Assessment of Cracking and Collapse Behavior for Old Brick Masonry Columns

In this paper an experimental and theoretical research focused on the structural behaviour of old brick masonry columns is presented. In order to gather data concerning the influence of the changes in the brick - mortar interaction stresses when the load is increasing up to the failure, six prototype columns made up with old XVII century bricks and lime mortar were prepared and tested. The instrumentation layout allowed to carefully detect the cracking load and to pick out some selected strain values. Afterwards, the obtained data were discussed on the basis of the well known hypotheses characterising the masonry stress fields and the collapse events. A simple modification to the classical Hilsdorf equilibrium equation motivated by the observed experimental behaviour led to a sensible interpretation of the nested phases of brittle failure passed by the masonry up to the collapse. In order to account for the changing interaction stress between the brick courses and the mortar layers, an influence factor was defined, capable to restore the equilibrium during the evolution of the column damage states. In fact, the introduced mortar influence factor holds an important position in defining the margin between the cracking and global failure phases, explaining so why a column can sustain a load higher than the first crack one. Moreover, thanks to some simplifications in the analyses, it was shown that this key parameter plays the role of an amplification factor linked to the load increase and that, consequently, it can be used in the quantitative evaluation of the remaining reliability of the masonry column after the stabilised cracking phase.