International Concrete Abstracts Portal

The use of tendons placed outside the concrete for the longitudinal prestressing of bridge decks is particularly well suited to the triangulated trusses that have now proven effective. Experience acquired first on the Tehran Stadium roofing (internal tendons) and then on the externally prestressed Bubiyan Bridge in Kuwait has highlighted the numerous advantages to be derived from the latter system. The technology has, therefore, been used again, with external tendons, in the construction of the Sylans and Glacieres viaducts located along the Macon-Geneva-Mont Blanc highway in France, on behalf of the Societe des Autoroutes Paris Rhin-Rhone. The two viaducts have a total length of 1500 m. Each consists of two parallel decks, 10.75 m wide. Typical spans are 60 m long. The deck is precast in forms as a series of 4.66 m long elements and erected by sequential cantilevering using a launching girder.

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.

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.

Approximately 40 percent of the bridges in the United States are classified as deficient and in need of rehabilitation or replacement. Of these bridges, many are classified as deficient because their load-carrying capacity is inadequate for today's increased traffic. This insufficient load-carrying capacity has resulted from poor maintenance, increase in legal load limits, deck overlays, changes in design specifications, and other factors. In response to the need for a simple, efficient procedure for strengthening existing bridges, the authors have been investigating the use of post-tensioning. They have investigated its use on simple span bridges as well as continuous span bridges. Various post-tensioning schemes have been tested on laboratory models and actual bridges. The paper briefly reviews the post-tensioning research that has been completed by the authors in the past few years. This work indicates that post-tensioning is a viable, economical technique for flexural strengthening of steel-beam composite-concrete deck bridges.

A laboratory investigation was performed to study deviation saddles, a type of tendon deviator used in externally post-tensioned precast segmental box girder bridges. Ten reduced-scale models of deviation saddles were fabricated and loaded to ultimate using a specially designed testing apparatus that applied load to each deviator just as would be applied to a deviator in a bridge. The objectives of the study were to: experimentally investigate the strength and ductility of deviators; evaluate deviator details in light of observed performance; define behavioral models for deviators; determine the effects of using epoxy-coated reinforcement; and establish design criteria. Data from the test series are presented, two analysis techniques are formulated, and design recommendations are made for design of tendon deviators.