PSI - Issue 64

Leonardo Paris et al. / Procedia Structural Integrity 64 (2024) 2222–2229 Leonardo Paris, Maria Laura Rossi / Structural Integrity Procedia 00 (2019) 000 – 000

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of alignments and lying planes of the surviving elements are linked to a hydrogeological context that has changed greatly over time, both due to natural events and anthropic influence. Today the Augustus bridge retains only one of the ancient four round arches, with raised doors and at variable heights compared to the upper frames of the pylons aligned in order to manage the difference in height between the left and right banks. Three of the four strings belong to an original phase with A-B-A rhythm; the fourth was added later as can be seen from the different distance C and also from the different technique of laying the ashlars on the intrados; a fifth arch was built at the end of the 19th century for the railway tunnel which resulted in the total loss of the stone cladding of the right shoulder. The surviving arch of the left shoulder, well preserved also from the point of view of the external coverings thanks to its raised position compared to the maximum level recorded during a flood of the river, was certainly affected by the sudden absence of the large central arch which had a main role in balancing the thrusts: marked fractures of the kidneys, a strong lowering of the key and a rotation of the pylon towards the river bed were found. The collapse of the main central arch and the third one is certainly due to the inability of the pylon, which supported them both, to resist the impetuous floods of the river. It should be highlighted that the lack of rostrums leads one to think that in the original construction phases the pillars were external to the riverbed: this would also justify the presence of a larger arch than the two lateral ones. Today the pylon is lying on the ground and broken into 3 large portions which show its core and the arrangement of the travertine blocks held together by metal clips. This detailed information package on the construction and installation technique of the various elements allowed an equally detailed analysis from the point of view of stereotomy: there are rigid rules in cutting and equipping the segments with a rhythm and metric always attributable to the unit of measurement of the time, the Roman foot (29.65 cm). It is therefore the stone ashlar that "modulates" the entire infrastructure. For this reason it was decided to approach modeling in a BIM environment starting from some basic typological elements - the central ashlar, the corner one, the upper one with frame, etc. - modeled parametrically as multiples of the 29.65 cm module, and positioned following the equipment strategies found, with alternating cutting and banding. This process allowed us to simulate in a digital modeling environment the different installation techniques of the ancient Roman construction site, from the initial phase with three arches - 66p-104p-66p -, to the addition of the fourth - 53p -, to the broken bridge in the state of ruin we know today. (Figure 3) Similarly from the point of view of the cognitive approach, but in a dissimilar way if we consider the modeling strategies in a parametric environment, the case of the Fabricio Bridge which connects the Tiber Island with the left bank of the Tiber was addressed (Orlandi, 2008). Bridge built in 62 BC with blocks of travertine with a structure of two round arches and the center of the impost lower than the plane of the pylons; has a single central pylon in which a smaller arch was inserted to limit the thrust of the current and guarantee the outlet of the flood - two other outflow arches were also present in the lateral shoulders, the one on the right incorporated into the buildings of the island, the one on the left and incorporated in 1870 in the construction of the Tiber walls (Betocchi, 1900; Cozza, 1907). Unlike the Augustus Bridge which has significantly changed its shape over time, the Fabricius Bridge maintains its original structure substantially unchanged apart from some changes in the facings and brick inserts instead of travertine blocks. The central pillar, framed by two pilasters and supported by a tuff slab, is protected upstream by a powerful asymmetric triangular water-cut square, on which there is a triangular cap and a semi-pyramidal closure that reaches up to the threshold of the central fornix; downstream there is a rounded, semicircular reverse, without an upper closure. The vaulted structures and the intrados of the arches are in rammed stone while the head arches are in travertine with a toothed ashlar arrangement. In this case the installation technique used for the arrangement of the ashlars is that of the square work with lime bedding to improve the adhesion between the joints - there are no traces or evidence of the use of metal clamps. Also in this case, a metric-proportional analysis of the structure was conducted by comparing the geometric matrices deduced from the survey model with the Roman foot. The two arches, net of specific deviations due to the longevity of the building, measure 86 feet in diameter, 10 feet apart on the impost plane; on the axis of symmetry is the fornix with an arch of 20 feet. Despite the clear reference to the unit of measurement of time in the general composition of the structure, there is no "modulating" principle such as to proceed with the same approach used in modeling the Bridge of Augustus, above all due to the various remodeling of the external cladding. The modeling in the BIM environment was therefore carried out following the classic principles of decomposition and recomposition of the various macroelements, using as a guide the geometric matrices attributable to the Roman foot and reference planes deduced from the analysis of the point cloud. (Figure 4)