Field observations and failure analysis of the Basilica S. Maria di Collemaggio after the 2009 L’Aquila earthquake

On Monday, the 6th of April, 2009, a devastating earthquake struck L’Aquila causing the partial collapse of the Basilica S. Maria di Collemaggio, an important symbol of the city. The mechanism of the transept structure’s failure, which left the external boundary walls almost undamaged, probably due to the sudden collapse of the large multi-lobed pillars at the end of the nave, is discussed in the paper by different points of view. A brief historical review of the monument restorations is followed by the analysis of the damage scenario recorded during the post-earthquake inspections. Finite element models of the Basilica, updated on the basis of available dynamic tests, have been used to perform a seismic assessment by response spectrum analysis according to the current Italian code, showing a high vulnerability, in the transversal direction larger than in the longitudinal one. The AQK earthquake accelerograms, recorded close to the site, have evidenced a prevailing component almost aligned with the longitudinal Basilica axis; the intensity is comparable with the one provided by the code, with exception in the vertical one greater than expected. Static nonlinear analyses have furnished the crack propagation in the masonry walls due to the increase of longitudinal horizontal loads. The presented failure description obtained by structural modeling is coherent with the direction of the registered polarized seismic action and it is compatible with the observed damage and with most of the debris positions coming from the collapse of the transept structures.