Assessment of cracking and collapse for old brick masonry columns

This paper presents experimental and theoretical research focused on the st ructural behavior of old brick masonry columns. To gather data on the role played by the evolution of brick-mortar interaction stresses when the load is increasing up to failure, six prototype columns made with 17th century b ricks and lime mortar were prepared and tested. The instrumentation layout allowed the writers to carefully detect the cracking load and to pick out s ome selected strain values. Afterward, the obtained data were discussed on the basis of the well-known hypotheses characterizing the masonry stress fi elds and collapse events. A simple modification of the classical Hilsdorf e quilibrium equation motivated by the observed experimental behavior led to a sensible interpretation of the nested phases of brittle failure endured b y the masonry up to the collapse. In order to account for the changing inte raction stress between mortar layers and brick courses, an influence factor was defined to restore the internal equilibrium during the evolution of th e column, damage states. In fact, the introduced mortar influence factor ho lds an important position in the definition of the margin between the crack ing and global failure phases, explaining why the collapse load of the colu mn is higher than the first cracking load. Moreover, thanks to some simplif ications in the analyses, it was shown that this key parameter plays the ro le of a strength amplification factor linked to the damage evolution, and t hat consequently it can be used in the approximate evaluation of the remain ing reliability of the masonry column after the stabilized cracking phase.