The Antrenas Tubular Arch Bridge, France

For the Antrenas interchange, the architects originally proposed a concrete arch bridge, with no intermediate supports at this relatively open and visible location. When the project was presented to SETRA, the French National Highway and Road Department, an alternative arch solution was proposed by the consulting engineer. The alternative was based on a previous, unrealized design for an arch made of concrete-filled steel pipes with a prestressed concrete deck slab; the slab being connected to the arch by a triangular spatial steel truss. One of the goals of the design was to eliminate vertical supports between abutments and arch. A polygonal arch was selected since the thrust in a circular arch is only marginally balanced by the weight of the tube. Even when filled with concrete, its self-weight would have only balanced one-tenth of the radial force generated by the arch thrust.