Steel and composite tied-arch bridges: a conceptual approach to structural design

The behaviour of tied-arch bridges with composite decks (bowstring bridges) is overly sensitive to the different choices made by the designer in the first phases of the conceptual approach, mainly in the design of the arch and the deck. The geometry of the arch shape, the design of the arch–tie joint and the construction sequence can significantly modify the global behaviour in terms of stress state and deformed configuration. After a general discussion on the different parameters affecting the behaviour of bowstring bridges with steel–concrete composite decks, the consequences of choices made in the conceptual design phases are discussed, analysing the effects of each choice on the global behaviour of the bridge with particular focus on the potential for significant concrete slab cracking and on the reinforcement amount to be used in the deck slab for limiting the crack widths. Acting on geometry, materials and construction sequence, the bridge behaviour can be optimised, and examples of this approach are shown for two actual cases of bridges in southern Italy.