SULL'ORIGINE DEI PONTI AD ARCO IN LEGNO: IL RUOLO DEI COLLEGAMENTI

In this study two timber bridges are analyzed, which constitute examples of structures with arch behaviour obtained assembling straight elements of reduced dimensions: Traian’s bridge on Danube River and the military bridge drawn by Leonard from Vinci. For each one, starting from available surveys in literature, a structural definition based upon an accurate interpretation of the behaviour of connections between the straight elements, needed for arch configuration, has been made. Leonard’s military bridge shows an arch configured by rectilinear rods, disposed on staggered planes, jointed by lashings. The exam of some papers in the Atlantic Code, relating to this bridge and to studies on details of connections made by lashings, has allowed the recognition of the military bridge joints as an application of the mechanism used by roman carpenters in building Caesar’s Bridge on Rhine River, following the interpretation given by Palladio. Joint’s behaviour schematization, introduced in a proper numerical modelling, has allowed verifying the real arch behaviour of the structure drawn by Leonard. For Traian’s Bridge, a new hypothesis on structure design has been proposed, different from those existing in literature, but more appropriate for technological level of its building times; the given interpretation recognizes in this work the first example of a bridge with an underlying polygonal centring. An experimental study on scale models has allowed introducing in the numerical modelling different degrees of stiffness of the joints between straight elements making the arch and double hanging posts. From numerical results the need of spacing transversal joists comes out at the aim of induce an equilibrated distribution of normal stresses between the three polygonal profiles of the arch structure.