THE FOURTH ARCH OF THE AUGUSTUS BRIDGE AT NARNI (ITALY): A CASE STUDY OF ROMAN ARCH WITH RIBS

The roman bridge said of Augustus (27 BC) is a monumental viaduct on the Nera River, north of the town of Narni. The bridge is made up of four arches, of which only the first survives to the south, has always been a source of admiration for the size and the shape and has been the subject of technical studies and artistic representations. The remains of the fourth arch, the northernmost, show a very particular ribbed appearance, noted by many authors, among all: Balance, Choisy and Galliazzo. Some authors indicate the fourth arch to be similar in some respects to other arched structures (bridges or basilicas roofs), for example to the roofing vault of the temple of Diana in Nimes or to the Roman bridge in Nimreh, ancient Namara, in Syria. The authors of the present study, through a bibliographic research, having verified the diffusion of this shape in other Roman contexts, believe that at to date there do not seem to be other similar examples. The fourth arch appears to be unique in relation to the arrangement of the voussoirs, which are set to be more connected than a simple overlap of elements. However, the different radial depth of the voussoirs could have caused less effectiveness, especially in relation to possible movements related to the thrust of the arch on the springer and in concurrence with the second (from the south) pier subsidence with possible detachment and the consequent collapse of the central voussoirs. The subsidence of the second pier, which finally collapsed in 1885, is indicated by many authors as the main cause of the collapse of the central arch. In this work, the study of the subsidence and collapse of the second pier is also investigated through 3D FEM modeling of the pier and of the entire bridge. Finally, a set of 3D FEM models was presented to analyze the structural feature of the fourth arch of the Augustus Bridge, the executive assembly modalities, especially in the keystone area, compared to the same arch without emptiness among the ribs.