| Popis: |
A frontier in earthquake engineering remains between engineered, modern code designed buildings and the stock of older traditional buildings to be protected by acting on their different critical features. In the Southern European building tradition, timber structures have been systematically adopted for floor and roof systems. In many cases, such structures, sometimes built centuries ago, have not undergone decay and can still successfully sustain the design loads. In the traditional design approach, however, resistance was provided towards gravity loads only, with no consideration of the possible occurrence of horizontal actions. In the case of roof systems, seismic damage observation has revealed a number of structural characteristics that are determinant in the response to seismic action, thus suggesting 1) to develop specific procedures for vulnerability analysis, and 2) to define rehabilitation and strengthening interventions specifically conceived and effective for seismic action. For the analysis of vulnerability, a recently developed procedure requires to evaluate, on the one side, the conceptual design and the structural details and, on the other, the interaction between the roof and the underlying structure. During an earthquake, the roof system can either play a positive linking role for the walls of the building below or, on the opposite, trigger its collapse if a suitable path is not provided to the inertia loads. With reference to this problem, a critical situation may arise in the very frequent case of a roof system constituted by a series of parallel trusses resting on two masonry walls; in such a situation, if the seismic motion acts in the direction of the roof trusses, the supporting walls are unfavourably loaded out-of-plane in bending. This situation frequently occurs in churches, in relation to the main nave walls and roof system and favored by the structural characteristics and dimensions involved. The recent Italian earthquakes of L’Aquila, 2009, and Emilia-Lombardia, 2012, have been particularly devastating to heritage church buildings due to the roof-wall interaction effect; the relatively moderate magnitude involved indicates the critical situation of these buildings and their strong need for improvement. The seismic behaviour of the masonry walls and roof system is affected by several factors; mainly, by the roof mass which depends on the span length and the typology, by the wall dimensions and mechanical properties, and by the restraint conditions between the roof structure and the supporting walls. This problem is discussed here in terms of the typical collapse mechanism, which permits to highlight the effect of the basic parameters on the general structural response through consideration of the limit equilibrium condition. The procedure is, then, applied to typical example cases, leading to the evaluation of the collapse acceleration. The proposed criterion is suitable for pointing out critical situations simply from visual inspection and basic measurements, and may therefore be effectively applied in vulnerability analyses. |
