Đề tài Numerical analysis of externally prestressed concrete beams - Bùi Khắc Việt

Acknowledgements First of all, the author would like to express his deepest gratitude and appreciation to his advisor, Prof. Umehara Hidetaka of Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, for his invaluable guidance, continuous motivation, patient explanations, useful comments and understanding that have been the great source of support throughout the course of this research. His tireless devotion, unlimited kindness even for the private matters has earned the author’s highest respect. Only his ingenious ideas, constructive suggestions and tireless guidance made the completion of this study possible. The author wishes to express thanks to the members of his dissertation examining committee, Prof. Umehara, H., Prof. ….and Prof. ….for going through the text of this thesis painstakingly and for making enlightening suggestions and comments that helped to refine the scope and content of this study. The author wishes to express special thanks to Prof. Tanabe Tada-Aki of Department of Civil Engineering, Nagoya University, for his continuous guidance and discussions, invaluable suggestions from the days of the author’s studying under his supervision up to present. That is in addition to his going through the text of the journal papers making enlightening suggestions and useful comments through the course of study. The author also wishes to express his sense appreciation to Associate Prof. Uehara Takumi of Department of Civil Engineering, Nagoya Institute of Technology, for his assistance during the course of study. Thanks are also expressed to Research Assistant Mr. Kimata, H., Dr. Ito, A., of Concrete laboratory of Department of Civil engineering, Nagoya University for his help in various kinds of matters. Special thanks are expressed to Dr. Shahid, N, Mr. Brohi, K. and all Vietnamese friends in Aichi Ken for their help and assistance to overcome the daily difficulties. The author likes to express special appreciation go to Mr. Hirahara, H., Mr. Ushida, K. and all members of Concrete laboratory, Nagoya Institute of Technology for their assist to overcome the daily difficulties and for their friendly attitude during the whole research period. Grateful acknowledgement is given to the Ministry of Education Science and Culture of Japan, since it has generously provided the financial support, which has made it possible for the author to pursue this course of study. The author would like to express special thanks to Dr. Do Huu Tri, Doctor General of the Research Institute for Transportation Science and Technology (RITST) of Vietnam, and to Dr. Nguyen Xuan Dao, former Doctor General of RITST for their continuous supports in various kinds of matter that allowing the author completes the course of study in Japan. Grateful appreciations are also expressed to all members of Department of Bridges and Tunnels of RITST for their help and continuous cooperation in the current research field. Last but not the least, the author would like to express his deep sense of gratitude to his wife for her patience, understanding and moral support, for her high limit state of endurance over the years, which made the full completion of this dissertation a reality. About all, the author wishes to express his deepest sense of respect, for which words are not enough, to his mother for her tenderness, love, care, sacrifices and encouragement. -1- Chapter 1 INTRODUCTION 1.1 GENERAL One of the latest developments in prestressed concrete technology has been the use of external cables, which may be defined as a method of prestressing where major portions of the cables are placed outside the concrete section of a structural member. The prestressing force is transferred to the beam section through end anchorages, deviators or saddles. This type of prestressing could be applied not only to new structures, but also to those being strengthened. Substantial economic and construction time saving have been indicated for this innovative type of construction. External prestressing system was used in the bridge construction in the early days of prestressing. However, due to a generally inadequate technology, external prestressing has received a bad image and was almost abandoned in the 1950’s. This is because the corrosion problem for the external cables was serious, and the internal prestressing system with the bonded cable was emphasized. With the development of partial prestressing techniques and protective system for the external cables, it is possible to have structures with external cables, whose performance is as good as the structures with bonded cables. In recent years, external prestressing revives in the construction of new structures and has a great development in the bridge construction. The deterioration of existing bridges due to increased traffic loading, progressive structural aging, and reinforcement corrosion from severe weathering condition has become a major problem around the world. The number of heavy trucks and the traffic volume on these bridges has both risen to a level exceeding the value used at the time of their design, as a result of which many of these bridges are suffered fatigue damage and are therefore in urgent -2- need of strengthening and repair. A method for strengthening and rehabilitation of such structures has become increasingly important. External prestressing is considered one of the most powerful techniques used for strengthening or rehabilitation of existing structures and has grow recently to occupy a significant share of the construction market. The adoption of external cables has been proposed as a very effective method for repairing and strengthening damaged structures. Although external prestressing is a primary method for rehabilitation and strengthening of existing structures, it is being increasingly considered for the construction of new structures, particularly bridges. Since the external prestressing system is simpler to construct and easier to inspect and maintain as compared with the internal prestressing system, the beams prestressed with external cables have attracted the engineer’s attention in recent years, and it has been proposed in the design and construction of new bridges. A large number of bridges with monolithic or precast segmental block have been already built in the United States, European countries and Japan by using the external prestressing technique. Recently, a new type of structures using the external cables or combination with either bonded cables or unbonded cables has been increasingly developed around the world such as externally prestressed concrete bridges consisting of concrete flanges and folded steel web or extra- dosed bridges with a short tower. In this chapter, the definition of post-tensioned prestressed concrete beams and classification of beams prestressed with external cables is initially presented. The application of external prestressing is discussed together with its advantages and disadvantages. The historical development of external prestressing is also discussed, following by literature reviews of the previous studies. A general overview of problem arisen from the application of external prestressing is highlighted. The differences between internally unbonded cables and external cables at all loading stage are also briefly presented and discussed. Finally, the objectives and scope of the present study as well as the organization of the course of study are defined and given at the end of this chapter. 1.1.1 Definition of post-tensioned prestressed concrete beams An initial distinction is helpful when dealing with definition of prestressed concrete structures. A post-tensioned prestressed concrete beams may be classified as either bonded or unbonded. Frequently, the prestressing cable is placed inside the concrete cross section and bonded by filling the ducts with cement grout after the desired prestressing force has been -3- applied; this is called as conventional prestressing or conventionally prestressed concrete beams. On the contrary, the ducts may be left empty, or filled with grease, in this case the bond between the concrete and the prestressing cable is eliminated, friction inside the ducts is artificially reduced to minimum value and the cables transfer their load to the concrete beam through the end anchorages and the deviators, the terms “unbonded” and “external“ prestressing are adopted. The term “unbonded prestressing” is used if when the cable is placed inside the cross section and friction between the duct and the cable is equal to zero. Whereas, the term “external prestressing” is used if the cable is placed outside the cross section and attached to the beam at some deviator points along the beam. Prestressed concrete beams may also be classified as either fully or partially prestressed. Fully prestressed beams contain only prestressing cables, whereas partially prestressed beams contain bonded non-prestressed reinforcement in addition to the prestressing cables in the tension zone. Depending on the extent of bondage between the concrete and the prestressing cables, all the prestressed concrete beams can be mainly divided into two groups, namely, prestressed concrete beams with bonded cables and prestressed concrete beams with unbonded cables. And in each group, beams can be divided into small subgroup. For example, beams prestressed with bonded cables may be classified either perfectly bonded or partially bonded Fig.1.1 Classification of prestressed concrete beams Unbonded Prestressed Concrete Beams Prestressed Concrete Beams Internally Unbonded Bonded Prestressed Concrete Beams Perfectly Bonded Partially Bonded css εε Δ=Δ ∫ Δ=Δ l css dxl 01 εε Externally Unbonded cscs l csss dxl K εεεε Δ+⎟⎠ ⎞⎜⎝ ⎛ Δ−Δ=Δ ∫01 ???=Δ sε Assumption for computing method of cable strain -4- cables, whereas beams prestressed with unbonded cables may be classified either internally unbonded cables or external cables. Fig.1.1 shows the classification of prestressed concrete beams. In this figure, the equations of cable strain for each group are also presented, except the equation for externally prestressed concrete beams, which is a main target of this study and will be presented in Chapter 3 and Chapter 6. 1.1.2. Classification of externally prestressed concrete beams Generally, external prestressing is defined as a prestress introduced by the high strength cable, which is placed outside the cross-section and attached to the beam at some deviator points along the beam. Fig.1.2 shows a typical view of a concrete box girder bridge prestressed by external cables. Fig.1.2 Typical view of a prestressed concrete box girder bridge with external cables Fig.1.3 Classification of externally prestressed beams Deviators are located within the depth of cross section Location of deviators Deviators are located outside the depth of cross section Conventionally prestressed beams with external cables Externally prestressed beams with large eccentric cables Arrangement of external cables Cable Deviator Cross Section Deviator Cable Cross Section Arrangement of external cables -5- In an external prestressing system, depending on the location of deviators, there are two kinds of the beams. Deviators are placed within the depth of cross section, the term “conventionally prestressed beam with external cables“ is used, and otherwise the name “beam with large eccentric cables“ is adopted (see Fig.1.3). 1.1.3 Advantages and disadvantages of external prestressing External prestressing, initially developed for bridge strengthening, is now used for new bridges, particularly for precast or cast-in-place concrete segmental bridges. New design concepts and prestressing techniques have been developed to implement external prestressing, especially in France, the United States and Japan. These efforts were undertaken because of the following advantages of external prestressing: • External prestressing leads to simple cable layouts, with very limited angular deviations, reduce friction losses and improve the concreting conditions by eliminating ducts from webs. • Concreting of new structures is improved because there are no cables inside the section (external prestress only) or there are fewer cables (internal combined with external prestress). • Dimensions of the concrete cross section, especially, the webs can be reduced due to the partial or full elimination of internal cables (deadweight reduction). • Profile of external cable is simpler and easier to check during and after the installation. • Grouting is improved because of a better visual control of the operation and therefore, a better protection of prestressing cable is obtained. It is also possible to easily inspect the cable during the entire life of the structure. • External cable can be removed and replaced if the corrosion protection of the external cable allows the release of the prestressing force (for instance, cables made of individually lubricated sheathed strand. • Friction losses are significantly reduced because external cables are contacted to the structure only at the some deviator points and anchorages. • The main construction operations, concreting and prestressing are more independent of one another. Therefore, the influence of workmanship on the overall quality of the structure is reduced. -6- Though the above-mentioned advantages are attractive, some shortcomings are nevertheless encountered due to external prestressing, which are as follows: • Because the external cables are located outside the cross section of the beam, they have to be protected from corrosion by the high density polyethylene (HDPE) ducts, which results in a higher initial material cost for the prestressing system over that of the internal cables. • External cables do not participate in the local crack control. • The strain difference between the cable and the concrete may lead to movements of the cable over the deviators and thus to friction corrosion. • The external prestressing system is transferred the force to the beam via the anchorages and the deviator points along the beam. Therefore, the anchorages and the deviator points must carry the high concentrated forces under the applied load. Consequently, they become the critical regions of the structures. They must be designed to support large longitudinal or transverse forces, and their connection to the cross section usually introduces shear transfer in the form of concentrated load acting on the cross section. These elements should be carefully detailed and adequately reinforced. • In the deviation zones, the high transverse pressures are acting on the prestressing cable. The saddle inside the deviation zones made of metal tubes or sleeves should be precisely installed to reduce friction as much as possible and to avoid damage to the prestressing cable, which could lead to the strength reduction. • The behavior of anchor head of the external cable is more critical. Failure of the anchor head of an external cable means a complete loss prestress in that cable. Therefore, the anchor head should be carefully protected against corrosion. • At the ultimate state, failure with a little warning due to insufficient ductility is a major concern for externally prestressed structures. • Under the ultimate bending condition, more prestressing force is required to generate the ultimate strength similar to that of internally bonded cables. • External cables are subjected to vibrations and, therefore, their free length should be limited. • External cables might be susceptible to fire damage. -7- 1.2. LITERATURE REVIEW 1.2.1 Historical development and application of external prestressing External prestressing was a mode of construction in the early days of prestressing. Several bridges were built for example in Germany first, with the Adolf Hitler Bridge at Aue in 1936, designed by Franz Dischinger. In Belgium then, under the influence of Magnel with the Sclayn bridge in 1950. And in France between 1950 and 1952, the bridge at Villeneuve-Saint- Georges, designed by Lossier, the bridge at Vaux-Sur-Seine and port a Binson, built by Coignet, and bridge at Can Bia. These first attempts, however, did not produce excellent results. Most of these externally prestressed structures suffered from corrosion. This experience gave a poor image of external prestressing, and very few externally prestressed concrete bridges were built in the sixties and in the seventies except for a series of road bridges in Belgium were built between 1960 and 1970, and in England the Bournemouth Bridge and the Exe and Exminster viaducts. After lying dormant for some time, external prestressing has been rediscovered as an attractive application of prestressing. Under the influence of French engineers-Jean Muller in the United States and SETRA (Service Technique des Routes et Autoroutes) in France, external prestressing has been made possible by the development of the prestressing technology, and numerous structures have been designed and built with external prestressing around the world, especially, in Europe and the United States. One of the recent projects is the construction of the second stage expressway system in Bangkok, Thailand, which commenced in 1989 where external cables and dry jointed precast segmental desk were used1). The Shigenobu river bridge was the first externally prestressed segmental type bridge in Japan. And from the experience of that bridge, numerous bridges have been designed and constructed with external prestressing in Japan up to present. The development of high capacity cables has resulted in a reduction in the number of external cables, which eases design and construction. And above all, the experience of strengthening some classical prestressed concrete bridges, in which the initial prestressing forces were not great enough, has made it possible to put into use protective systems adapted to ensure resistance to corrosion of external cables. Furthermore, experience in strengthening these bridges, which perforce had to be placed the cables outside the concrete section, made designers aware of the advantages of external prestressing. This led them to consider its use in building new bridges. The principal -8- advantages are the considerable simplification of the cable layout and the large reduction in losses of prestress due to friction. In recent years, the external prestressing technology is widely used in the construction of concrete bridges. Highways and elevated railways are being constructed using the external prestressing with precast segments. Another application of external prestressing is the strengthening or rehabilitation of existing concrete structures, which is restored for economical of legal reasons instead of being demolished. Furthermore, the application of this technology have paved way to many innovative structures. Extra-dosed bridge is one such example where the cable is placed above the girder over the supports in continuous bridges, similar to the cable stayed bridge, but with a short tower. External prestressing has been applied also in composite bridges such as steel beams with a concrete top slabs, or other a) Extra-dosed bridge with a short tower b) Bridge with large eccentric cables Fig.1.4 New type of bridges using external cables -9- combinations of steel elements and concrete slabs. Recently, external prestressing is increasing