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The concept of precast segmental construction with external tendons has been developed extensively since 1978, starting with the construction of the Long Key Bridge (Florida). Since this first experience, many other structures (more than 5,500,000 ftý of deck) have been designed and successfully built using the same method. The performance of all bridges now in operation has been excellent. However, some questions were raised in the minds of engineers, unfamiliar with the method, regarding the behavior of structures prestressed with external tendons beyond the range of design loads (serviceability limit state). Because continuous reinforcement is not usually provided across the match cast joints between segments, concern was expressed that adequate ultimate behavior and sufficient strength could not be obtained. To provide a satisfactory answer to these legitimate questions, a special computer program (DEFLECT) has been developed to analyze accurately the response of the structures prestressed by external tendons. Moreover, several tests are available to confirm the behavior of such structures while verifying the validity of the DEFLECT computer program. This design tool has been used to predict the structural behavior of simply supported and continuous structures beyond joint opening, up to ultimate capacity of the girders with and without thermal loads. Several different prestressing methods have been analyzed with different bonding conditions at the point of deviation of the external tendons. It was found systematically that structures prestressed with either internal or external tendons behave essentially the same way at all loading stages up to ultimate.

Experience gained in the design of several externally prestressed bridges is reported. New cables especially developed by contractors are discussed. Also, the main experimental results gained at the author's institute for reinforced concrete are given.

A simple methodology for the solution of beams prestressed or partially prestressed with external or unbonded tendons in the linear elastic cracked and uncracked range of behavior is described. It leads to equations allowing the computation of stresses in the concrete section, the tensile reinforcing steel, the compression reinforcing steel, and the prestressing steel. In particular, it is shown that the stress in unbonded tendons is a function of the applied loading, the steel profile, and the ratio of the crack width (or crack band width) to the span. These factors can all be accounted for through the use of a strain reduction coefficient ê for the uncracked range of behavior and a similar coefficient êc for the cracked range of behavior. It is shown that, when the strain reduction coefficients ê and êc are taken equal to unity, the solutions developed here revert to the solutions developed earlier for partially prestressed beams with bonded tendons.

SP120 External prestressing--that is, the use of unbonded prestressing tendons outside the concrete section of a structural concrete member--offers substantial economic savings and a dramatic increase in construction speed, making it an extremely attractive option for the construction of new concrete structures, particularly bridges. It is also a logical choice for the rehabilitation and strengthening of many existing structures. ACI's symposium volume, External prestressing in Bridges, provides the last word on this important technological development Based on experience in both the U.S. and Europe, the book addresses: *The state of the art, *Technologies for construction, *Applications in new construction and rehabilitation, *Testing and experimental observations, *Analysis and design considerations. Covering practical construction information as well as analysis and design, this is required reading for anyone involved in building bridges.

After 10 or so years of research and practical application, the external prestressing of concrete is now becoming a normal procedure, soon to be codified in some countries. Among the technical solutions applied to a large number of works, in the U.S.A. and France particularly, some emerge as being the best examples both from the point of view of performance and economy. This is the case where normal strands in HDPE ducts are used with an injected grout. This technique can be used equally well with removable external prestressing and with external prestressing that is partially bonded. In difficult cases, such as very long structures (bridges with lengths exceeding 200 m or 600 ft) or structures with high curvature (tanks and various vessels) or where it is difficult to bring large jacks up to the anchorages, an external cable, formed of projected strands, gathered together in the same HDPE duct and isolated from one another, is a very effective and elegant solution with an unequalled degree of protection and with the opportunity to check the prestressing force throughout the entire life of the structure.