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.