Damping Factors and equivalent SDOF definition in the displacement-based assessment of monumental masonry structures
The recent Italian seismic decrees (OPCM 3274/2003 and OPCM 3431/2005) have strongly modified the safety concept for existing buildings. This code introduces the idea of design and verification through performance-based analysis. Unfortunately, in case of monumental heritage, the OPCM 3431 decree does not propose a specific methodology. For masonry historical structures, the available literature on this topic generally deals with the out-of-plane overturning mechanism, considering the wall as an oscillating rigid block (e.g., façades or bell towers). As a matter of fact, for these structures or macroelements, the fundamental periods are large, even for the initial elastic range, and they can increase because of cracking in masonry, typically considered as no-tension material. The effects of these features on the dynamic response and the related prediction of displacement capacity need to be investigated. For this purpose, non-linear dynamic analyses on the equivalent SDOF systems are performed, employing different input ground motions. These analyses may be useful to evaluate the influence of factors such as: 1) the feasibility of the equivalent SDOF system definition for the displacement-based procedure (as for mainly rocking system, the model is derived by Housner’s non-linear oscillator, having elastic behaviour, with softening range); 2) the equivalent viscous damping relationship and the consequent displacement response spectrum reduction for the long-period range.