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 Copyright 1992 by VSL INTERNATIONAL LTD., Switzerland - All rights reserved - Printed in Hong Kong. E XTERNAL P OST-TENSIONING 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