ULTRABRIDGES

The new footbridge spanning over a water channel, has the shape of a circular bow with a cross-section of hollow box girder. The total length of the girder is 22.1 m and consists of five pre-cast elements; each has length of 4.4 m. The total height of the girder is only 0.4 m. The side walls of the girder are inclined towards each other under the angle of 25°. The thickness of the side walls is 100 mm. The thickness of the top deck is only 80 mm and it is supposed to carry load from 12-t vehicle.

         The minimal concrete class used for deck is C170/200. A high-quality calcinated bauxite has been used as the aggregate. The grain size of the aggregate is in range 0-6 mm. White Portland cement with rapid hardening process has been chosen for the bridge as a binder. The water-cement ratio is very low, around 0.17. The proper hydration and thixotropic behaviour in fresh state is assured by other admixtures and additives such as super-plasticizer and un-hydrated micro-silica. The required colour of the bridge is creamy beige. For this reason a pigment compound must be accounted in the admixture. The pigment should not decrease final strength of concrete significantly. The amount of the pigment should not be larger than 5 % of cement content. The whole deck is reinforced both by straight steel fibres and classical reinforcing bars. Only this combination of bars and fibres maximises utilization of UHPFRC in terms of ductility and prevention from a brittle behaviour.

         The bridge deck is post-tensioned by five bonded strands. Each strand has 13 wires. Hence the whole cross-section is under compression of 17 MPa. The anchorage of the pre-stressing system is same at the both ends of the deck. Active pre-stressing is carried out only at one end of the bridge. Due to large slenderness of the bridge and relatively large pre-stressing force, special attention must be paid for potential stability issues, splitting, bursting of tendons or in the field of dynamics. Especially dynamics is the problematic issue for the bridge since the natural frequency is only 2.3 Hz.

         Unlike the deck, the railing is made from High-Strength Concrete C110/130 and elements are cast separately and fixed later on the already post-tensioned deck. The railing does not contribute to overall carrying capacity of the bridge. The railing has a bionic shape and struts are randomly distributed. Thanks to high strength of the concrete, the struts are only 50 mm thick. Each strut is reinforced with two steel bars. The railing is attached to the deck by a special rail anchor system, which is already cast into the deck.