LONG SPAN BRIDGES: THE EVOLUTION OF DECK STIFFENED SYSTEMS AND A CASE STUDY ON THE SEISMIC BEHAVIOUR OF A MAILLART- ARCH- TYPE BRIDGE, VIADOTTO OLIVIERI (SA)

This doctoral thesis deals with the design evolution of long span bridges and the corresponding technical improvement which has led to change the role of deck stiffened system through centuries. All typologies (suspension bridges, cable –stayed bridges, deck arch bridges, bowstring arch bridges) usually adopted to cover long distances, have been taken into account, reading in the same perspective the process which allows their structural optimization. Through the back analysis of about 100 existing bridges, this work investigates on main changes occurred for deck stiffened system, underling the corresponding effects on long span bridge structural behaviour. The central thread of this dissertation is to underline that to cover 150m longer spans, it has been necessary to pass form flexural regime of structures having shorter spans, which act like simple beam-bridges, to the extensional regime of slender long span bridges, whose strict succession of deck cross sections guarantee them to carry loads mainly through tensile or compression strength: bending moments are greatly reduced, while torsional effects are often counteracted by rigid deck box sections. In order to emphasize the effects on structural response that deck characterization could have, peculiar case studies have been analysed. Modern design approach, here explained, inevitably having a relevant effect on bridges aesthetic and their relationship with the surrounding contest, leads to an innovative conception of deck – stiffened system, completely modifying classical basis bridge design.