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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.

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.

Over 600 bridges composed of adjacent prestressed concrete box beams were built in the early 1950s in Pennsylvania. The box beams were placed side by side and had an asphalt wearing course on top. Span lengths ranged form 30 to 60 ft. Their design was very conservative by today's standards. The concretes used in these bridges have high chloride contents; water leaks down through the joint between box units and the strands often have inadequate cover. Thus, it is not surprising that many of the box beams are deteriorating due to corrosion of their prestressing strands. This project was directed toward developing economical repair schemes for these bridges. The literature survey did not reveal any schemes specifically applicable to adjacent box beams. Two external reinforcement repair schemes were developed and trial installations were made on a bridge near York, Pa. Both schemes included the removal of deteriorated concrete, placement of external reinforcement beneath the beam, and application of shotcrete to the soffit of the beam. In Scheme 1, the external reinforcement consisted of epoxy-coated reinforcing bars. This repair method restored ultimate flexural capacity but did not restore lost prestress. It was the least costly of the two methods. In Scheme 2, post-tensioned, epoxy-coated strand was used. This restored the full ultimate flexural capacity and most of the lost prestress. Difficulties were encountered in installing anchors for the post-tensioned system, but its performance was good. The bridge was tested after repair. The external reinforcements were found to be fully composite with the original beams. The tests also revealed the lateral distribution of wheel loads. In spite of the poor condition of the bridge, the wheel loads were well distributed laterally, leading to a structure that was stronger and stiffer than expected.

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.