Assessment of Cracking and Collapse for Old Brick Masonry Columns

This paper presents experimental and theoretical research focused on the structural 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 bricks and lime mortar were prepared and tested. The instrumentation layout allowed the writers to carefully detect the cracking load and to pick out some selected strain values. Afterward, the obtained data were discussed on the basis of the well-known hypotheses characterizing the masonry stress fields and collapse events. A simple modification of the classical Hilsdorf equilibrium equation motivated by the observed experimental behavior led to a sensible interpretation of the nested phases of brittle failure endured by the masonry up to the collapse. In order to account for the changing interaction stress between mortar layers and brick courses, an influence factor was defined to restore the internal equilibrium during the evolution of the column damage states. In fact, the introduced mortar influence factor holds an important position in the definition of the margin between the cracking and global failure phases, explaining why the collapse load of the column is higher than the first cracking load. Moreover, thanks to some simplifications in the analyses, it was shown that this key parameter plays the role of a strength amplification factor linked to the damage evolution, and that consequently it can be used in the approximate evaluation of the remaining reliability of the masonry column after the stabilized cracking phase.