** ** 4 No No 0.2***) 0.1***) 0.25 60 65 55

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

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

6 E XTERNAL P OST-TENSIONING

system will prove to be a viable solution - Saddle arrangements: Various solutions Further innovation may be expected or for prestressing steel. Problems could have been used in practice (see Fig. 9). indeed can already be seen on the occur due to the fact that only the outer In most cases saddles consist of a pre- horizon. This will include progress in strand surface is protected, the king bent steel tube cast into the surround- materials (e.g. corrosion protection wire and inner surfaces of the six ing concrete or attached to a steel systems), in design proçedures, in surrounding wires having no coating. At structure by stiffening plates. The structural concepts and in construction the anchorages, the coating is locally connection between the free tendon technology. interrupted by the indentations of the length and the saddle must be carefully The following few examples Will wedge teeth. It is also possible that, as detailed in order not to harm the highlight what could be expected: with zinc coated strands, problems may prestressing steel by sharp angular - Bridge superstructures with underlying occur in the anchorages. Special care deviations during stressing and in external tendons: It is not an entirely must be taken to prevent damage to service; also, the protective sheathing novel idea to arrange external tendons the coating during handling and must be jointed properly. underneath the bridge girder. For installation. If tendon replacement is a design example, this concept has been used c) Protective sheathing: The protective requirement, the saddle arrangement for steel bridges, such as the Neckar sheathing represents an envelope must be chosen accordingly (e.g. Valley Bridge at Weitingen, Federal Re- around the prestressing steel. Suitable double sheathing; see Alt. 3, Fig. 9). public of Germany [31], (Fig. 10), and materials are steel or plastic tubes for the Bridge Obere Argen, Federal - Minimum radii: L imits must be respect- (polypropylene [PP] or polyethylene Republic of Germany [32], [33]. In both ed because otherwise either the [PE]). In order to achieve an effective cases, the extremely difficult local soil prestressing steel or the protective protection system, proper solutions are conditions led to such a design. In [34] sheathing could suffer. Although some required for coupling these tubes with Wittfoht proposes underlying external tests exist indicating reasonable values, each other, with the anchorages and tendons as a standard solution for box which may be used for preliminary with the saddles. girder bridges for road or rail traffic. designs, more research work is Menn describes a similar system for Injection of the remaining voids inside required in this respect. It is therefore slab bridges, by which the feasible the sheathing with cernent grout has advisable to verify the feasibility of a span range can be extended up to proven to be economical and reliable. In particular practical solution by tests. about 40 m [35], (Fig. 11). A compre- the case of restressable anchorages, hensive test programme for determining cernent grout must be replaced at least 1.4. Future developments the structural behaviour has been focally by grease or similar soft plastic carried out at the Swiss Federal material. Particularly in France, grease The revival of external post-tensio- Institute of Technology (ETH), Z urich, and wax products have been applied, ning has been a stimulus for engineers. Switzerland [36] (Fig. 12). instead of cement grout, on the entire tendon length [14], [30]. Besides being rather expensive, these products are difficult to inject (e.g. preheating up to 100° C required) and special measures are needed to prevent leakages (see also para. 6.2.5.). In this category, individually greased and plastic-sheathed monostrands offer many advantages. They are manufac- tured under factory conditions. The prestressing steel is therefore effectively protected against corrosion during transportation, storage on site and installation, provided that proper care is taken not to damage the sheathing. Monostrands can be used either individually or in bundles as multistrand tendons. In the latter configuration they are usually placed inside a plastic or steel tube. The remaining voids are filled with cernent grout.

1.3.5. Saddles at points of deviation When designing saddles it is important to consider the following: Figure 9: Various saddle arrangements

7 E XTERNAL P OST-TENSIONING

New corrosion protection systems: In related fields of application, new concepts have already been imple- mented. It is known that corrosion does not occur in a dry environment (relative humidity  40%). This fact has been utilized in steel construction. The designers of the suspension bridge over the L ittle Belt (1970) and later on of the Far  Bridges (1985), both in Denmark, introduced a dehumidification and ventilation system for the interior of the large steel box sections, thus protecting the inside surfaces against corrosion [37]. In Sweden, the Swedish State Power Board used a specially designed dehumidification and ventilation system for all containment tendons of Forsmark 1-3 Nuclear Power Sta- tions. It is obvious that the conditions in a well-attended power station are more favourable than in an ordinary bridge structure. In the future, however, further developments in this direction may be expected. figure 10: Neckar Valley Bridge at Weitingen, Federal Republic of Germany Monitoring of tendons: External tendons make possible monitoring of the tendon force and of the soundness of the tendons. Refined techniques for monitoring the integrity of the corrosion protection systems and for inspecting the tendons are being developed.

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

8 E XTERNAL P OST-TENSIONING 2. Design Considerations for Bridges with External Tendons

2.1.General

The purpose of this chapter is to highlight some special aspects that should be considered in the design of externally post-tensioned bridge superstructures. As for any design, it is normally at the conceptual stage that the “fate” of a structure with regard to economy and durability is determined. A straightforward structural system, good detailing, and the early integration of the construction proc- ess are the major elements of a successful design. In this respect, bridges with external tendons are no exception. To obtain a satisfactory behaviour of a structure both under serviceability and ultimate limit state conditions, it is essential to recognize the peculiarities of girders with external tendons. Fig. 13 shows moment-curvature curves for a typical bridge cross-section with either bonded or unbonded prestressing. For Figure 13: Moment-curvature curves for a typical bridge cross-section with bonded and comparison, a curve for non-prestressed with unbonded prestressing bonded reinforcement is also shown. The cross-sections of the reinforcement and Good crack distribution can only be - Girder with unbonded tendons: prestressing were chosen such that all obtained if the flexural resistance at a three sections would reach the same Because relative longitudinal displace- section exceeds the cracking moment. ultimate moment. As can be seen from ments between concrete and steel are This principle is well known from minimum Fig. 13, there is no fundamental differ- not prevented by bond, the tendon reinforcement requirements. For case a) ence between girders with bonded or with force increases only due to deformation of Fig. 13 this principle is only barely met unbonded tendons below decompression of the entire structural system. Similar and hence deformations may take place moment. The section with unbonded to slabs with unbonded tendons [38], in just one or a few cracked sections. tendons has a larger initial prestressing the tendon force increase depends This may lead to an undesirable strain force and, therefore, a higher decompres- primarily on the geometry and the localization with a subsequent premature sion moment than the section with overall deformation of the structure as failure. bonded tendons. With regard to the well as the tendon profile. fatigue behaviour, Dischinger [10] already For long tendons and slender struc- As a consequence of the described mentioned the advantage that for tures this increase will be relatively behaviour, externally post-tensioned unbonded tendons only negligible stress small, even for large overall deforma- structures are inherently more sensitive to fluctuations occur in the prestressing secondary effects since, unlike bonded steel under live load. tions of the system. Therefore in Fig. 13, the tendon force increase has been systems, they do not have the capability A closer look at the behaviour of the neglected and hence the decompres- to adapt to local overloads by local sections is required following decompression moment is equal to the ultimate yielding. Hence, while a realistic assess- sion: moment. Of course, friction at deviation ment of secondary effects is not of - Girder with bonded tendons: After points would somewhat improve the be- primary importance for bonded systems, decompression, the tendon force haviour, but unless intermediate an- this is quite different for externally post- increases up to the yield strength. The tensioned systems. tendon force increase and the associ- chorages or partial bond at sufficiently ated increase of the internal lever arm closely spaced locations along the In practice, continuous bonded of the section provide a yield strength tendons and/or additional bonded reinforcement and partial bond of external considerably larger than the decom- reinforcement are provided, it shoutd be tendons, at tendon deviations due to pression moment of the section. Owing recognized that decompression friction or cernent grout will contribute to to the bond between concrete and essentially means ultimate. increasing the ratio of flexural resistance steef, the flexural behaviour at a section At any rate, the strength of an exter- to cracking moment and thus result in a is more or less independent of adjacent nally post-tensioned girder at one par- more forgiving behaviour of the structure. girder zones. ticular section depends on the behavi- Finally, it should be mentioned that our of the entire structural system, or at prior to grouting of the tendons a structure least parts of the system if intermediate with bonded tendons behaves similarly to anchorages are used. 9 E XTERNAL P OST-TENSIONING

an externally post-tensioned system. In A reasonable prestressing force may after all losses in order to get an estimate one case a bridge failed during construc- be estimated using the load balancing of the ultimate resistance. Alternatively, a tion [39] primarily because the special method. If a substantial part of the dead rigorous analysis can be performed by in- conditions of the construction stage were load is balanced by the prestressing, a tegrating the strain increments in the overlooked. This failure clearly exhibited satisfactory behaviour of the structure structure along the tendon axis (Fig. 15a), the effects of strain localization and points may be expected regarding both deflec- or an estimate of the tendon elongation to the need for a careful evaluation of all tions and cracking. can be obtained from the consideration of possible effects of loads and imposed de- As soon as the tendons and their a rigid body mechanism (Fig. 15b). formations when designing externally profiles are selected, the tendon force Integration of the strain increments post-tensioned systems. diagram can be determined. As external along the tendon axis requires an iterative tendons are generally arranged in a non-linear analysis [14], [40]. For a given 2.2. Serviceability and polygonal shape, the force diagram will load increment and an assumed tendon ultimate limit states have steps at the deviation points. Fig. 14 force the strain increments at each shows the forces applied to a deviation section and the associated tendon force 2.2.1. Serviceability limit state point. increase can be computed. Repeating the Usually, the amount of prestressing is Long-term losses due to relaxation of calculations with the new value of the selected at a relatively early stage in a the prestressing steel, as well as creep tendon force will result in an improved project. This selection is influenced by and shrinkage of the concrete, cause a estimate and after a few iterations a considerations regarding serviceability decrease in the tendon force. As long as reasonable approximation will be ob- and economy of the structure: no relative displacements between tained. Similar computations can then be tendon and concrete occur at the devia- made for the next load increment and so - Under dead load only, the structure tion points, either because of the pres- on. shall remain substantially uncracked or ence of high friction coefficients or existing cracks shall be closed. On the because of partial bond, these losses other hand, the requirement of an may be estimated section by section as uncracked structure for dead and live for bonded tendons. However, for low load inctuding secondary effects might friction coefficients there will be some lead to undesirable long-term hogging slippage between tendon and concrete deflections under dead load only. and the losses may be estimated from However, under such loading condi- mean axial deformations due to creep tions cracked sections can normally be and shrinkage of the entire structure. accepted if the stresses in the rein- As mentioned in Section 2.1. structures forcement are limited such that the with external tendons may be sensitive to cracks close again after removal of the secondary effects. Therefore, it is load. essential to assess tendon forces and - From an economic point of view, one secondary effects due to temperature, should ideally provide just enough creep, shrinkage and other effects as non-prestressed reinforcement and re- realistically as possible when performing stressing steel as necessary to obtain checks at serviceability limit states. The the required resistance. effects of prestressing may be considered either by the primary and secondary moment method or by the load balancing method. The first method is generally used for the final design of a structure because it allows for an easy consideration of friction losses. On the other hand, the second method is prlmarily suited for preliminary designs if friction losses are neglected. Figure 15: Tendon elongation +L

2.2.2. Ultimate Iimit state Tendon size Minimum radius As mentioned in Section 2.1. the (VSL tendon unit) (m) tendon force increase in externally post- up to 5-19 or 6-12 2.50 tensioned structures will generally be up to 5-31 or 6-19 3.00 rather small unless intermediate anchor- up to 5-55 or 6-37 5.00 ages or partial bond at sufficiently closely spaced locations are provided. Hence, for ultimate timit state considerations one Table III: Recommended minimum tendon Figure 14: Forces at deviation points may opt to neglect any possible tendon radii force increase and use the tendon force

10 E XTERNAL P OST-TENSIONING

However, the wobble factor k can Rigid body mechanism considerations ment of secondary effects is usually normally be neglected since the tendons have been successfully applied in essentiat for externatly post-tensioned designing post-tensioned slabs with systems because of their inherent are straight between the points of unbonded tendons [41], [42], [43], [44]. sensitivity to such effects (see Section deviation. Typically, a nominal failure characterized 2.1.). Hence, while a liberal attitude to- Based on test results, site experience by a maximum deftection of about two wards secondary effects due to imposed and the technical literature, the friction percent of the stab span is assumed and deformations of any sort may be assumed coefficient varies as follows: the resulting elongation of the tendon is when designing a bonded structure, a  determined from geometry. Knowing the more cautious approach is necessary for - bare, dry strands over steel elongation and the stress-strain relation- externally post-tensioned structures. saddle 0.25-0.30 ship of the tendon, the tendon stress increase and the tendon force can be - bare, greased strands over determined. While this procedure is well 2.3. Particular aspects steel saddle 0.20-0.25 estabtished for slabs, some caution is - bare strands inside plastic recommended for the application to 2.3.1. Saddles tube running over saddle 0.12-0.15 bridge girders until more information The design and detailing of saddles at - greased and plastic-sheathed on maximum deflection values is avai- points of tendon deviations is a delicate lable. monostrands inside plastic task. An early coordination between the If the tendon force increase is taken tube over saddle 0.05-0.07 into account, second order effects as designer and the tendon supplier is exemplified by Fig. 16 have to be advisable. It is of utmost importance that considered as well. However, if an the forces transferred at the saddle appropriate number of deviation points is locations are carefully evaluated. It is provided, the influence of such second recommended to use higher safety order effects may be kept small. factors as the proper functioning of these Knowing the tendon force, the ultimate elements is essential for the entire resistance of an externally post-tensioned structure. Typical examples of saddles structure can be determined using are shown in Fig. 9 and 18. conventional methods. Similar to bonded structures, an external tendon can either 2.3.2. Minimum tendon radii be treated as part of the integral load resisting system (Fig. 17a) or it may be Minimum tendon radii as recom- considered to be separated from the mended in Table Ill must be respected in concrete and its action can be modelled order to avoid damage of the prestressing by applying the equivalent anchorage, steel and the plastic sheathings as well as deviation and friction forces onto the the outer tubing. It is also known that concrete (Fig 17b). However, in contrast friction problems may occur if the tendon to bonded structures, a realistic assess- radii are too small.

2.3.3. Prestress losses due to friction Similarly to conventional prestressing, the force -friction retationship can be described with the following formula:

-( α+kx) P(X) = P 0 e  where

P(X) = Post-tensioning force at a distance x Figure 16: Influence of girder deflection from the stressing anchorage on tendon eccentricity P 0= Post-tensioning force at the stressing anchorage e = Base of Napierian logarithms  = Coefficient of friction α = Sum of angular deviations (in radians) of the tendon in ail planes over the distance x k = Wobble factor (inaccuracies in Figure 17: Ultimate resistance placing) per unit length Figure 18: Examples of deviation points

11 E XTERNAL P OST-TENSIONING

3. VSL External Tendons

3.1. Introduction 1.3.2., strands have a relatively high 3.2.2. Selection criteria breaking strength, which results in a The main technical criteria for selecting The VSL External Tendons are in reduced consumption of steel. Another the tendon type are: essence an adaptation of the well-proven main advantage of VSL External Tendons VSL Post-Tensioning System [45] to the is the modular principle, which enables - Environmental conditions and tendon requirements for external tendons. Thus, any desired tendon size and tendon exposure: as for internal, bonded they do not represent a completely new anchorage to be made up from standard prestressing, for external tendons it technology, but simply a further develop- units. Thus, the construction principle is seems logical to select the degree of ment of a technology relying on many always the same. On the other hand, the corrosion protection according to the years of practical experience. system is sufficiently flexible to allow for environmental conditions and the The main characteristic of VSL adaptation to any requirement. This exposure of the tendons. Based on the External Tendons is the use of strands for means, therefore, that the information classes given in Table II (p. 6), it is the tensile members. As shown in para. presented in Chapters 3 and 4 is merely recommended to Use Type 1 for representative and does not constitute classes 1, 2 and 3 and Type 2 for class any limitation to the possible range. 4.

3.2. Types of VSL External - Need for tendon force adjustment Tendons and Technical Data during lifetime of the structure: in this case Type 2 is recommended. 3.2.1. General - Tendon friction during stressing opera- The VSL External Tendons consist of tion: as shown in para. 2.3.3., the the following main elements (Fig. 19): friction with tendon Type 2 is much smaller than with Type 1. In the case of - a bundle of prestressing strands (either long tendons running over several bare or individually greased and plastic- spans with sizeable angular changes, sheathed) as the tensile member, Type 2 offers technical and economical - a plastic or steel tubing for the strand advantages. bundle, Table IV summarizes the selection - end and intermediate anchorages, and criteria. It should be mentioned that other couplers, factors may influence the decision such - a grouting compound. as price, availability of materials, local practice, etc. VSL offers two main types of external tendons (Fig. 20) which can be character- 3.2.3. Strands ized as follows: For VSL External Tendons, cold-drawn Type 1: Bundle of strands inside a 7-wire prestressing strands of  13 mm steel or plastic tube; the grouting com- (0.5”) and 15 mm (0.6”) normally of low pound normally consists of cement grout. relaxation quality are used. The Type 2: Bundle of greased and plastic- geometrical and mechanical properties sheathed monostrands inside a steel or are given in Table V. plastic tube; the grouting compound Whereas for tendon Type 1 bare consists of cement grout, except in the strands are envisaged, greased and anchorage zones, where especially suited plastic-sheathed strands (monostrands) corrosion preventive compounds are are used for Type 2. The grease has used. favourable characteristics with regard to Figure 20: Cross-sections of VSL External Tendons Type 1 and Type 2

Figure 19: Composition of the VSL External Tendon (schematical) 12 E XTERNAL P OST-TENSIONING

3.2.6. Anchorages its long-term stability and its suitability in Type 1 Type 2 providing corrosion protection of the The anchorage principle of VSL prestressing steel. The sheathing is of Externat Tendons corresponds in polyethylene (or alternatively polypro- essence to the VSL Post-Tensioning Environmental conditions (see Table II): pylene) and has a minimum thickness of System. Figures 21 to 29 represent a class 1 n 1 mm for straight tables and 1.5 to 2 mm variety of possible anchorages. Different class 2 n for curved tables. parameters, such as required adjustabil- class 3 n class 4 n ity, replaceability, load monitoring, 3.2.4. Characteristic breaking loads of VSL External Tendons installation procedure, access to the end Table VI gives the nominal breaking anchorages (e.g. for the strengthening of Need for tendon force adjustement: loads for the VSL External Tendons structures), static considerations and n environmental conditions as per para. no according to the four strand types as yes n detailed in Table V. The characteristics of 3.2.2., influence the selection of the the strand may, however, slightly deviate particular anchorage type. Tendon friction from these values, depending on the The exposed surfaces of the anchor- shorter tendon and manufacturer and applicable standard. ages are properly coated for corrosion small ∑α n protection. longer tendon and 3.2.5 Tubing high ∑α n The strand bundle (consisting of either 3.2.7. Grouting compounds bare or greased and plastic-coated Table IV: Main technical criteria for The tubing around the strand bundle strands) is usually encased in a plastic selection of tendon type tube. Alternatively, steel tubes may be constitutes its primary corrosion protec- used. In certain areas, such as deviation saddles or parts of the tendon embedded in concrete, regular corrugated steel duct as normally used for post-tensioning 13 mm (0.5”) 15 mm (0.6”) tables may be chosen. The latter, Strand type (A) (B) (D) however, is only applicable when the (C) Euronorm ASTM ASTM tendon does not need to be replaceable Euronorm 138-79 A 416-85 A 416-85 (see also para. 3.2.6.), and tendons Type 138-79 Super Grade 270 Grade 270 1 are used. Super Nominal diameter (mm) 12.9 12.7 15.7 15.2 In general, the plastic material is 2 140 polyethylene and meets the requirements Nominal steel area (mm ) 100 98.7 150 of appropriate standards such as DIN Nominal mass per m (kg) 0.785 0.775 1.18 1.10 8074 and 8075, ASTM D 1248 and 3035 or equivalent. Alternatively, polypropylene Yield strength (N/mm2) 1,580 1,670   1,500  1,670      may be used. The ratio of internal 2 1,770 diameter to wall thickness is approxi- Ultimate strength (N/mm ) 1,860 1,860 1,860 mately 16:1. In general carbon black is 186.0 183.7 265.0 260.7 added as ultraviolet stabilizer. This Min. breaking load (kN) material is chemically inert against 1) 0.1 % proof load method 2) 1 % extension method practically any foreseeable agent (see e.g. DIN 16934). It has shown excellent Table V: Strand types durability behaviour in structural applications. In the case of steel tubes, a higher internal diameter/wall thickness ratio can Ø 13 mm (0.5”) Strand Ø 15 mm (0.6”) Strand be used (approx. 30:1 to 50:1). The Breaking load (kN) Breaking load (kN) dimensions used are primarily dictated by Max. Max. the availability of standardized tubes. The Cable number of Strand Strand Cable number of Strand Strand outer surface of the tubing is normally type strands type A type B type strands type C type D provided with a paint giving sufficient corrosion protection. 5-3 3 558 551 6-3 3 795 782 The plastic or steel tubing represents 5-4 4 744 735 6-4 4 1,060 1,043 the prime barrier against corrosive attack. 5-6 6 1,116 1,102 6-6 6 1,590 1,564 5-7 7 1,302 1,286 6-7 7 1,855 1,825 It is connected to the anchorages and the 5-12 12 2,232 2,204 6-12 12 3,180 3,128 saddles, thus providing an effective and 5-19 19 3,534 3,490 6-19 19 5,035 4,953 continuous envelope around the 5-22 22 4,092 4,041 6-22 22 5,830 5,735 prestressing steel. 5-31 31 5,766 5,695 6-31 31 8,215 8,082 5-37 37 6,882 6,797 6-37 37 9,805 9,646 5-43 43 7,998 7,899 6-43 43 11,395 11,210 5-55 55 10,230 10,104 6-55 55 14,575 14,339

Table VI: Characteristic breaking loads 13 E XTERNAL P OST-TENSIONING

tion. In addition, the tendon is completely and fulfills the same requirements filled with a grouting compound. As as the one used in traditional post- Due to the fact that the envelope mentioned under 3.2.1.. normally cement tensioning. With its alkaline properties, reduces or eliminates the diffusion of grout is used. it provides active corrosion protection. gases and liquids, carbonation of the The grout is made from Portland cement The grout completely fills the interstices cement grout is inhibited. between the strand bundle and the outer Notes: *Bursting steel not shown for clarity. tubing. *Figures 26 to 29 have been omitted.

Features : Replaceable stressing or dead end anchorage where no adjustability and load monitoring is required. Available for 0.6" diameter bare strand tendons (Tendon Type 1).

Figure 21: Anchorage Type Ed

Features : Large-sized guide pipe enabling push-through for trumpet/anchor head assembly. Fully adjustable, detensionable and replaceable anchorage (Tendon Type 2). Ring nut can be used instead of split shims. Can also be used with bare strands, if only load monitoring, small adjustements or replaceability is required.

Figure 22: Anchorage Type A

Features : Replaceable stressing or dead end anchorage where no adjustability and load monitoring is required (Tendon Type 1)

Figure 23: Anchorage Type CSd

14 E XTERNAL P OST-TENSIONING

Feature: Replaceable stressing or dead end anchorage where no adjustability and load monitoring is required (Tendon Type 1)

Figure 24: AnchorageType ECd

Composition: Anchor block with wedges, retainer plate on passive side to secure wedges, steel case. Tube for strand overlenght if detensionability or adjustability 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 30: Intermediate tendon supports

Figure 31: Test installation

3.3. Experimental evidence Several tests have been conducted by VSL (Fig.31). The first test aimed at tendon was slightly stressed from both during the development of the VSL determining the groutability of a bundle of anchorages prior to grouting. The tendon Extemal Tendons. These tests have monostrands, especially in the saddle was then stressed to 70% of the breaking provided valuable data for material area. The second test involved stressing load, again applying a 600 mm relative selection and procedures, anchorage the tendon in stages up to 70% of the displacement. The fourth test was similar design, and friction losses in saddles. For breaking load. To simulate actual to the third, but incorporated the improve- the Bois de Rosset Viaduct project conditions, a relative displacement of ments obtained through the earlier tests. (para.6.2.9.), four tests were performed 600 mm was applied. In the third test, the 15 E XTERNAL P OST-TENSIONING

4. Application of the VSL External Tendons

4.1. Manufacture and The strands are inserted by pushing the Type 2 Tendons (greased and plastic- installation tube over the prepared strand bundle or coated monostrands) are stressed in two by pushing individual strands through the stages. In the first stage, an initial force is Basically there are two different tube. applied which removes the slack in the methods used for the installation of VSL tendon. Then the tendon is grouted. After External Tendons The bearing plates or anchorage the grout has attained the required bodies and the supports at the deviation strength, the stressing operation is - Installation of completely prefabricated points are fixed to the structure. The continued. In the second stage, the tendons. prefabricated tendon is then placed into stressing force is raised in uniform fashion its final position either manually or by to its final value. - Installation of the empty tube in the final mechanical means using hoists or Depending upon the anchorage type position followed by insertion of the winches. Intermediate temporary fixings chosen, the tendon force can be checked, strands. along the straight lengths are provided to adjusted or released using the same keep the tendon in its correct position. multtstrand stressing jack. 4.1.1. Prefabrication The method of complete tendon 4.3. Grouting prefabrication is usually applied for short, 4.1.2. Fabrication in the final light tendons where easy access on site position The VSL anchorages incorporate a allows the placing of the entire tendon. Besides the fixing of bearing plates and grout connection which can be used as Prefabrication may be carried out deviation points, it is necessary to provide inlet or as outlet. Further grout connec- either in a factory or in a prefabrication temporary intermediate tendon supports tions are provided at the deviation points. area on site depending on the means of along the straight length of the tendon Grouting commences at the lower end transport, the time between manufacture prior to the placing of the tubes (Fig. 30). of the tendon and proceeds at a steady and installation, and the availability of The tube (steel or PE) is prepared in rate until grout of the same consistency is adequate space on site. The standard suitable sections and placed in its final ejected at the deviation points and finaly lengths of tube are connected to position by attaching it to the previously at the other end of the tendon. For long achieve the required total length. PE fixed supports. The tube sections are tendons, the grout is injected at subse- tubes are connected by butt-welding, connected by welding or by using quent inlets along the tendon. When steel tubes by welding or with couplers. couplers. At the ends the tube is tightly using greased and plastic-sheathed connected to the anchorages. strands inside a steel or PE tube (tendon When the tube is securely fixed in the Type 2), the tendon is grouted after initial final position it is ready to receive the tensioning by injecting cement grout into strands. The strands are inserted by the tubing only. The anchorage zones are pulling the prepared strand bundle (as filled with a non-hardening corrosion one unit or in groups) through the tube by preventive compound. a winch. If the tendon consists of bare strands, the VSL push-through machine 4.4. Completion Work can be used by taking the strands directly from the coil and pushing them through The exposed surfaces of the anchor- the tube one by one (Fig. 32). ages are properly coated for corrosion protection. 4.2. Stressing As a result of the simplicity of the construction principle, the high quality of The VSL External Tendons are the materials used and the excellent stressed with the appropriate VSL corrosion protection, VSL External multistrand jack. All the strands are Tendons are virtually maintenance-free. Figure 32: Pushing strand stressed simultaneously but individually The use of anchorages type Am, locked off (Fig. 33). The stressing (anchor head with thread) enables the operation normally follows the procedures attachment of a VSL Load Cell (Fig. 34), established by the specifications, by local allowing the monitoring, checking and codes of practice or by the FIP recom- small adjustments of the tendon force mendations. during the whole life of the structure. Type 1 Tendons (bare strands) are Anchorages type As, and Ar, provide for stressed at a steady rate in one or several adjusting and detensioning of the tendon increments until the required stressing force and, if required, replacing of the force is reached. Grouting is carried out entire tendon. after completion of the stressing operation.

Figure 33: Stressing a VSL tendon

16 E XTERNAL P OST-TENSIONING

5. VSL Service Range

5.1. General

The VSL Organizations provide a comprehensive range of services in connection with externally post-tensioned structures, including: - Consulting services to owners, engineers and contractors - Preliminary design studies - Assistance with the design of externally post-tensioned structures - Detaifed design of external tendons - Supply and installation of external tendons - Supply of materials, equipment, and supervisory personnel - The use of other VSL Systems, such as slipforming, soil and rock anchors, heavy lifting, bearings, expansion joints etc. The extent of VSL’s services will usually be clarified in discussions between the owner, engineer, contracter, and the VSL Organization. In many cases the application of several VSL Systems is possible on a Figure 34 : Stressing of an external tendon equipped with a VSL Load Cell single project. This enables the use of labour and material to be rationalized with consequent cost savings. - Pamphlet “Life Extension and Strength- At this point, reference may be made to ening of Structures” The first solution, in most cases, will other VSL publications which are of - Various Job Reports prove to be the better one and therefore importance in the construction of exter- should be selected as a rule. The - VSL Newsletters. nally post-tensioned structures: foremost reason is quality assurance. The durability over the lifetime of a structure - Pamphlet “VSL Post-tensioning” [45] 5.2. Tender Preparation mainly depends on quality of materials - Technical report “Concrete Storage and on quality of workmanship. The The basic requirements for a tender for Structures” [21] experience available with VSL, whose one or more of the above services, in so personnel is engaged exclusively on post- - Technical report “The Incremental far as they concern the carrying out of tensioning, is the most suitable for the Launching Method in Prestressed detailed design, supply, installation and effective manufacture and installation of Concrete Bridge Construction” execution, are detailed drawings and tendons. Another reason is economy. A specification documents. This applies specialist worker can achieve a better rate - Technical report “The Free Cantilever- both for structures which are about to be of progress both by his experience and by ing Method in Prestressed Concrete constructed and also for Clients propos- the advanced type of equipment he has at Bridge Construction”. als which require further technical his disposal. He will require less time to development, or to which alternative solve unforeseen problems on site. In addition, the following VSL publica- proposals are to be prepared. In addition, considerable savings are tions are available that may also be of A VSL tender for external post- possible if a Main Contractor investigates interest in connection with externally post- tensioning may consist of: tensioned structures: jointly with VSL how best to use the available material, plant, equipment and - Pamphlet “VSL Slipforming” - Supply of material plus manufacture methods for a specific project. VSL has, and complete installation of the cables, - Pamphlet “VSL Soil and Rock for this purpose, built up its own design Anchors” [2] including provision of personnel and engineering staff. The combination of equipment, or engineering skill with detailed knowledge - Pamphlet “VSL Heavy Lifting” of the possibilities and special features of - Pamphlet “VSL Messtechnik/Measuring - Supply of material, provision of supervi- the VSL Systems has proved to be an Technique/Technique de mesure” sory personnel and provision of attractive service to Main Contractors for equipment. optimizing their construction work. 17 E XTERNAL P OST-TENSIONING

6. Examples from practice

6.1. Introduction

The purpose of this chapter is to give the reader information about a number of structures in which VSL External Tendons have been used, and at the same time to illustrate in what structures external tendons are applicable. The descriptions include concrete and steel bridges, buildings, silos and other structures. The chapter is split into a first part presenting structures that have been designed for the use of external tendons from the beginning, while the second part comprises structures which had to be strengthened by means of external tendons. The job reports show that external post-tensioning is applied in various countries; a certain predominance of Figure 35: Exe and Exminster Viaducts France and the USA cari,, however, be observed because these countries are at 6.2. Bridges originally separated from each other by an embank- the forefront of the development of this designed with external ment approx. 380 m long, cross the Exe technique. Wherever available, details tendons Valley. The Exe Viaduct spans the river regarding the design of the structures and Exe and the Exeter Canal, while the the reasons for the selection of external 6.2.1. Exe and Exminster Viaducts Exminster Viaduc$ carries the motorway post-tensioning are also presented. near Exeter, Great Britain across a double-track railway line (Fig. In both parts, the projects are listed Owner Department of Transport, 35). chronologically. The designs of the older South Western Road The Exe Viaduct has a length of 692 m structures may differ from what is outlined Construction Unit and comprises eleven spans (53.50 - 9 x in the previous chapters, but at the same Engineer Freeman Fox and Partners, 65.00 - 53.50 m). The Exminster is 302 m time they may make evident the improve- London long and has five spans (53.50 - 3 x 65.00 ments in the state of the art achieved over Contractor Cementation Construction - 53.50 m). The superstructures of both the past years. Ltd., Croydon bridges consist of two parallel, twin-cell Post- Losinger Systems Ltd., box girders of 7.00 m width and 2.80 m tensioning Thame depth, having 5.00 m cantilever wings on Construction each side. Diaphragms are provided in the Span 1 Span 2 Span 3 Period 1974-1976 boxes at 7.50 m and 10.00 m distances. The viaducts were designed with the These two viaducts are part of the objective of obtaining the lightest possible motorway M5 linking Birmingham in the structure, as soil conditions were found to Midlands to Plymouth in the south. Not far be poor. Therefore, the dimensions of the from Exeter the two structures, which are superstructure were minimized and the Stage 1 post-tensioning tables placed inside the

Stage 2

Stage 3

Figure 36: Construction procedure Figure 37: Tendon layout scheme

18 E XTERNAL P OST-TENSIONING

boxes. The construction procedure was 6.2.2. Seven Mile Bridge, Florida, chosen accordingly. Construction of the USA superstructure was split into two phases. Owner Florida Department of First the boxes were constructed on Transportation, Tallahassee, falsework, while the cantilever slabs were Florida added later, using movable shuttering. Engineer Figg and Muller Engineers, The technique applied in the construc- Inc., Tallahassee, Florida tion of the boxes was as follows (Fig. 36): Contractor Misener Marine Construc- - Stage 1: Construction of span 1 plus tion, St. Petersburg Beach, 115th of span 2 by successively Florida concreting diaphragms, base slab and Erection of webs, soffit slab. superstruc- - Stage 2: Installation and stressing of ture and the first half of the span 1 tables, con- post- VSL Corporation, creting of the remainder of span 2. tensioning Los Gatos, California Figure 38: Saddle detail - Stage 3: Moving of scaffolding and Construction from span 1 to span 3 formwork, Period 1979-1982 installation and stressing of tendons covering spans 1 and 2, concreting of The Seven Mile Bridge, which leads span 3. from south of Marathon to Little Duck - repetition of cycle. Key, is the longest in the chain of road structures connecting the mainland of The tendons were made up from Florida to Key West. With a total length of monostrands and are not encased in any 10,931 m (35,863’) it is also the longest additional sheathing between the dia- concrete box girder bridge in the world. It phragms. In each span, there are 16 VSL consists of 266 spans, most of them tendons type 6-19 dyform (breaking load having the standard 41 .15 m (135’) span approx. 5,700 kN each) having normal length. The superstructure with its single- VSL anchorages type E at both ends. The cell box section has a total width of tendons caver two spans and overlap at 11.89 m (39’) and a constant depth of the piers. Thus the maximum tendon 2.13 m (7’). length is approx. 170 m (Fig. 37). Profiling The design especially aimed at speed was achieved by means of saddles cast of construction, in addition to economy. into the diaphragms. The saddles Thus some structural details are quite consist of mild steel tubes of 110 mm unusual, even somewhat bold, with outside diameter welded into a steel box regard to concept and durability. These (Fig. 38). details are: The tendons were prefabricated in the - No gluing material to bond or seal the Figure 39: Construction phase workshop and introduced into the box joints of the match-cast segments. through a hole in the deck slab. Tendons Multiple keys only were provided to near the top of the box or inclined transfer shear forces. temporary prestressing strands. The pier tendons showed a considerable sag - Segments stressed together with segment, accompanying the assembled between diaphragms, due to their dead external tendons running inside the box spart, was also transferred to the shuttle load. As this would have created prob- and connected to pier diaphragms and barge, which then moved beneath the lems during stressing, intermediate props deflector blocks. erection truss (Fig. 39). were temporarily placed between The truss was a very sophisticated - No overlay or wearing surface on the neighbouring diaphragms. gantry combining innovative engineering segments; the traffic runs directly on Originally, stressing at both tendon with proven techniques and practical the precast concrete. ends was required to compensate for the experience. VSL proposed to use such a friction losses, as in the design a friction The segments of the superstructure truss as a variant to the construction coefficient u = 0.30 was adopted. Tests were cast in a yard in Tampa, Florida, scheme given in the contract documents. on site, however, showed an effective of approx. 400 km (250 miles) north of the The following are the main factors that led between 0.05 and 0.10. A revised bridge site. The five sets of forms allowed VSL to alter that scheme: calculation with these values proved that for an average production rate of three - The segment alignment is taken off the unilateral stressing was therefore accept- spans (i.e. 24 segments) per week. After critical path. thus increasing speed of able for obtaining the required forces. proper curing, the segments were barged construction and adding flexibility to the Post-tensioning work started in June to site. There, the segments of a span operation. 1975, and was completed in Cctober were placed aboard a shuttle barge, then - A cleaner division is achieved between 1976. The total quantity of strand incorpo- winched together, aligned to the required the work performed by the general rated in both viaducts is 1,100 tonnes. alignment and connected with four contracter and by VSL.

19 E XTERNAL P OST-TENSIONING

- VSL can control all its operations Additional details about the Seven Mile type EE 5-12 are installed, providing overhead. Bridge and its construction can be found forces from 600 to 1,200 kN. The tendons in [ 46] . are encased in PE tubes and were The truss, designed and operated by grouted after stressing (Fig. 42). VSL, in a typical operation lifted the pier 6.2.3. Châtelet Viaduct, Charleroi, As the Belgian standard in force at that segment into place, then cantilevered Belgium time did not contain any prescriptions for itself forward to position its centre on that Owner I.A.C. (Intercommunale this type of construction, full size model newly placed pier segment and finally Autoroute Charleroi), tests were performed with tendons of the raised the lifting frame bearing the Charleroi above-mentioned unit in order to assess preassembled span from the barge. Engineer Office J. Rondas, Brussels the fatigue behaviour of the tendon itself Concrete blocks were then inserted into Contractor Joint Venture Socol S.A., and especially of the anchorages. In this the gap and the post-tensioning tendons Brussels/ Ateliers de Con- way an anchorage design fulfilling the re- stressed to 15% of the ultimate force. struction Jambes-Namur; quirements was found. Another detail After the closure concrete had reached a later Ateliers de Construc- checked was the tightness of the tendon strength of 17.5 N/ mm²² (2,500 psi) tion Jambes-Namur/ Société system at every point, in particular at the overnight, the tendons were fully Pieux Franki S.A., Liège anchorages and the saddles. stressed. Post- Civielco B.V., Leiden, The tables being practically straight, VSL started erection of the first span of tensioning Netherlands saddles were required only near the the bridge on May 30, 1980. On average, Construction anchorages, for reasons of space. These three spans were installed per week, with Period 1981-1982 saddles consist of thick-walled steel tubes a maximum of six spans in a six day In order to take the transit traffic off the welded to the web of the steel girder. The single shift period. The structure was city centre and to connect the industry PE tubes of the tendons are fitted into the completed in May 1982. zones at the periphery, an outer ring road saddles and the joints tightly sealed. Each span contains 4 VSL tendons was built around Charleroi, an industrial 5-27 (breaking load approx. 4,960 kN centre in southern Belgium. In the suburb each) and 2 of the unit 5-19 (Fig. 40). The of Châtelet the ring road crosses the tendons are anchored in the pier dia- valley of the river Sambre, which is phragms by VSL stressing anchorages densely built-up and through which the type EC. In the five central segments of a railway line Paris-Cologne also passes. span, the tendons are deflected in To cross the valley a 1,097 m long deviation saddles. In the pier segment, viaduct had to be constructed. semi-rigid duct was embedded to bring The design adopted was put forward in the tendon to the anchorage, while a the tender stage as an alternative to the short piece of galvanized pipe guides the tender design. It comprises two independ- tendons through the deviation saddles. ent superstructures, each made up of two Between these, the tendons are encased 3.00 m deep steel girders carrying a in plastic pipes. Plastic pipe and duct or 16.00 m wide light-weight concrete deck. pipe are connected with rubber boots and Most of the 25 spans, the lengths of hose clamps (Fig. 41). All tendons were which vary between 34.98 and 58.60 m Figure 41: Detail of deviation saddle prefabricated and pulled in by hydraulic (except the main span which measures winch; they were cement-grouted for 68.40 m), are simply supported beams. corrosion protection. In view of the bad soil conditions, the Transverse post-tensioning was lightest possible structure was sought. For also applied, in the deck slab of the pier this reason the steel girders are post- segments only. The tables used are tensioned with external tables. At the internal, consist of four strands  13 mm base of each girder, up to 6 VSL tendons (0.5”) and are provided with VSL anchorages.

Figure 42: Cross-section and detail with Figure 40: Tendon layout tendons

20 E XTERNAL P OST-TENSIONING

designed for twin-track operation, is 2.13 m (7’) deep and has a deck width of 9.22 m (30’ -3”). Both structures are longitudinally post- tensioned with external VSL tendons 5-12 to 5-27 which run in the interior of the box. These are between 21.34 and 43.59 m (70’ and 143’) long and have EC anchorages at both ends. The strands were Figure 44: Prefabrication yard placed in polyethylene ducts, which were grouted with cernent mortar after stressing. Deviator blocks are provided in every 3.05 m segment. In these and in the end blocks, the strands lie in steel pipes which are connected to the polyethylene pipes by means of rubber hoses clamped on the pipes (Fig. 45). Transversely, the deck was pretensioned in the casting bed. Figure 45: Detail of pipe connection at deviator block Figure 43: General view of one bridge 6.2.5. Loir Bridge, La Fléche, France under construction Owner Direction Départementale de I’Equipement de la 6.2.4. MARTA Bridges, Atlanta, Sarthe, Le Mans Ga., USA Engineer SETRA, Bagne/ Bureau Owner Metropolitan Atlanta Rapid d’Etudes Dragages et Transit Authority (MARTA), Travaux Publics, Paris Atlanta, Ga. Contractor Dragages et Travaux Engineer Figg and Muller Engineers, Publics, Tours Inc., Tallahassee, Fla. Post- VSL France s.a.r.l., Contractor J. Rich Steers Inc., tensioning Boulogne-Billancourt New York, N.Y. Construction Figure 46: One bridge half nearly Post- VSL Corporation, Period 1982-1983 completely rotated tensioning Atlanta, Ga. The construction of a by-pass road in Construction the town of La Flèche made a new bridge Period 1982-1983 over the river Loir necessary. In view of the large areas of land liable to flooding The two bridges described below, one and the bad soil conditions, a very low span a depth of 1.75 m was adopted. The designated CS-360, the other one CN- road level was adopted in order to avoid deck slab width is 10.75 m. 480, are the first precast segmental embankments. This fact, of course, had Each half superstructure was con- concrete box girder railway bridges built in also a decisive influence on the design of structed parallel to the river with two 9 m the USA (Fig. 43,44). Originally the struc- the bridge. long form-works, which were advanced tures should have been constructed of in- A three-span bridge with the smallest like travellers of a free cantilever bridge. situ concrete; the successful contracter, possible depth of the superstructure Thus segments of about 5 m length were however, took advantage of a value- obviously was the only solution corre- obtained. After completion, each half was engineering clause and had a redesign sponding to the given conditions. The rotated to the final alignment (Fig. 46). prepared which resulted in the least length of the centre span was fixed at Construction lasted from March 1982 to amount of expenditure and saved time. 64 m, while the lateral spans were chosen February 1983. Thus up to four spans were completed at 26 m each. In order to equalize the Post-tensioning consists of two families per week. Construction of both bridges masses between the lateral span and half of tendons. For the quasi free cantilever- fasted for 64 weeks. the centre span, lightweight concrete was ing stages, internal tendons VSL type EC/ CS-360 is 1,594-10 m (5,230’) long and selected for 59 m of the centre span while EC 6-12 were chosen, one tendon was additional concrete was required in the has spans of 21.34 to 30.48 m (70’ to anchored at the top of the web at each last 10 m of the side spans. 100’), while CN-480 has a length of segment end. In the bottom slab of the For hydraulic reasons, the depth of the 579.12m (1,900’) and spans between centre span 4 VSL tendons EC/ EC 6-12 single-cell box girder superstructure had had to be placed, within the concrete 22.86 and 43.59 m (75’ and 143’). The to be limited to 2.80 m at piers; at mid- section. In addition, 8 external tendons single-cell box girder superstructure,

21 E XTERNAL P OST-TENSIONING

Figure 47: Longitudinal section with external tendons

VSL 6-19 run near each web. These are provided with standard VSL anchorages type EC (Fig. 47). The strands were installed by means of the VSL Push- through Method. The sheathing of the external tendons consists of steel tubes. To allow for possible force monitoring or the replacement of an external tendon, these were grouted with grease. This, however, proved to be a very expensive method due to the cost of the grease and of the steel tubes. The external tendons are deviated in diaphragms and cross-beams provided at various sections of the superstructure. Deviation is obtained by means of a curved piece of rigid steel tube which is connected to the tendon sheathing by a connecting sleeve welded to both tubes.

6.2.6. Bridge O.A. 33, Marseille, France Owner Direction Départementale de I’Equipement des Bouches-du-Rhône, Figure 48: Bridge O.A.33 under construction Marseille Engineer Bureau d’Etudes Dragages This is the location of the bridge desig- 48) as this variation offered the lowest et Travaux Publics, Paris nated O.A. 33. The bridge consists of two price. Fabrication was carried out behind Contractor Dragages et Travaux superstructures each having three traffic the Marseille abutment which is the Publics, Marseille lanes. The two structures have spans of lowest point of the structure. The pier Post- VSL France s.a r.l., 23-29-43-2x40-43-38-20 m and 31.69- diaphragms and internal diaphragms for tensioning Boulogne-Billancourt 2x40.01 -38.61-2x43.02-33.01 -27.01 m the external tables were constructed at Construction respectively. They are curved both in the the same time as the respective incre- Period 1983-1985 horizontal and the vertical plane. The ments. single-cell box girders have a depth of In the tender a post-tensioning layout, 2.85 m, the width of each deck slab being partly or entirely external, could be In Marseille, the motorway A55 leaves 14.42 m. proposed. For execution the following the centre of the City northwards. Soon it Both superstructures were built using three groups of tables were selected (Fig. crosses an industrial and railway area. the incremental launching method (Fig. 49):

Figure 49: Tendon scheme

22 E XTERNAL P OST-TENSIONING

Engineer Figg and Muller Engineers, of 3.66 m (12’) by inclined struts ending at Inc., Tallahassee, Fla. the intersection point between web and Contractor Paschen Contractors, Inc., bottom slab. The main structure is Chicago, Ill. followed on each side by 18 spans of Post- VSL Western, 41 .15 m (135’) standard length twin tensioning Campbell, Cal. single-cell box girders (Fig. 52). Construction All box-sectional parts were made up Period 1983-1987 from precast segments joined together The Sunshine Skyway Bridge replaces with an adhesive, some with in-situ a section of the bridge structure leading concrete. In the main structure, the across Tampa Bay between St. Peters- segments were hoisted from barges and burg and Sarasota. The replacement placed at alternate ends in the free Figure 50: Tendons inside the box became necessary, as in May 1980 a cantilevering manner. In the 41.15 m tanker veered out of the navigation (135’) spans, however, the erection truss channel and rammed one of the main from the Seven Mile Bridge (see para. piers, thus sending 400 m (1,300’) of 6.2.2.) was reused after adaptation and - Permanent tendons which were bridge into the water. In October 1982, complete spans were placed by the stressed before a new increment was the owner opened bids for a replacement contracter, who had bought the truss from jacked forwards. These tables are bridge. VSL. polygonal and parabolic and are within The winning design for the high-level The superstructure is post-tensioned the concrete section. and main approach spans consisted of a longitudinally and partially transversely. - Temporary tendons that were stressed concrete box girder structure with a The latter tables consist of VSL tendons before jacking and afterwards were 365.76 m (1,200’) table-stayed main SO/ SO 6-4 placed in the concrete section destressed and removed. After span. It should be noted that all stays are in flat corrugated ducts of high-density completion of the first superstructure, VSL Stay Cables System 200 with up to polyethylene. Longitudinal post-tensioning these temporary tendons were reused 82 strands of  15 mm (0.6”). The rebuilt in the free cantilevered part consists of in the second one. All of these tendons crossing was opened to traffic in April tendons within the concrete section were external. Some were straight, 1987 (Fig. 51). while others followed a polygonal The table-stayed part with spans of profile to give, in conjunction with the 164.59-365.76-164.59 m (540’ -1,200’ - first group, a central prestress. 540’) is adjoined on each side by three - Permanent tendons which were spans each of 73.15 m (240’) and one stressed after the increments were span of 42.67 m (140’). The superstruc- jacked forwards. They are all external ture of this main part consists of a 4.47 m and either straight or polygonal in (14’ -8”) deep single-cell box girder with steeply inclined webs. The deck slab is layout. Figure 52: Cross-section of twin, single 28.78 m (94’ -5”) wide. Along the central All tendons are of the VSL unit 6-12 box girder structure (breaking force 3,024 kN each) and have axis it is supported in the box at intervals stressing anchorages type EC at both ends. The temporary tendons and the final external tendons were placed in polyethylene tubes, while steel pipes were used for the final internal tendons (Fig. 50). All final internal tendons were grouted with cement grout. The external tendons are deviated in concrete frames provided in the boxes, into which curved steel tubes are placed. A short piece of PE tube is placed on the ends of the steel tubes and fixed in the concrete. To this the PE tube of the tendon is joined by means of a joint welded on both ends of the PE tubes.

6.2.7. Sunshine Skyway Bridge across Tampa Bay, Florida, USA Owner Florida Department of Transportation, Tallahassee, Fla. Figure 51: Sunshine Skyway Bridge 23 E XTERNAL P OST-TENSIONING

installed during free cantilevering while has two river piers. The height above the stresses desired by the engineer. the continuity cables run inside the box. river varies from 24.38 m (80’) at the north All tendons were cement-grouted for These are VSL units 5-17 and 5-27, up to end to 58.22 m (191’) at the south end, corrosion protection. A special detail was 238 m (781’) long. In the 41.15 m (135’) making it one of the world’s steepest required for the trumpet to allow for spans, VSL tendons 5-19 to 5-27 are bridges (Fig. 53). structure movement during stressing arranged inside the box similarly to the The new bridge makes use of some while maintaining a seal capable of cables of Seven Mile Bridge (see para. innovative structural techniques. Although withstanding the high grouting pressure. 6.2.2.). Anchorages used were standard the main river span appears to be a EC type. After stressing (double end traditional arch, it does not function as 6.2.9. Bois de Rosset Viaduct near stressing for longer tendons) the cables such. The structural loads are distributed Faoug (VD), Switzerland were cement-grouted. in cantilevered action through the use of Owner Département des Travaux the half arches on either side of the main Publics du Canton de Vaud, 6.2.8. High Bridge, St. Paul, Mn., span and the tensioned tables beneath Lausanne USA the deck. This unique structural system Engineer CETP Ingénieurs-Conseils allows steel members to be lighter than SA, Lausanne/DIC Owner State of Minnesota, conventional arches and this contributes Ingénieur Conseil, Aigle St. Paul, Mn. to the graceful aesthetic qualities of the Contractor Joint Venture of Engineer Strgar-Roscoe, Inc., structure. Frutiger SA, Yvonand/ Wayzata, Mn./ Eight VSL tendons 5-27, 166.12 m Ramella + Bernasconi SA/ T.Y. Lin International, (545’) long, tie the cantilevered arches Reymond SA San Francisco, Cal. together at the south pier. Similarly eight Post- Contractor Lunda Construction, black tendons of the same unit, 146.61 m (481’) tensioning VSL International SA River Falls, Wi. long, were used at the north pier. The Crissier Post- VSL Corporation, tendons, which are straight, were Construction tensioning Burnsville, Mn. anchored at both ends by means of period 1988-1990 Construction normal E type stressing anchorages Period 1985-1987 provided with a special cover cap. Each The Bois de Rosset Viaduct consists of tendon was pulled from ground into a two parallel structures with 15 spans each The new High Bridge, which replaces a galvanized steel pipe of 101.6 mm (4”). (23.00-34.20-11x42.75-51.30-38.50 m). It structure that was built in 1889, is the first Stressing was carried out in three stages: has a total length of 617.25 m, a width of bridge in the USA to use a combination Stage I stressing closed the gaps in the 2x13.0 m, and crosses a railway line at a of table and steel tension-tie design for a slotted connections at the ends of the height of approx. 10 m. The composite deck-tied arch bridge. It is 839.72 m wide flanges, Stage II took place after the superstructures consist of steel trough (2,755’) in length with a width varying concrete deck was cast and Stage Ill fine- girder sections connected to a transverse- from 20.02 to 27.13 m (65’ -8” to 89’). It tuned the arch to the camber and ly post-tensioned concrete deck slab. The structures are longitudinally post- tensioned with four VSL External Ten- dons in each span. Each tendon consists of 12 individually greased and plastic- sheathed VSL Monostrands which are grouted inside a thick-walled polyethylene tube. Tendon lengths range from 196 m to 216 m, and are located inside the steel troughs, routed over a maximum of five upper deviation saddles and ten lower saddles.

Figure 54: Cross-section of superstructure of Bois de Rosset Viaduct project Figure 53: High Bridge, St. Paul, Minnesota

24 E XTERNAL P OST-TENSIONING

This project represents the first use of b) Surrounding the masonry with stressed entirely surrounds the flue gas pipe and is monostrand external tendons in Switzer- steel bands. The installation of these anchored in a steel buttress. To minimize land, and the installation is monitored as bands and the disc springs required be- the effect of friction losses, successive part of a long-term observation program. tween band segments are expensive as pairs of tendons are alternately anchored Four tendons are equipped with perma- exterior scaffolding is needed around at buttresses on opposite sides. Each nent VSL load cells, and all tendons are the chimney. The same is also true for tendon is stressed to 100 kN. Tendon adjustable and replaceable. additional stressing of the bands at a spacing is 1 m. Before reaching the As mentioned in para. 3.3, extensive later date, which is necessary as the buttress, one table end undergoes a testing was performed for the develop- springs do not provide long-term com- deviation in a special construction, so that ment of this tendon system with regaid to pensation for creep of the gap-filling materials, procedures, anchorage design, compound. it can be anchored in the buttress. The and friction losses in the saddles. For two chimneys a new method was deflection device and the anchor buttress applied in 1986/87, in particular to avoid are coated with an anti-corrosive paint 6.3. Other structures the above-mentioned disadvantages. This (Fig. 56). originally designed with method consists of surrounding the flue The installation of the special bricks, external tendons gas pipe at regular intervals with external, the PE protective pipes containing the individual post-tensioning tendons. These monostrands, and the buttresses was 6.3.1. Flue gas chimneys, Federal must fulfill the following requirements: carried out as part of the brickwork of the Republic of Germany - easy installation, flue gas pipe. Thus, no special outside For environmental reasons, coal-fired scaffolding was required. For stressing, - easy stressing operation, power plants in the Federal Republic of however, a scaffold was used, which Germany are equipped with flue gas - possibility of additional stressing, could be displaced vertically in front of the sulphur removal systems. The purified - possibility of monitoring the tendon flue gases are normally expelled through force, the cooling tower, together with the - easy replacement. cooling steam. In the case of a breakdown, however, the flue gases are The tendons used are VSL Monos- diverted past the desulphurization system trands 15 mm. They rest on special and fed into the chimney. bricks containing a groove into which the tendon is fitted. These bricks are thermal- The shaft of the chimney ( approx. 10 to 17 m) is made of reinforced concrete; ly insulated from the flue gas pipe so that the tendons are subject to a maximum inside is the flue gas pipe made of acid- temperature of 40°C which both grease resistant ceramic masonry. The flue gas and PE coating are able to withstand pipe is surrounded by thermal insulation without problems. Nevertheless each made of foam glass (Fig. 55). Upon monostrand is additionally inserted into a breakdown of the desulphurization protective pipe of PE in order to prevent system, the temperature of the flue gas the coating of the tendon from bearing Figure 55: Cross-section of the flue gas rises quickly by 90°C to 180°C. The shock directly onto the bricks. Each tendon chimney of the sudden change in temperature leads to high compressive and tensile stresses in the heat-resistant ceramic masonry. The tensile stress exceeds the stress limit allowed in the standard DIN 1056. To ensure the serviceability of the flue gas pipe, special methods must therefore be taken. Up to now the following methods have been used: a) Reinforcing steel with anti-corrosive coating. The disadvantages of this solution are that the reinforcing steel does not prevent cracks and that bonding problems can occur as a result of the differing thermal expansion coefficients of reinforcing steel and ceramic brick.

Figure 56: Deflection device with anchorages

25 E XTERNAL P OST-TENSIONING

row of buttresses. The post-tensioning after checking of the design, it was found and because of the length and the weight force cari be checked at a later date, if that no temperature gradient had been of the tendons. Thus only the push- required, the anchor heads being considered and that the cover of the through method was applicable. Tests externally threaded for this purpose. prestressing tendons was insufficient. The made by VSL enabled the best way of Before the system was actually applied structure therefore had to be repaired in operating to be found. They showed that on site, tests were carried out in order to: the shortest possible period. A complete two intermediate pushing posts were - verify that the edge of the bricks would interruption of the highway being unac- required. According to the access not cause any long-term damage to the ceptable, the owner had to allow for the possibilities, pushing sections of 135, 160 PE coating of the monostrands repair work to be done under light traffic. and 135 m were selected. Two pushing - test and practise proper installation, Therefore, the consultant proposed to machines had to be placed at the first stressing and replacing of the tendons apply external longitudinal prestressing intermediate post in order to obtain the after the cracks had been grouted with required pushing force (Fig. 58). The tests gave fully satisfactory results resin. In addition, the following measures When a certain number of strands had which were confirmed during application. had to be taken: been introduced, the pushing force available was no longer sufficient for 6.4. Bridges with - Construction at either end of the bridge overcoming the friction in the steel tubes subsequently added external of a prestressed concrete cross-beam, and an auxiliary strand running between tendons incorporating the anchorages of the the first two posts was used, to which the new tendons and transmitting the strands to be installed were coupled. The 6.4.1. Roquemaure Bridge near additional forces to the superstructure first machine pushed the auxiliary strand Avignon, France of the bridge. while the second machine pulled it. After - Construction of a working chamber pulling, the auxiliary strand was pushed Owner Autoroutes du Sud de la behind each cross-beam, from which back to the first post and the operation France, Védène the strands would be fed into the ducts then repeated. When the pushing Engineer Etudes Ouvrages d’Art and where the cables could be operation was finished, the openings in (Bouygues), St. Quentin en stressed. the tubes were closed by previously Yvelines - Installation of hangers beneath the mounted coupling sleeves. Additional bridge deck carrying the cable ducts. Before the tables were stressed each Post- VSL France s.a. r.l., The longitudinal prestressing force individual strand was pretensioned to tensioning Boulogne-Billancourt 1 N/mm² by means of a monojack to bring Execution 1975-1976 required amounted to 54,000 kN after losses. VSL thus proposed to use 8 all strands to the same length. This bridge is part of Motorway A9 tendons of the unit 5-55 (ultimate force Orange-Narbonne in Southern France, 9,169 kN) running from one end of the which it carries across the river Rhône bridge to the other without any coupler near Avignon. The structure is 420 m long (table length thus 430 m!). Four spare and has spans of 50-4x80-50 m. The ducts were also installed in case addi- 21.60 m wide superstructure consists of a tional prestressing should be needed. double-T section with a depth between VSL was awarded the post-tensioning 5.40 m at piers and 1.80 m at mid-spans contract because, besides a reasonable (Fig. 57). It was built in 1971 to 1974 by price, VSL could prove its experience, in the free cantilevering method, with cast- pushing through strands and it had in-place segments up to 6.12 m in length. available equipment for this method, In 1975 a surveillance campaign revealed including several high capacity jacks. the presence of major cracks (as wide as Placing of the tendons was the most de- 8 to 10 mm) at the mid-span sections. manding part of the job. Placing preas- After examination of the damage and sembled strand bundles was excluded from the beginning because of the limited space available in the working chambers Figure 58: Pushing trough strands

Figure 59: Stressing of additional tendons Figure 57: Cross-section of Roquemaure Bridge with added external tendons in the stressing chamber

26 E XTERNAL P OST-TENSIONING

Stressing had to be done on two cables grouted cracks. Furthermore, it was stressed at the anchorages behind the simultaneously and at both ends for established that the stress variation in the pier diaphragm. Elongations measured symmetry reasons. The time available for post-tensioning steel considerably 440 mm on average. Post-tensioning stressing 4 x (of the 8) tendons was fixed exceeded the allowed value. work lasted for seven days. All cracks at 6 hours and therefore great mobility of Thus a static strengthening was closed after stressing. the equipment was required. Five VSL required, not only corrosion protection The rehabilitation work overall took five jacks ZPE-1000 (one as spare) and corre- measures. In view of the limited space months. sponding accessories, as well as five available inside the box cells, which did pumps were engaged (Fig. 59). The not allow for adding reinforcement, 6.4.3. Bridge over Wangauer Ache jacks, each weighing 2.5 tonnes, were strengthening the larger span by means near Mondsee, Austria mounted on specially constructed of post-tensioning tables offered the best Owner Republic of Austria, Federal hydraulic carriages. Stressing was done solution. Straight unbonded tendons were Road Administration, Vienna in steps of 5 N/mm². The cable extension selected, the number of which had to be Engineer amounted to 3,150 mm. The cables of the the smallest possible. (Repair) Kirsch-Muchitsch, Linz end cross-beams (16 No. EE 5-12 on A total of 24 tendons VSL type 5-16 Contractor Hofman u. Maculan, each side) were stressed in groups of two (breaking force 2,833 kN each) with (Repair) Salzburg at the same time as the longitudinal threaded anchor heads and an average Additional tendons. length of 75 m were required. At the post- In view of the quantity of material to be abutment side these were anchored in tensioning Sonderbau GesmbH, Vienna injected and of the length of the cables, anchor blocks added to the web prolonga- Execution 1987-1988 the use of a special grout mix with tions, while buttresses were provided retarded hardening and consisting of behind the pier diaphragm, which itself This bridge is part of the highway clinker and resin was required by the also had to be post-tensioned to take the Vienna-Salzburg. It was built in 1962- client. Grouting was executed in sections, additional forces. 1964. In recent years improvements have which made movable equipment neces- The monostrands were placed in PE been made several times, but a thorough sary. The grout mix was injected over a ducts which were provided with two inspection revealed a large number of distance of 180 m before the installation movable joints to absorb temperature deficiencies, making a general rehabilita- had to be moved. Two tables were movements. As strand deviations could tion necessary. In particular, a lack of grouted per day, requiring 12 m3 of not be avoided and inaccuracies in the longitudinal prestressing force was boring had to be expected and in view of grouting material. detected, which had become obvious in the large elongations, the likelihood of opened construction joints. Thus, rehabili- 6.4.2. Ruhr Bridge Essen-Werden, damage on the strand coating due to tation had to include also the installation Federal Republic of Germany transverse pressures caused by strand of additional tendons. Since a closure of deviations was evaluated in tests. The Owner City of Essen the motorway was not acceptable, one coating remained safe in these tests. Engineer structure was strengthened first, followed Borings had to be carried out with high (Repair) Prof. Dr. G. Ivanyi, Essen by the other. accuracy. Boring distances were 10 to Contractor Polensky & Zollner AG, 12 m (in pier diaphragm). Oblique boring The twin bridge has spans of 25-6x28- (Repair) Bochum (in plan view) was also required through a 2x41.25-3x28-25 m, i.e. a total length of Additional span diaphragm and subsequently 384.50 m. Each superstructure has a post- SUSPA Spannbeton GmbH, through a web. double-T cross-section, 13.05 m wide and tensioning Langenfeld The strand bundles were prepared in 2.20 m deep. Since the existing longitudi- Execution 1985-1986 the workshop. The diaphragm tables nal post-tensioning was bonded, and no were placed by means of a movable spare ducts were available, the additional This two-span post-tensioned concrete crane. The longitudinal tendons were post-tensioning had to be placed on the bridge (spans 66.40-47.00 m) has a multi- cell box superstructure with a deck width over the intermediate pier of 34.41 m. This width increases on both sides towards the abutments (Fig. 60). In the bottom slab and in the web of the larger span, numerous cracks due to bending had developed making rehabilita- tien measures necessary especially in view of the corrosion protection of the post-tensioning tables in the cracked area. Grouting the cracks (which were up to 0.4 mm wide) was disregarded since it was established that the cracks originated from temperature gradients; thus new Figure 60: Plan view of Ruhr Bridge cracks would have appeared near the Figure 61: Anchorages inside the box (showing cracks) prior to concreting

27 E XTERNAL P OST-TENSIONING

distances of 40 to 50 m. Two grouting pumps were required. 6.5. Other structures with subsequently added external tendons 6.5.1. Clinker Silo, Jakarta, Indonesia Owner PT Perkasa lndonesia Cement Enterprise (Indoce- figure 62: View of added tendons ment), Jakarta Engineer VSL INTERNATIONAL LTD. outside of the webs. However, the total (Repair) Berne, Switzerland length of 386 m was considered to be a Contractor PT John Holland Construc- problem. (Repair) tions Indonesia, Jakarta Four VSL tendons EE 5-12 per web Additional were selected as the additional post- post- tensioning. Polyethylene tubes of 90 mm tensioning PT VSL Indonesia, Jakarta diameter and 4 mm wall thickness were Execution 1985 chosen as sheathings (Fig. 62). At the ends of the superstructures, end dia- The silo, which is located at Tanjung phragms, each post-tensioned by 3 VSL Priok, Jakarta’s harbour, is used for the tendons EP 5-7, were provided. storage of clinker awaiting export. The The ducts were fixed to the webs at the structure has an internal diameter of Figure 63: View of rehabilitated clinker quarter points of the spans by means of 19.80 m and a height of 30 m. Its wall is silo clamps. In between, additional cable 400 mm thick. It was built of reinforced supports were provided in order to avoid concrete in the early seventies. Because tendons comprising four strands. Since wobble. These supports were hung from the reinforcement was inadequate, it had there were no buttresses, VSL anchor- the deck slab. In order to prevent the to be repaired. ages type Z 5-4 were used (two on every strands from abrading the polyethylene The repair consisted of placing external tendon). In total 60 hoop tendons were duct at clamping points, steel tubes were tendons around the silo. In view of the required. The tables were assembled by placed inside the polyethylene ducts at high ambient temperature and the hand and temporarily hung from steel those points. These steel tubes also act aggressive environment, it was decided to supports. The anchorages were placed as stiffeners at the joints of the polyethy- use greased and PE-coated strands on concrete pads. After stressing, the lene ducts. (monostrands) 13 mm bundled to anchorages were covered with concrete The strands were installed by the VSL  Push-through Technique. Two pushing machines were placed one behind the other and driven by a hydraulic pump of corresponding power. The strands had to be cut to length by hand before they could be pushed through. However, only the first 3 to 4 strands could be fully pushed through without squeezing. Therefore, strand installation was completed by hand from a joint opened in the duct about 250 m from the push-through machines. Before stressing all joints were checked for tightness. Stressing was done from both ends. A friction coefficient of only 5% was observed. Grouting was provided for corrosion protection, not bond. It was performed from the lowest point in the middle of the table, towards both ends. Vent hoses were provided at

Figure 64: View of Pier 39 parking structure

28 E XTERNAL P OST-TENSIONING

so that now the repaired structure out. Top-of-slab cracking adjacent and Approx. 10% of the tendons were appears to have four buttresses (Fig. 63). parallel to many beams, and beam replaced. Most stressing work was carried deflections of up to 38 mm (1 1/2”) in out from the outside of the building (Fig. 6.5.2. Pier 39 Parking Structure, many instances were found. Also several 66). The repair job started in August 1986 San Francisco, USA strands popped out of beam ends. All and was substantially complete by April Owner Pier 39 Associates, strands were subsequently examined; 1987. San Francisco, Cal. they all showed some signs of corrosion. It should be emphasized that short- Engineer Bijan, Florian & Associates Several strands had even failed. comings encountered in early application (Repair) Inc., Mountain View, Cal. The central issue to the rehabilitation of of unbonded strands in commercial Contractor the structure, besides economy, was that buildings and parking structures have (Repair) and a new system had to be built around or long been recognized and fully rectified. Additional added to the existing structural members Today, post-tensioning of such structures post- VSL Corporation, while the parking garage remained is commonly the most economical and tensioning Los Gatos, Cal. essentially operational. The beams, and performance-healthy mode of design and Execution 1986-1987 to a lesser degree the slabs, were the construction. primary targets of rehabilitation. This structure, which is part of a Two major options of rehabilitation shopping centre, was originally con- were reviewed in detail, one using steel structed in 1978/79 in the Fisherman’s members (trusses or channels) and the Wharf area; it has space for 1,000 cars other post-tensioned tendons. The latter (Fig. 64). It has five parking levels was adopted. including the roof, and a rectangular plan The structural design followed UBC with overall dimensions of 118.90 x 63.00 1982. Two tendons per beam, each m (370’ x 196’); at one corner there is a consisting of six strands  13 mm (0.5”) square recess of 20.90 x 54.60 m (65’ x were added, one on each side of the web. 170’). At mid-spans and over columns the The original structural system con- tendons are deviated by means of sisted of post-tensioned beams, 914 mm deflectors. These were made of  114 (36”) deep and spanning 21.00 m mm (4 1/2”) extra-heavy pipe. In order to (65 1/2’), which frame into columns to obtain the best possible corrosion protec- form a parallel plane frame in the trans- tion, the strands were coated with epoxy. verse direction. One-way post-tensioned The 51 mm (2”) corrugated PVC pipe slabs, 114 mm (4 1/2”) thick, span the was used as tendon sheathing (Fig. 65). longitudinal direction with spans of 5.80 m The work was carried out to allow continuous use of the garage by the (18’). The beams contained seven  public. Deflectors, end brackets and 15 mm (0.6”) monostrands while the slab precast members were erected on a night had 13 mm (0.5”) monostrands at shift, tendons in a day shift. The slab 660 mm (26”) centres. When in 1985 tendons were inspected and replaced by severe slab cracking at the roof level and removing 1.22 m (4’) closure strips substantial water leakage from the roof located at approximately one-third points were noted, further inspection was carried along the longer side of the structure.

Figure 65: Scheme of added beam tendons Figure 66: Stressing added tendons

29 E XTERNAL P OST-TENSIONING

7. Bibliography and References

[1] VSL Stay Cables for Cable-Stayed [12] Schambeck H.: Über das Langzeitverhal- [24] Verzeichnis der allgemain bauaufsicht- Bridges, January 1984. VSL INTERNA- ten einer 50 Jahre alten Spannbe- lich zugeiassenen Spann-stähle (List of TIONAL LTD., Berne, Switzerland. tonbrücke (On the iong-term behaviour the generally officially approved of a 50-year old post-tensioned concrete prestressing steels). Mitteilungen Institut [2 ]VSL Soil and Rock Anchors. Pamphlet bridge). Bauingenieur, 1987, pp. 557- für Bautechnik (IfBt), Berlin, 1/1988, pp. issued by VSL INTERNATIONAL LTD., 559. 12-15. Berne, Switzerland, 1987. [13] Broar över Angermanalven vid Sandö [25] Heavy Duty Composite Bar HLV made of [3]Specht M. et al.: Spannweite der (Bridge across Anger-Manälven to Polystal for Prestressing Tendons and Gedanken, zur 100. Wiederkehr des Sandö). Kungl. Väg-och Vat- Soil Anchors. Pamphlet by joint venture Geburtstages von Franz Dischinger (The tenbyggnadsstyrelsen, Sweden, 1949. Strabag Bau-oAG, Cologne/Bayer AG, span of ideas, on the centenary of the Leverkusen. birth of Franz Dischinger). Springer- [14] Virlogeux M.; La précontrainte extérieure Verlag, Berlin, 1987. (External Post-tensioning). Annales de [26] Gerritse A., Schürhoff H.J.: Prestressing I’Institut Technique du Batiment et des with Aramid Tendons. 10th International [4] Freyssinet E.: Une révolution dans les Travaux Publics (ITBTP), No. 420, Congress of the FIP, New Delhi, In dia, techniques des bétons (A revolution in December 1983. 1986, Proceedings Vol. 2, pp. 35-44. concrete techniques). Librairie de l’Enseignement Technique, Editeur Léon [15] Storrer E: Le pont de Sclayn sur la [27] Recommendations for acceptance and Eyrolles, Paris, 1936. Meuse (The Scfayn bridge across river application of post-tensioning systems. Meuse). Annales des Travaux Publics de Fédération Internationale de la Précon- [5] Dischinger F.: Elastische und plastische Belgique, 1959, No. 2, pp. 179-196 and trainte (FIP), London, UK, 1981. Verformungen der Eisenbetontragwerke No. 4, pp. 603-618. und insebesondere der Bogenbrücken [28] Thielen G., Jungwirth D.: Corrosion (Elastic and plastic deformations of [16] Müller Th.: Umbau der Strassenbrücke Protection of Prestressing Tendons. structures in reinforced concrete and über die Aare in Aarwangen (Recon- IABSE Symposium Paris-Versailles especially of arch bridges). Der Bauin- struction of the road bridge across the 1987. IABSE Report Vol. 55, pp. 73-78. genieur, 1939, pp. 53/286/426/563 and river Aare at Aarwangen). Schweizeris- International Association for Bridge and following. che Bauzeitung, 1969, No. 11, pp. 199- Structural Engineering (IABSE), Zurich, 203. Switzerland. [6] Schonberg M., Fichtner F.: Die Adolf- Hitler-Brücke in Aue (Sa.). (The Adolf [17] Mu//er J.: Construction of the Long Key [29] Dorsten V., Hunt F.F..Preston H.K.: Hitler Bridge in Aue [Sa.]). Die Bautech- Bridge. Journal of the Prestressed Epoxy Coated Seven-Wire Strand for nik, 1939, No. 8, pp. 97-104. Concrete Institute, November-December Prestressed Concrete. Journal of the 1980, pp. 97-111. Prestressed Concrete Institute, July- [7] Lippold P., Spaethe G.: Rekonstruktion August 1948, kpp. 120-129. der Bahnhofsbrücke in Aue (Reconstruc- [18] Podolny W., Muller J.: Prestressed tion of the railway station bridge in Aue). Concrete Segmental Bridges. John Wiley Bauplanung - Bautechnik, 1965, No. 9, & Sons, New York, 1982. [30] Chabert A. et al.: Injection à la cire pp. 435-438, No. 10, pp. 505-512 and pétroliére de cables de précontrainte No. 11, pp. 542-547. [19] Virlogeux M.: Bilan de la politique (mise en oeuvre en première mondiale d’innovation dans le domaine des au viaduc de la Boivre). (Grouting of [8] Hofmann G.,: Thürmer E.: Erfahrung bei ouvrages d’art (Evaluation of innovation post-tensioning tendons by means of der Sanierung der Bahnhofsbrücke Aue policy in civil engineering works). petroleum wax [world’s first application at (Experience with the rehabilitation of the Travaux, March 1985, pp. 20-34. the Boivre Viaduct]). Travaux, March railway station bridge Aue). Die Strasse, 1985, pp. 41-44. 1986, NO. 6, pp. 174-180. [20] Ivkovic M. et a/.: New Prestressed Concrete Hangar at the Belgrade [31] Wössner K. et al.: Die NeckartalbrUucke [9] Dischinger F.: Weitgespannte Tragwerke International Airport in Yugoslavia. 10th, Weitingen (Neckar Valley Bridge Weitin- (Large-span structures). Der Bauin- International Congress of the FIP, New gen). Der Stahlbau, 1983, No. 3, pp. 65- genieur, 1949, No. 7, pp. 193-199, NO. Delhi, India, 1986, Proceedings Vol. 1, 77 and No. 4, pp. 113-124. 9, pp. 275-280 and No. 10, pp. 308-314. pp. 239-244. [32] Hofmann E. Becker A.: Talbrücke [10] Dischinger F.: Stahlbrücken im Verbund [21] Concrete Storage Structures - Use of the <> - Entwurfs-varianten mit Stahlbetondruck-platten bei gle- VSL Special Construction Methods, May aus Ideen- und Angebotswettbewerb ichzeitiger Vorspannung durch hochwer- 1983. VSL INTERNATIONAL LTD., (Valley Bridge <> - tige Seile (Steel bridges combined with Berne, Switzerland. Design variants from idea proposing and reinforced compression slabs and post- tender competition). Bauingenieur, 1987, tensioned with high-strength cables). Der [22] Pelle K., Schütt K.: Post-Tensioning pp. 219-229. Bauingenieur, 1949, No. 11, pp. 321-322 System for Flue Gas Pipes of Power and No. 12, pp. 364-376. Plant Chimneys. FIP Symposium Israel [33] Talbrücke Obere Argen (Valley Bridge 1988. Obere Argen). Pamphlet issued by the [11] Müller P.: Brücken der Reichsautobahn joint venture «TalbrUucke Obere aus Spannbeton (Bridges of the Reich [23] Adeguamenti antisismici (Antiseismic Argen>>. motorway in post-tensioned concrete). equalizers). Pamphlet by ICOS S.p.A., Die Bautechnik, 1939. No. 10, pp. 128- Milan, Italy, September 1978. 135. 30 E XTERNAL P OST-TENSIONING

[34] Wittfoht H.: Outstanding and Innovative [45] VSL Post-tensioning. Pamphlet issued [57] Aalami B.O., Swanson D.T.: Innovative Construction Methods in Concrete by VSL INTERNATIONAL LTD., Berne, Rehabilitation of a Parking Structure. Structures - Recent and Future Trends Switzerland, 1980/1986. CONSTRUCTION INTERNATIONAL, (from the Viewpoint of the Main Contrac- February 1988, PP. 30-35. tor). 10th international Congress of the [46] Closing the gaps with assembly line FIP, New Delhi, India, 1986, Proceed- span placement. Engineering News [58] Post-tensioning turns inside out. ings Vol. 1, pp. 349-357. Record, Mc Graw-Hill, New York, Engineering New Record, Mc Graw-Hill; September 3, 1981. New York, March 12, 1987, p. 32FC. [35] Menn Ch.: Brückentrager mit Unterspan- nung (Bridge girder with underlying [47] FIP 9th Congress Stockholm 1982 June tendons). Schweizer Ingenieur und 6-10 (Extract from Annales des Travaux Architekt, 1987, pp. 200-204. Publics de Belgique No. 2 - 1982). Ministry of Public Works/Belgian Con- [36] Menn Ch., Gauvreau P.: Scale model crete Society, pp. 6/7. study of an externalty prestressed concrete slab bridge. International [48] MARTA Rapid Transit Bridges Going Up Conference on Cable-Stayed Bridges, Full Speed. Journal of the Prestressed Bangkok, Thailand, 1987, Proceedings, Concrete Institute, July-August 1983, pp. pp. 919-926. 184/185.

[37] Haas G. et al.: Die Stahlüberbauten der [49] MARTA Rapid Transit Bridges. Journal Far-Brücken, Dane-mark (The steel of the Prestressed Concrete Institute, superstructures of the Far Bridges, March-April 1985, pp. 188-194. Denmark). Der Stahlbau, 1985, No. 12, pp. 353-363. [50] US rapid transit bridge built span-by- span with precast elements. VSL News [38] Ritz P.: Beigeverhalten von Platten mit Letter August 1985, pp. 20/21. VSL Vorspannung ohne Verbund (Flexural INTERNATIONAL LTD., Berne, Switzer- behaviour of slabs prestressed with un- land. bonded tendons). Institut für Baustatik und Konstruktion ETH Zürich, Bericht Nr. [51]Virlogeux M., Placidi M., Hirsch D., 80, Birkhauser Verlag Basel und Lacoste G., Mossot J., Fesnais P., Colas Stuttgart, Mai 1978. M.: Le nouveau pont sur le Loir a la Fléche (Sarthe). (The new bridge across [39] Wittfoht H.: Betrachtungen zur Theorie river Loir at La Fléche [Sarthe]). Travaux, und Anwendung der Vor-spannung im July-August 1983, pp. 3-23. Massivbrückenbau (Considerations on the theory and application of post- [52] Bridge halves constructed parallel to the tensioning in concrete bridge construc- river and connected together after tion). Beton-und Stahlbetonbau, 1981, rotating. VSL News Letter May 1983, pp. No. 4, pu. 78-86. 21/22. VSL INTERNATIONAL LTD., Berne, Switzerland. [40] Zimmermann J.: Tragverhalten und Systemtragfähigkeit von Trägern mit [53] Autoroute A.55, Ouvrage d’art 33 Vorspannung ohne Verbund (Behaviour (Motorway A.55, Structure No. 33). ai-id ultimate strength of beams with Brochure issued by Dragages et Travaux unbonded prestressing). Thesis, Publics. Technische Hochschule Aachen, 1985. [54] Unconventional table layout for [41] Post-tensioned Slabs, January 1981/ incrementally launched bridge with 1985. VSL INTERNATIONAL LTD., varying spans. VSL News Letter August Berne, Switzerland. 1985, pp. 7/8. VSL INTERNATIONAL LTD., Berne, Switzerland. [42] DIN 4227, Teil 6: Spannbeton, Bauteile mit Vorspannung ohne Verbund [55] Skyway bridge boasts a record and (Prestressed Concrete, Structural parts innovations. Engineering News Record, with unbonded prestressing). Vornorm Mc Graw-Hill, New York, September 11, Mai 1982, Beuth-Verlag, Berlin und köln. 1986.

[43] SlA E 162: Betonbauten (Concrete [56] Ivanyi G., Fastabend M., Lardi R., Pelle Structures). Draft March 1987. K.: Statisch-konstruktive Verstarkung Schweizer lngenieur- und Architekten durch zusatzliche Vorspannung (Static- Verein, Zürich, pp. 13 and 29. constructive reinforcement by means of additional post-tensioning). Bautechnik [44] CAN 3 - A 23.3 - M 84: Oesign of 1987, No. 6, pp. 181-187. Concrete Structures for Buildings. Canadian Standards Association, Toronto, 1984, p. 170. 31

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AUSTRALIA CZECH REPUBLIC INDIA PERU USA VSL Prestressing (Aust.) VSL Systemy (CZ) s. r. o., Killick Prestressing Ltd., Pretensado VSL del Peru VSL Corporation Pty. Ltd. PRAHA BOMBAY SA, ATLANTA, GA THORNLEIGH, NSW 2120 Tel 42 - 2 - 242 252 96 Tel 91 -22-5784481 Fax 91 - LIMA Tel 1 - 404 - 446 - 3000 Tel 61 -2-4845944 Fax 42 - 2 - 242 254 31 22 - 578 47 19 Tel 51 -14-760423, Fax 1 - 404 - 242 - 7493 Fax 61 - 2 - 875 38 94 - 76 04 26 INDONESIA FRANCE Fax 51 - 14 - 76 04 77 VSL Corporation DALLAS, TX PT VSL Indonesia, Tel 1 - 214 - 647 - 0200 VSL Prestressing (Aust.) VSL France S.A. JAKARTA Tel 62 - 21 - 570 Fax 1 - 214 - 641 - 1192 Pty. Ltd. EGLY 07 86 Fax 62 - 21 - 573 12 PORTUGAL VIRGINIA, OLD Tel 33-1 -69261400 17 VSL Prequipe SA, LISBOA Tel 61 - 7 - 265 64 00 Fax 33 - 1 - 60 83 89 95 Tel 351 - 1 - 793 85 30 VSL Corporation DENVER, Fax 61 - 7 - 265 75 34 ITALY Fax 351 - 1 - 793 09 01 CO GERMANY VSL Italia S. r. I. Tel 1 - 303 - 239 - 6655 VSL Prestressing (Aust.) VSL Vorspanntechnik (D) MONTESE SINGAPORE Fax 1 - 303 - 239 - 6623 Pty. Ltd. GmbH Tel 39-59-981413 VSL Singapore Pte. Ltd., NOBLE PARK, VIC ELSTAL Fax 39 - 59 - 98 14 12 SINGAPORE VSL Corporation Tel 61 -3-7950366 Tel 49 - 33234 - 8340 Tel 65 - 235 70 77/9 HONOLULU, HI Fax 61 - 3 -795 05 47 Fax 49 - 33234 - 83416 JAPAN Fax 65 - 733 86 42 Tel 1 - 808 - 682 - 2811 VSL Japan Corporation, Fax 1 - 808 - 682 - 2814 AUSTRIA GREAT BRITAIN TOKYO SOUTH AFRICA Sonderbau GesmbH, WIEN Balvac Whitley Moran Ltd. Tel 81 - 33 - 346 89 13 Fax Steeledale Systems (Pty.) VSL Corporation MIAMI, FL 81 - 33 - 345 91 53 Tel 43 - 222 - 892 02 80 DERBYSHIRE DE55 4PY Ltd. Tel 1 - 305 - 592 - 5075 Fax 43 - 222 - 892 02 80 33 Tel 44 - 773 54 16 00 JOHANNESBURG Fax 1 - 305 - 592 - 5629 KOREA Fax 44 - 773 54 17 00 VSL Korea Co., Ltd., Tel 27-11 -6137741/9 VSL Corporation BOLIVIA SEOUL Fax 27 - 11 - 613 74 04 Prestress VSL of GREECE Tel 82-2-5748200 MINNEAPOLIS, MN Bolivia Jauregui Ltd., LA VSL Systems S.A., ATHENS Fax 82 - 2 - 577 00 98 SPAIN Tel 1 - 612 - 456 - 0985 PAZ Tel 30 - 1 - 363 84 53 VSL Iberica S.A., MADRID Fax 1 - 612 - 456 - 9281 Tel 591 - 2 - 321 874 Fax 30 - 1 - 360 95 43 MALAYSIA Tel 34 - 1 - 556 18 18 Fax 591 - 2 - 371 493 VSL Engineers (M) Sdn. Fax 34 - 1 - 597 27 01 VSL Corporation GUAM Bhd. PHILADELPHIA, PA BRAZIL VSL Prestressing (Guam), KUALA LUMPUR SWEDEN Tel 1 - 215 - 750 - 6609 Rudloff-VSL Industrial Ltda. TUMON Tel 60 - 3 - 242 47 11 Internordisk Spannarmering Fax 1 - 215 - 757 - 0381 SAO PAULO Tel 67-1-6468061 Fax 60 - 3 - 242 93 97 AB, DANDERYD Tel 55 - 11 - 826 04 55 Fax 67 - 1 - 649 08 50 Tel 46-8-7530250 VSL Corporation Fax 55 - 11 - 826 62 66 NETHERLANDS Fax 46 - 8 - 753 49 73 SAN JOSE, CA Civielco B.V, AT LEIDEN Tel HONG KONG Tel 1 - 408 - 866 - 5000 31 -71 -768900 BRUNEI DARUSSALAM VSL Hong Kong Ltd. SWITZERLAND Fax 1 - 408 - 374 - 4113 Fax 31 - 71 - 72 08 86 VSL Systems (B) Sdn. Bhd. Quarry Bay, HONG KONG VSL (Switzerland) Ltd. VSL Corporation BANDAR SERI BEGAWAN Tel 852 - 590 22 88 NEW ZEALAND LYSSACH Tel 673 - 2 - 22 91 53, Fax 852 - 590 02 90 Precision Precasting Tel 41 - 34 - 47 99 11 WASHINGTON, DC -221827 (Wgtn.) Ltd., OTAKI Fax 41 - 34 - 45 43 22 Tel 1 - 703 - 451 - 4300 Fax 673 - 2 - 22 19 54 VSL Redland Concr. Prod. Tel 64 - 694 81 26 Fax 1 - 703 - 451 - 0862 Fax 64 - 694 83 44 THAILAND CHILE Ltd. VSL (Thailand) Co., Ltd., VIETNAM Sistemas Especiales de Quarry Bay, HONG KONG NORWAY BANGKOK VSL North East Asia Construccion SA, VSL Norge A/S, Tel 66-2-237 32 88/89/90 HONG KONG SANTIAGO Tel 852 - 590 03 28 STAVANGER Fax 66-2-238 24 48 Tel 852 - 590 22 22 Tel 56 - 2 - 233 10 57 Fax 852 - 562 94 28 Tel 47-4-563701 Fax 852 - 590 95 93 Fax 56 - 2 - 231 12 05 Fax 47-4-562721