Augustus Bridge in Narni (Italy): Seismic Vulnerability Assessment
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This is an Accepted Manuscript version of the following article, accepted for publication in NTERNATIONAL JOURNAL OF ARCHITECTURAL HERITAGE, 2017, 1:5, 717-746. It is deposited under the terms of the Creative Commons Attribution-NonCommercial License (http://creativecommons.org/ licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited https://doi.org/10.1080/15583058.2017.1300712 Augustus Bridge in Narni (Italy): Seismic Vulnerability Assessment of the Still Standing Part, Possible Causes of Collapse, and Importance of the Roman Concrete Infill in the Seismic-Resistant Behavior a a b a Elisa Bertolesi , Gabriele Milani , Fulvio Domenico Lopane , and Maurizio Acito a b Department of Architecture, Built Environment and Construction Engineering, Technical University of Milan, Milan, Italy; Department of Civil Engineering, Newcastle University, Newcastle, UK ABSTRACT The final results of advanced FE analyses performed on a Roman arch bridge, namely the Augustus Bridge (Ponte di Augusto) in Narni, center Italy are presented. The bridge, one of the most impressive Roman artworks, has been injured by several traumatic events during the millennia, the result of which is its present ruined condition. The aims are manifold, starting from a better understanding of the causes at the base of the partial collapse occurred on the central pier, passing through a seismic assessment of the ruined still standing part and ending with a discussion on the role played by Roman concrete on the stability against horizontal actions. An advanced material model exhibiting damage, plastic deformation, and softening in both tension and compression is adopted for Roman concrete. Both the case of a foundation settlement of the central pier and the application of a seismic excitation are investigated, by means of nonlinear static and nonlinear dynamic analyses. Numerical simulations are carried out within the FE code ABAQUS by means of detailed 3D models, using historical documentation and previous results of the latest research carried out on materials, assuming realistic models on both the uniaxial stress-strain relationships under nonlinear load- unload conditions by using independent damage parameters in tension and compression, and the multiaxial behavior ruled by a regularized Drucker-Prager strength criterion. The methodological approach turns out to be potentially valid for all existing Roman bridges. Results highlight the vulnerability of the ruins, that the collapse of the central part was probably due to settlement of the central pier and that Roman concrete plays a crucial role in increasing the stability against earthquake actions. KEYWORDS Earthquake; masonry arch Roman bridges; nonlinear dynamic analysis; nonlinear static analysis (pushover); Roman concrete; spectral response analysis 1. Introduction This was not the only bridge built in the 2nd century BC. Such a century was without any doubt a period of The Ponte di Augusto (Italian for “Bridge of great works, many streets were paved with flint in Augustus”) in Narni, Italy is one of the most important Rome, graveled and fixed out of the cities and many artworks ever built by the Romans, most of which are arch bridges were built in many places. The Ponte still in service nowadays. Milvio belongs also to this period: it was built, using Roman bridges were an essential part of the great road stone, in 109 BC. system of ancient Rome, which at the time of Diocletian The construction of bridges and viaducts grew and reached a length of about 53,000 miles (approximately extended gradually to the entire Roman Empire during 80,000 km—hence the saying “all roads lead to Rome”), the 1st century BC. With Caesar’s death in 44 BC and and which gave rise to several road systems of many the rise of Augustus, and the consequential beginning modern nations around the Mediterranean sea. of the Imperial Age, there has been, in Rome and Several historical sources, as Cecchi (2003) and throughout the Empire, an impressive reorganization Galliazzo (1994), indicate the Ponte Emilio as the first of many urban plants and energetic restoration of artifact of stone built in Rome in 179 BC. Most prob- numerous road layouts with the resulting construction ably, the bridge initially had a mixed structure, with or rebuilding of many bridges. stone piers and wooden superstructure, which was From the late Republican bridge, meant as civil replaced with stone arches in 142 BC. structure of crossing, in the Augustan age the bridge CONTACT Gabriele Milani [email protected] Department of Architecture, Built Environment and Construction Engineering, Technical University of Milan, Piazza Leonardo da Vinci 32, Milan 20133, Italy. started to be considered as a monument, a tangible Galliazzo 1994) trace the cause of the collapse of the II, expression of unity and power of the Roman Empire. III, and IV arch, to a flood of the Nera river, that swept the From this period is the Bridge over the Nera river at bridge following the collapse of a dam upstream. Narni on the Via Flaminia, which numerous historical In Cecchi (2003), however, based on analyses and references, as Cecchi (2003) and Galliazzo (1994), studies conducted on the bridge, the author traces the describe as one of the marvels of Roman engineering. causes of the collapse to a progressive foundation set- By way of example Figure 1 shows the images and tlement of the pier II. As a matter of fact, on the basis the relative sizes of some of the main and most impor- of geometrical surveys and historical constructive con- tant Roman arch bridges with a Roman concrete filling, siderations, it is believed that the original configuration which clarify the high level of engineering reached by was the one with 4 arches with the springers of circular the Romans in the construction of bridges, with works arch II at the same height. The lowering of about 2.9 m that were overcome only in the modern era, when the of the dosseret block of pier II, according to Cecchi construction of bridges has been able to rely on materi- (2003), is due to the foundation settlement of the pier als such as steel and reinforced concrete. itself occurred during 2,000 years. In Figure 2, the Ponte di Augusto is illustrated, in At present, a large amount of technical literature dealing the most reliable reconstruction of the original config- with the historical/structural analysis of masonry arch uration (Cecchi 2003; Galliazzo 1994). bridges and masonry arches in general is at disposal, as The numeration of piers and arches is done in Oliveira, Lourenço, and Lemos (2010), Cavicchi and ascending order from left to right (or from South to Gambarotta (2005), Cavicchi and Gambarotta (2006), North).The existing parts of the bridge are darker, Reccia et al. (2014), Milani and Lourenço (2012), while the hypothetical reconstruction of the original Audenaert and Beke (2010), Fanning, Boothby, and bridge is clearer. The right arch is modern, the abut- Roberts (2001), Martín-Caro and Morales (2007), Pelà, ment was perforated to allow the passage of the railway Aprile, and Benedetti (2009), Pippard (1948), Gilbert and line Rome-Ancona. Melbourne (1994), Gilbert (2006), De Felice and De Santis The bridge now presents a ruin configuration (2010), Audenaert et al. (2008), Drosopoulos, Stavroulakis, (Figures 1 and 2), with a single arch survived, very and Massalas (2006), Fanning and Boothby (2001), and similar to the one celebrated in past centuries by impor- Zeman et al. (2008). Sometimes, to be practically more tant artists (among them Jean-Baptiste-Camille Corot efficient, such literature simplifies either the real 3D geo- (1796–1875), who depicted the bridge over the years metry to 1D/2D problems or uses inadequate material 1826–1827 in a painting exhibited in the Louvre models. Obviously, such structural models involve differ- Museum—Paris). In this, and in other images that depict ent levels of accuracy and simplifications, which limit their the bridge in the 17th century, the bridge appears as range of applicability to specific cases. The most common shown in the dark part of Figure 2, except for the hole idealizations of masonry material behavior are elastic, plas- for the passage of the railway of the twentieth century, tic and nonlinear (for a detailed discussion the reader is and the final collapse of the pier II, occurred in 1885. referred to, e.g., Reccia et al. 2014; Milani and Lourenço In addition to the impressive majesty and dimension 2012), but the most diffused theoretical approach, particu- of the bridge, to determine the significant structural larlyinthe case of masonryarch bridges, still remains interest to the Ponte di Augusto is its state of ruin, limit analysis (Cavicchi and Gambarotta 2005, which allowed an in-depth study. Considerably inter- 2006; Drosopoulos, Stavroulakis, and Massalas 2006; esting, from the structural point of view, are the geo- Gilbert 2006). Limit analysis provides very quickly failure metrical and mechanical insights reported in Cecchi mechan-isms and an estimation of the load carrying (2003), concerning the concrete filling and the traver- capacity of the structure. Besides the historic rules, the tine coating. With this information it was possible to classic approach to determine the stability of arch bridges have an appropriate level of knowledge to identify the is probably due to Pippard (1948), but nothing is said vulnerability of the structure, both in the current ruin about the role played by out-of-plane horizontal loads. configuration and in the plausible original configura- Transversal effects may be very important from a prac- tion, which enabled the bridge to be in service for more tical point of view, playing a crucial role in the decrease of than 1,000 years after the construction. the load bearing capacity or in case of earthquakes (Milani Although there had already been a partial collapse and Lourenço 2012; Pelà, Aprile, and Benedetti 2009).