Interpretation of collapse modalities for a timber roof truss

Roof structures in historical buildings are often based on complex timber trusses; this is typically the situation of theatre buildings, where a wide room is present and large spans have to be covered. Several studies have been done on such structural systems showing that, in the case of complex roof systems, connections between timber elements were conceived in the past mainly to resist vertical loads corresponding to symmetrical patterns, with no consideration of eccentric loads and horizontal forces. In the paper, this problem is investigated with reference to the specific case study offered by a theatre, where a queen post truss was adopted for the roof system and the sudden collapse of one truss has occurred. The theatre, which is located in northern Italy, was built at the beginning of the 19th century. Although some modifications were made through time, the roof structure has preserved the original configuration. A precise identification of the real structural conditions has been obtained on the basis of the results of laser scanning and photogrammetric surveys. On this basis, numerical models have been developed in order to analyse a variety of load scenarios compatible with the observed structural collapse, with the final purpose of highlighting possible deficiencies in the original structural configuration, in line with the collection of similar cases which are reported in the literature.