This article deals with the mechanical interpretation of the in-situ diagonal compression test on masonry panels, through a non-linear numerical modeling, and proposes a methodology for the evaluation of the tensile strength and the shear modulus of masonry. The results of a wide experimental campaign on 24 masonry panels in the region of Tuscany (Italy) are presented; the obtained material parameters are classified according to the masonry typology. A critical review of the frequently used methodologies for the interpretation of the diagonal compression test, regulated by ASTM and RILEM (ASTM E 519-02, 2002; RILEM TC-76 LUM, 1994), has been made, showing the inaccuracy and incompleteness of both. The aims of this research are to simulate the behavior of different masonry typologies and to give a numerical interpretation of the tests, in order to determine the tensile strength of the panel. This parameter is very important for the seismic safety check of masonry panels in existing buildings, according to many seismic codes.

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The seismic response of existing un-reinforced masonry (URM) buildings is strongly dependent on the characteristics of wooden floors and, in particular, on their in-plane stiffness and on the quality of connection between the floors and the URM elements. It is generally well-recognized that an adequate in-plane-stiffness and proper connections can significantly improve the three-dimensional response of these buildings, obtaining a better distribution and transfer of forces to the lateral load resisting walls. However, the extensive damage observed during past earthquakes on URM buildings of different types have highlighted serious shortcomings in typical retrofit interventions adopted in the past and based on stiffening the diaphragm. Recent numerical investigations have also confirmed that increasing the stiffness of the diaphragm is not necessarily going to lead to an improved response, but could actually result to detrimental effects. The evaluation of the in-plane stiffness of timber floors in their as-built and retrofitted configuration is still an open question and a delicate issue, with design guidelines and previous research results providing incomplete and sometimes controversial suggestions to practicing engineers involved in the assessment and/or retrofit of these type of structures. In this contribution, the role of the in-plane stiffness of timber floors in the seismic response of URM buildings is critically discussed, based on the relatively limited available experimental and numerical evidences. A framework for a performance-based assessment and retrofit strategy of URM buildings, capable of accounting for the effects of a flexible diaphragm on the response prior to and after the retrofit intervention, is then proposed. By controlling the in-plane stiffness of the diaphragm, adopting a specific strengthening (or weakening) intervention, the displacements, accelerations and internal force demands can be maintained within targeted levels. This will protect undesired local mechanisms and aim for a more appropriate hierarchy of strength within the whole system.