Keywords: masonry structure, pushover analysis, Hoek-Brown criterion.

In Italy the large and diversified existing building heritage is largely made up of masonry structures that utilise construction techniques and materials that can diversify substantially according to the period of construction, local construction techniques, materials, etc. Given the heterogeneity of masonry arising from the various components making up the structure, it is considerably complex to define the mechanical characteristics of the components, in order to arrive at an assessment of the reliability and stability of the building. At this time of important changes in the approaches to the calculation of structures, which are easy enough for the design of new structures, however they are very complex for existing structures. For these structures, in fact, especially in the analysis procedure, it is difficult to acquire all the data necessary to conduct a more precise and detailed non-linear analysis.

The tables, from the Italian code classify the masonry typology, that are to be used in the presence of partial experimental data, but they are of limited validity , unless some parameters, such as the mortar class, the type of masonry and the strength values of uncracked specimens are known. The introduction in the calculations of the safety factors of high intensity to ensure adequate security in the presence of such a lack of knowledge, makes the non-linear analysis less effective in economic terms and for the results achieved.

The objective of the work described in this paper is to compare the results obtained using the values proposed in the code for a level of knowledge LC3 for a masonry structure with the values experimentally obtained from the mechanical tests with flat jacks, the compression tests on stone elements and the evaluation of their shear strength. The evaluation of the shear strength of masonry is essential for checking the collapse arising from shear and the lack of diagonal tensile strength in the masonry piers.

The in-situ determination, for existing buildings, of the parameters required to perform the above tests (in-situ testing with a sliding test along the mortar layers), are difficult from a constructive point of view as well as economically burdensome. The Italian code proposes to derive the value of the shear strength indirectly from the compressive strength of the elements and the strength of the mortar.

The novelty of this research is in applying an analogy between masonry and rock mass, utilizing data obtained directly from in-situ tests. Considering, masonry made with limestone blocks with natural elements and, or limestone interspersed with mortar layers, it has been possible to assimilate it into the rock mass, assuming the following common parameters: 1) uniaxial strength of samples (extracted intact), 2) the percentage of the intact elements; 3) discontinuities arising from the presence of mortar joints; 4) the size of mortar joints; and 5) the moisture level in the masonry. With this analogy and with the above parameters it is proposed to apply the criterion of a non-linear failure envelope that is transformed into the Mohr Coulomb linear criterion. To determine the shear strength, the method of Hoek and Brown as used for rock mass is applied.

Push-over analysis is performed as a case study using the values of shear strength proposed by the code and the shear strength obtained with the criterion of nonlinear envelope transformed into the linear Mohr Coulomb criterion. The results obtained for the two procedures are of the same order of magnitude.

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