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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.

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.

The state of Texas is involved in two projects that use precast concrete segmental erection methods with external post-tensioning tendons. Design and construction features of these projects, along with construction problems related to external tendons, are described. The future of segmental construction and use of external tendons in Texas are discussed.

The bridge that links Re Island and the mainland was completed in April, 1988. It crosses a 3000 m wide sea channel that separates the island from the town of La Rochelle, on the west coast of France. The deck of the bridge is a concrete box girder built by the balanced cantilever method with precast segments. The prestressing tendons are partly inside the concrete and partly external.