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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.

Evaluation of existing bridges is an important part of the strategy of dealing with the deteriorating infrastructure. Load and resistance parameters are random variables because of uncertainties in load components, material properties, and dimensions. Therefore, the reliability of a structure is a convenient measure of its performance. The load and resistance models are first summarized. The major load components in bridges are dead load and live load. The live load model is based on the weigh-in-motion studies. Girder distribution factors are derived using special computer procedures for bridge analysis. Behavior of composite girders is considered using a nonlinear model. The basic characteristic of the section is the moment-curvature relationship. The reliability is measured in terms of the reliability index. The approach is demonstrated on evaluation of a prestressed concrete girder bridge. Three cases are considered: original design condition, damaged with corroded strands, and repaired by external prestressing. The load components and load-carrying capacities are evaluated and then the reliability indexes are calculated for the three cases.

A research project is currently underway at the Swiss Federal Institute of Technology, Zurich, to establish the feasibility of an alternative structural system for short-span highway bridges. Concerns over the long-term durability of structural systems currently used in the 25 to 40 m span range provided the primary motivation for the study. The proposed system consists of a solid concrete slab that is externally prestressed. The external tendons are deviated at the third points of each span using struts. A 1:3-scale model bridge has been constructed and is currently being tested to verify the behavior of the bridge under permanent, service, and ultimate static loads, as well as dynamic and fatigue loads. The favorable results obtained thus far have confirmed the feasibility of the proposed structural system, and will serve as a basis for extending the concept to spans greater than 40 m in length.

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.

The development of external prestressing has been one of the major trends in construction during the last 10 years, along with the development of concrete cable-stayed bridges and the increasing use of high-strength concrete. After a historical review, the main principles of the design of externally prestressed bridges are presented. The paper then details the influence of the construction method on the external tendon organization: bridges built span by span, bridges built by the cantilever method or by methods which are mechanically equivalent, and bridges built by the incremental launching method. Some practical problems are presented, such as handling heavy jacks. A last chapter is devoted to composite structures, with concrete top and bottom slabs and steel webs, prestressed by external tendons. French experimental constructions of this type do not appear economically interesting, and prestressing classical composite structures is not yet considered as good a solution for the same reason. But external prestressing is now widely developed for concrete bridges in the United States, France and, more recently, in Belgium, Switzerland, Venezuela, Germany, and Czechoslovakia.