Experimental modal and FEM analysis of a historical building

Abstract

The CEDRAV building in Cerreto di Spoleto, originally a monastery, was built in the 14th century on the top of a rock ridge. The two seismic events that occurred on September 26th, 1997, with magnitude Ms=5.4 and Ms=6, respectively, caused some damages to it, but the event occurred on 14 October 1997 caused the main damages. This study includes analyses mainly related to the part of the data acquired corresponding to tests and configurations using temporary deployments. The building is very complex and rigid. In fact, translational and torsional frequencies are close to one another, coupling occurs and damping ratio is low. Coupling of the frequencies and the low damping are two factors that cause beating effect when shaking is strong enough. For example, the approximate beating period is computed for the June 28th 2000 earthquake as (taking T1 = 1/9.7 = 0.104 s, and T2 = 1/10.4 = 0.096 s) Tb = 2.5 s, observable in the time history plots. On the basis of the experimental results obtained by means of temporary arrays, a permanent network was designed and installed, which recorded several seismic events. A preliminary analysis in frequency domain, based on the determination of auto and cross power spectral density, has been first performed; subsequently the modal structure identification by means of the Ibrahim time domain technique and the Frequency Domain Decomposition technique in the frequency domain, have been carried out. A frequency domain analysis of the structure has also been performed by means of the Frequency Response Function (FRF) analysis. The experimental results have been finally compared with those obtained from a FEM.