Reducing the seismic vulnerability of timber roofs: a case study

The earthquakes occurred in Italy in the last decades have shown a strong influence of the timber roof structures on the response of masonry buildings. The roof system may play a positive role linking and stabilizing the walls or, on the opposite, may suffer damage and often trigger collapse of the masonry structure. Architectural heritage, and especially church buildings, have particularly suffered collapse related to malfunctioning of the roof system, often generating pounding on walls, or resulting in progressive collapse from the roof to the underlying slabs or vaults. In order to reduce the risk of damage to roof structures, often themselves precious wooden artifacts, and to the building, it seemed necessary to define specific criteria for assessing seismic vulnerability and suitable intervention criteria, compatible with conservation requirements. Previous work was carried out, within a national research program (Reluis-DPC), with the objective of developing synthetic criteria permitting to assess vulnerability by inspection, with simple observations and some limited measuring operations. An assessment procedure was defined, based on estimating factors that influence seismic behaviour, like unrestrained thrusts, conceptual design and typology, dimensions of timber elements, conditions of supports and of carpentry joints, and state of conservation. Corresponding vulnerability indicators and grading criteria were developed. A complex roof, covering a large, century-old mansion which had not been recently maintained, has now been examined as case study. Its seismic vulnerability has been estimated according to the procedure; criticalities have been pointed out; on this basis, the analysis of possible intervention strategies and consequent reduction of vulnerability levels has been performed, with costs-benefits considerations. . The application of the seismic vulnerability assessment procedure has resulted capable of indicating clearly and in an organized form the critical aspects of the structure and the interventions necessary for its improvement; seismic improvement has appeared feasible in terms of costs and benefits.