Role of model identification and analysis method in the seismic assessment of historical masonry towers

The seismic assessment of historical monuments typically involves many issues, the most crucial of which are: retrieving geometrical and mechanical data; implementing a suitable numerical model; choosing the more appropriate type of analysis and properly interpreting the results. The more well-managed these issues are the better the reliability of the seismic assessment. The Civic Tower of L’Aquila (Italy) is considered in this paper as a typical case in which lack of data and discrepancy of sources make the seismic assessment hard to be suitably performed. The key role of the identification of the numerical model was firstly evidenced. Different numerical models were implemented to identify the best-fitting geometrical and mechanical properties of the structure. The considered case-study also led to the discussion of some issues concerning the identification of the numerical model of historical towers. Code-compliant linear and non-linear dynamic analyses were then carried out to compare the seismic performance of the tower assessed through the different methods based on regulations. A concrete damage plasticity model for masonry and a set of spectrum-consistent earthquakes were adopted. The stress maps, the horizontal displacement peaks (useful to avoid the pounding phenomenon with the adjacent Margherita palace), and the evolution of damage were investigated. The study highlighted that the widely adopted response-spectrum modal analysis (RSMA) may underestimate -even remarkably-the displacement de-mand compared to the more rigorous time-history linear analysis (THLA), this being an inconsistency of the code-compliant linear analyses. On the other hand, the results of the non-linear time-history analysis (NLTHA) confirmed the displacement demand predicted by the THLA, while the damage evolution under tensile stress was found in agreement with the crack pattern detected on the structure. The influence of the vertical component of the earthquake was also investigated, founding that the vertical component of the ground motion does not significantly affect the results.