Hypothesis on the Decrement of the First Natural Frequencies of the Santa Maria Di Collemaggio Basilica from Three Years Monitoring: The Role of the CLT Roof

The long term dynamic monitoring of the Santa Maria di Collemaggio basilica revealed an unexpected trend of the first natural frequencies decreasing from their initial values estimated in 2018. The decrement of the natural frequency could originate from several factors. The structural system derives from the arrangement of four building materials: masonry (predominant), reinforced concrete (RC), timber and steel. Masonry is unlikely to suffer rapid decaying of their mechanical properties. Reinforced concrete (RC) and the embedded steel should not present decaying phenomena in the first years after built. Like- wise, the steel plates which connect the Cross-Laminated Timber (CLT) panels should not manifest a significant progression of corrosion in an indoor environment. Conversely, timber's physical and mechanical properties, such as swelling and shrinkage, density, modulus of elasticity, strength have a time-dependent response, even in the long-term. Likely, the decaying of the natural frequencies depends on the CLT roof's time-dependent be- haviour due to the potential modification of its mechanical properties and boundary conditions (interaction with the steel plates, e.g.). This paper presents selected results from the basilica's three-years continuous dynamic monitoring and discusses an elementary mechanical model representative of the dynamics transverse to the nave walls. The mechanical model described by a limited set of parameters drives the assessment of the CLT roof's possible role in causing the detected decrement of the natural frequencies.