International Concrete Abstracts Portal

Design considerations of externally prestressed large-span reinforced concrete girders with tendons completely outside the cross section are dealt with. The analyzed systems are two-chord structural systems. The lower, downward convex tensioned chord usually consists only of prestressing tendons, while the upper, compressed chord is a reinforced concrete straight-line or upward convex polygonal girder. The desired configuration of tendons is achieved by compressed elements interconnecting the two chords at suitable distances. In such a way, the rise of tendons can be several times larger than the height of the reinforced concrete section, thus greatly increasing their efficiency compared to the classical internally or externally prestressed girders. An important characteristic of such structural systems is that adding a very small prestressing force reduces the deformation. Therefore, the dead load deflection can be easily controlled by the suitable choice of prestressing force. The time-dependent deflection is not considerably greater than the elastic one, even for a very high creep and shrinkage, as it is also primarily governed by the shape and deformation of tendons. Because of such properties, these structural systems are exceptionally favorable for roof structures of medium and very large spans but can also be successfully used for highway bridges. Due to the significant reduction of the chords' cross-sectional areas and the bending stiffness of such structural systems, the design has to be done using the second-order theory. The criteria for cases when it is notnecessary are discussed. Besides the theoretical analysis, some experiences in design and construction of the new hangar at the Belgrade International Airport in Yugoslavia, whose 135.80 m (445 ft) span main roof reinforced concrete girders are externally prestressed with tendons free in space outside the concrete cross section, are also presented.

The behavior of two segmental concrete girders incorporating external tendons was compared to that of a similar girder with internal tendons. The girders were 31 ft long and consisted of 11 match-cast segments. Test variable was the location of the tendon ducts. In the first girder, the ducts were embedded in the girder cross section. The ducts of the second girder were external to the concrete cross section except at pier segments and intermediate deviation diaphragms. The third girder was similar to the second except that portions of the external ducts were embedded in a second-stage concrete cast. The segments included multiple shear keys and were dry jointed. All ducts were grouted. Each girder was simply supported over a 30-ft span and loaded statically to destruction under a two point load. The first and third girders attained their respective flexural strengths predicted by the classic bending theory for monolithic girders with bonded tendons. The second girder exceeded the flexural strength predicted by the provisions of the AASHTO specifications for members with unbonded tendons.

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.

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.