SEISMIC VULNERABILITY OF TIMBER ROOF STRUCTURES: AN ASSESSMENT PROCEDURE

The objective of synthetic procedures for assessing the seismic vulnerability of structures is to sort out by inspection and limited elaboration the characteristics and critical features that may induce an inadequate seismic response. A procedure for assessing the vulnerability of timber roof structures, which are present in the majority of traditional masonry buildings in many countries and highly condition the building response, has been the object of a long-term research project and has been developed as follows: First, a visual inspection is performed, according to a predefined sequence of operations with the guidance of a form, or template, to be filled. Visual inspection is the basis of any in situ assessment of timber structures, aimed at providing information about their structural soundness. In this case, focus is on the characteristics that influence most strongly the response to seismic action. Information is collected following the template’s tree-like structure, with branches following in increasing detail the different aspects to be considered. The exam goes from the description of the structural configuration down to individual timber members and to joints and construction details, covering geometry, the materials, and the state of conservation and including possible modifications of the original layout. In a second step, this information is used for checking and grading selected structural characteristics that condition the seismic response and can be considered as vulnerability indicators. Four vulnerability classes, from A to D, have been defined for grading purposes, from the lowest vulnerability level, equivalent to a satisfactory situation, to the highest one, requiring thorough strengthening interventions. Main indicators are, The conceptual design: usually, roof structures were conceived for carrying vertical loads; their capability to respond to horizontal actions like earthquake-induced inertia forces is often less developed depending on the structural typology; correct sizing of elements is also considered here; The carpentry joints, which must maintain connection in dynamic conditions; The system of constraints between timber structure and supporting walls, which is critical because loss of support often triggers progressive failure of the whole building system; The state of the structure: this point includes any condition specifically related to the current situation of the structure that may affect vulnerability, including the state of conservation and previous strengthening interventions, if any. A particularly critical issue is the effectiveness of supports. Its evaluation has required developing specific models and synthetic parameters suitable for use in vulnerability assessment. Finally, the implementation of the procedure template as a software tool is under way.