Twisted steel bars are often used to strengthen masonry buildings and, in particular, to improve the transverse connection between different structural elements. The performance of the retrofitted structure depends on several parameters such as the mechanical properties of the base material (masonry), the bar diameter, the pre-drilled hole diameter. The paper presents the numerical assessment of the tensile behavior of twisted stainless-steel bars installed in brickwork masonry. Nonlinear numerical models in Abaqus software were developed and validated on the basis of experimental results, considering different bar diameters (8, 10, 12 mm). A parametric study was conducted to examine the factors that could influence the performance, namely the embedment depth, the pre-hole diameter, and the mechanical properties of the base material. The outcomes reveal the sensitivity of the twisted bars to the aforementioned parameters. In particular, the twisted bars exhibited a higher capacity in stiffer base materials, with longer embedment and smaller pre-drilled hole. However, none of the above parameters proportionally affect the ultimate strength, but lead to a limited improvement in the ultimate capacity (up to 40% in the considered range).

This paper presents the results of a parametric study on the response of unreinforced and retrofitted masonry specimens. The adopted strengthening technique is the steel-reinforced plaster, which is very commonly used but it is not supported by a proper theoretical and experimental characterization in the scientific literature. The aim was to investigate the main parameters that affect the structural performances of the walls. Several numerical models were implemented using the finite element method to analyze the influence of the bricks’ arrangements, the mechanical properties of the mortar joints, the number of connectors, and the mechanical properties and thickness of the plaster coating. A concrete damage plasticity model was adopted to describe the bricks, the mortar joints, and the plaster behaviors. For the unreinforced specimens, the outcomes confirmed that the mortar strength had a significant influence on the performance of the wall, together with the presence of potential weaknesses in the bricks, while the bond effect was negligible. For reinforced walls, the connectors do not have a significant influence on retrofitted wall capacity but may prevent instability if a proper number is considered. Furthermore, the strength of the plaster coating does not affect the collapse load significantly, while increasing the fracture energy, which can be produced, for instance, by using fiber-reinforced concrete, increases the capacity of retrofitted walls, with more limited damage. Finally, an increase in the plaster thickness may be beneficial in terms of collapse load, even though greater thickness may increase the seismic masses significantly.