In September 2002 the Municipality of Portogruaro, a town in North-Eastern Italy, asked the University of Trento to install and operate instrumentation to monitor the tilt of its Civic Tower. This building is an ancient masonry bell-tower, of height 51 m overall, which today shows an inclination measured as 1.197 m at the north-east corner. Although the inclination was and is striking, at the time there was no real specific concern about stability because the Tower had always been known to be leaning, as documented in local chronicles. Also, there was no evidence of inclination still in progress, and such an occurrence was judged at the time to be very unlikely.

The monitoring system started recording the inclination in October 2003. In September 2004, the University reported to the Municipality that the changes in inclination recorded during this period were too small to raise concern over the short-term safety of the Tower.

In September 2005, after almost two years of recording, the University told the Municipality of Portogruaro that analysis of the data acquired to date allowed calculation of a possible linear trend of 1.7 mm per year. In particular, the direction of the possible motion was very close to the maximum lean direction. Nonetheless, the leaning progression was too small and the monitoring period too short to state with certainty whether the trend calculated was a sign of an ongoing process.

During the third year of monitoring, the University recovered some unpublished historical documents. Among these, an original design project revealing an elevation built in 1879, and documents reporting old measurements of the inclination dating 1962 and 1997. Based on this new information and on the fresh instrumental data, the University concluded, in the annual monitoring report issued in September 2006, that an increasing inclination was very likely.

This final judgment, in sharp contrast with the initial view, might appear surprising. In reality, the change of opinion is the result of a rigorous and quantitative logical analysis of the information available at the time.

This paper follows this logical route again, showing how the initial view evolved to final awareness via a probabilistic model, based on Bayesian logic. In the next Section we introduce the Tower at issue, its history and the monitoring system installed; Section 3 formulates the algorithm used to update the posterior judgment based on the information acquired; the application of this procedure to the Civic Tower is reported in Section 4; finally, the outcomes are discussed at the end of the paper.

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Figure 1. Overview of the Tower (a); North and East

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