The Piston-Stayed Bridge

The Tervate bridge, situated over the river Yser in Belgium, introduces a new type for mobile stayed bridges: the piston-stayed bridge. The design of the bridge considers the statics and kinematics as a whole and not as an accumulation of individual design issues. This design approach stems from the idea of maximum member reduction which favours the introduction of elements that fulfil more than one function. Thus, the engineering design innovation lies in the use of one single element, i.e. the piston stay, for actuation as well as support of the mobile bridge section. The bridge, designed for vehicular traffic, pedestrians and cyclists, consists of a 16 m long central mobile section, adjoined on either side by an 11 m long static section. The deck of the static bridge sections consists of a 0.25 m reinforced concrete slab spanning the two steel girders. In the mobile section, a light steel orthotropic deck spans the two steel girders. A pair of masts is situated at the hinging point of the mobile bridge section. Each of the masts is connected to the mobile section by a fixed-length stay and to the static bridge section by a variable-length piston stay. The fixed-length stay forms a non-deformable triangle with the mast and the mobile section. The piston stay powers the opening and closing mechanism, and forms a deformable triangle with the mast and the static section. In effect, the piston stay uses the mast as a lever to tilt the bridge open or closed, while the dead weight of the mobile section ensures that both stays are always in tension. The Tervate bridge is the first instance of this new mobile stayed bridge typology.