Assessment and strengthening of an historic timber roof in Milan

An historic timber roof structure, belonging to the church of St. Ambrogio in Rozzano, close to Milan is here examined. The main structure, from XVII cent., is made by four timber trusses, with double posts (“alla palladiana”) and composed tie beams, spanning around 12 m. In XIX century the posts were shortened, in order to not rest on horizontal tie beams, according to truss theory, obtaining very highly deformable structures. In this way, also the out-of-plane behaviour is affected, requiring an additional truss structure realized in timber planks connecting the posts in the longitudinal direction. A comparison with similar interventions made in the same period in Venice (Arsenale) on the same type of trusses could help to support this assumption. Following a laser-scanner survey, useful for determining the global positioning of different elements and having a first estimate of the dimensions, a detailed survey of geometrical deviations (such as out-of-plumb, deflections, swinging) has been recorded. Moreover, a non-destructive survey campaign has been planned for investigating the timber quality and estimating its resistance, with the aim of maintaining the ancient structures, keeping them in use in the future. In this way, a strengthening intervention can be designed in order to obtain a “reinforced timber truss”, by the insertion of external iron cables in order to increase the stiffness of the existing trusses and eventually the load capacity of the roof. Particular care are taken in assuring the continuity in the connections among the pieces of the composite tie beams, and some wedges between posts and tie beams will be inserted assuring the continuity of the truss. Some simple FE analyses show the effectiveness of the adopted solution.