Constitutive stress–strain law for FRP-confined tuff masonry

The beneficial effect provided by external Fiber-Reinforced Polymers (FRP) confinement on the compressive behaviour of masonry columns has been widely demonstrated by experimental results. Conversely to FRP-confined concrete, for which some reliable stress–strain theoretical models have been developed, in case of confined masonry any unified model has been yet provided. In this paper, after the elaboration of a database containing experimental results on FRP-wrapped masonry columns tested under axial loads, new design-oriented stress–strain constitutive relationships for FRP-confined masonry are proposed. These laws—which have general validity for whatever confined masonry—were derived from the existing Lam and Teng’s model originally developed for FRP-confined concrete, and here adapted to confined tuff masonry. Two functions rule the shape of theoretical stress–strain curve: a second order parabola defines the elastic behaviour, while a straight-line describes the post-elastic behaviour. The entire axial stress–strain response is completely defined by unconfined masonry mechanical properties, composite material characteristics and by a single unknown parameter. This latter takes into account of the dispersion of experimental data and has been evaluated by means of the regression analyses procedure based on the least-squares optimization criteria. The outputs of the proposed constitutive laws were compared with the experimental results currently available in literature. A good matching is observed between the experimental results and the theoretical predictions in terms of axial stress–strain curves.