Numerical and operational modal analyses of the “Ponte delle Torri”, Spoleto, Italy

The Ponte delle Torri is a large historical construction that connects Colle Sant’Elia with Mount Monteluco in Spoleto, Italy. It was probably built in the XIII century, possibly on Etruscan or Roman ruins. The bridge superstructure is made up of a pedestrian deck, provided with a water canal on one side, supported by lancet arcades and stone piers known as “towers”. As testified by recent documentation and onsite inspections, the bridge’s state of damage is rather widespread and extended, urging local and national institutions to start suitable structural monitoring activities, which were included within the RoMA (Resilience enhancement of a Metropolitan Area) project. The project comprises many monitoring techniques (highresolution photogrammetry, 3D laser scanning, FBG monitoring, satellite InSAR, etc.) to assess the bridge cracks pattern and its structural health by a multidisciplinary approach that permits mutual validation. In particular, the present paper describes the numerical and operational modal analyses performed for the study of the structure and the methodology for the validation of the implemented Finite Element Models (FEMs). As a first step for the study of the bridge, the modal analysis of a preliminary FEM was executed in order to have an idea of the dynamic behavior of the structure and to guide the design of the on-site sensors positions. In such positions ambient noise was measured with velocimeters. Then, more refined FEMs were constructed and validated. In particular, mono-, bie tri-dimensional FEMs were generated through different numerical techniques and their resulting modal analyses were compared with the results of Operational Modal Analysis (OMA) techniques applied to the recorded velocimeters data. eas, such as cultural heritage assets, also through the integration of the widest variety of monitoring techniques able to provide a more complete understanding of ancient structures (De Canio et al. 2015). The procedure applied is based on the combined use of finite element modelling and in situ environmental experimental testing. In particular, part of the experimental results have been used to calibrate and validate the finite element model. 2 THE “PONTE DELLE TORRI” OF SPOLETO 2.1 History and main characteristics The “Ponte delle Torri” is a huge ten-arcade masonry infrastructure that connects two hills, Colle Sant’ Elia and Monteluco. In ancient times, it used to be an aqueduct, probably built in the 13 century on previous structures exploited as foundation, and a bridge at the same time. It is constituted by a roadway and a water canal in which the water used to flow exploiting the slope of the artificial track, located on top of a continuous wall prolonging the height of the parapet on the south side of the roadway. The deck is supported by ogive arches on stone piers, historically named “Torri”, or towers in English (Figure 1). The water network has undergone several changes: in 1891, the “Ponte delle Torri” was a key element of the water system of Spoleto and was used as a road link between Monteluco and Colle Sant’Elia. The waters came to the bridge through three pipelines, and, before being transported to the city, served as the driving force of two mills. Figure 1. General view of the Tower Bridge of Spoleto. 2.2 The bridge structure The bridge has a stone structure with shoulders and piers (“towers”) made up of rubble walls in a square matrix assembled by mortar and lime. The structure is only apparently regular. In fact, piers and arches have all different shapes and sizes, as well as the walls towards Colle Sant’Elia and Monteluco are also diverse in the masonry texture, due to their different construction timing and subsequent rebuilding interventions. The piers differ from each other even in their development in height: piers closer to Monteluco are more massive than the others and are reinforced by arches raised about half of their height, while clear spans between piers closer to Colle Sant’Elia are longer than the others (Figure 2). Figure 2. Longitudinal section of the bridge from south. Piers are number from West to East. With reference to the side of the “Rifolta”, and in particular in the lower portions of the piers, it is built as “booklet masonry”, while, on the side of the Rocca it is built as well-arranged masonry stones, characterized by perfectly vertical corners made with squared elements often well linked and interconnected by a thin layer of mortar. The towers are joined at the top by arches made up of rather squared stone blocks. The wall bearing water canal has a thickness of about 2.00 m, while its height is variable (greater towards Monteluco and smaller towards Colle Sant’Elia). The bridge has an overall length of about 230 m, while the highest tower is over 70 m (Sansi 1984). 2.3 Analysis of the current state The current state of damage, and more particularly the cracks in the bridge, are closely related to the time of construction, the quality of the materials, as well as to subsequent structural interventions related to its architectural modifications and functional