Marranzini, Dante (2023) Multi-scale characterization of existing timber structures for seismic vulnerability and durability assessment. [Tesi di dottorato]

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Abstract

Italy is one of the European countries most exposed to seismic risk, due to the frequency and intensity of earthquakes affecting its territory, as well as the high vulnerability of the built environment. For this reason, seismic prevention and risk mitigation policies assume great importance in national territorial planning. However, without an adequate vulnerability assessment of the built environment, it is not possible to accurately identify the areas and structures most exposed at risk or design effective mitigation measures. Seismic vulnerability assessment methods vary according to the scale of analysis, as the objectives and level of detail are different. In addition, the multi-scale approach is critical for providing useful information at various decision-making levels and for implementing appropriate mitigation measures. While organization devoted to territorial planning need quick tools for the identification of most urban areas at-risk, the professional community required practical and operational tools to support the knowledge and diagnosis of structures, aimed at vulnerability analysis at the scale of the individual building. However, it is important to stress that at any scale of analysis, the structural health assessment is a fundamental prerequisite for assessing structural vulnerability. Unlike other building materials, wood is particularly susceptible to degradation phenomena, which can significantly affect its structural performance and thus greatly reduce the service life of timber structures. In this context, the main focus of the thesis is the characterization of existing timber structures, aimed at the assessment of structural vulnerability, using methodologies at different scales. At the territorial scale, a quick method (Global Durability Factor Method -GDFM) is proposed for estimating the state of preservation of timber structures, considering the environmental and technological aspects of the building, using the support of specially designed survey forms. This methodology is integrated with the semi-quantitative Indicator-Based Approach (IBA) of seismic vulnerability indicators, which is widely applied in large-scale vulnerability analysis. In this thesis, the IBA method is applied to the specific case of large span timber buildings, thus identifying seismic vulnerability criteria among building characteristics such as ii geometric and technological aspects. Again, the applied methodology is supported by a survey form, the CARTIS GL form whose 3D section update contribution is shown (Reluis Project WP2 Task 2.3.4) Subsequently, the explained methods (GDFM and IBA) are applied to a set of case studies, in order to evaluate their efficiency and identify possible critical issues, as well as to test the effectiveness of the illustrated survey forms. Thus from a set of 10 large span timber buildings, those most vulnerable to seismic action are identified, while the service life is determined for a set of 4 timber buildings, recognizing those in the worst state of preservation. At the detailed scale, contributions and useful tools for the survey, diagnosis and mechanical identification of timber are provided. First, a form for the survey and diagnosis of historical timber trusses is proposed. It is the result of the scientific collaboration activity with the ICOMOS IIWC (Italian International Wood Committee). In the end two experimental campaigns aimed at the mechanical characterization of Acacia dealbata Link logs and ancient wood members in Larch Pine and Chestnut are shown. Several investigation techniques have been applied, showing the procedure and test results. From the performance of non-destructive test and destructive bending tests, correlation models useful for in situ mechanical identification have been developed. In addition, innovative surveying techniques (photogrammetry and laser scanner surveying) have been applied on the ancient timber members with the aim to define digital models useful for estimating geometric properties and conducting remote visual strength grading analysis. The applied procedures and devices are extensively described, showing pros and cons of the two methods. In addition, long-term bending tests have been carried out on a small number of ancient timber specimens, evaluating the wood creep behaviour comparing the main findings with what is prescribed by the standards. In the end, the results obtained from the statistical analysis of bending test data have been compared with the performance profiles provided by the standards for visual strength grading, and a comparative evaluation has been carried out.

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