Effect of FRP wraps on the compressive behaviour of slender masonry columns

In the last decade, Fibre Reinforced Polymer (FRP) wrapping technique has become a common method to retrofit masonry piers or columns with poor structural performances. The passive confinement effect induced by the external wrap allows increasing the compressive strength and ductility of the member. Several studies highlighted as the efficacy of this technique is affected by several key parameters, including the shape of the transverse cross section, stress intensification at the strength corner of sharp sections, amount and mechanical properties of adopted composite. Despite this technique has been widely studied from both theoretical and experimental point of view, most of studies focused on short columns and little information is available on the influence of second order effects on its structural efficacy. This paper presents a simplified method able to assess the effect of FRP confinement on slender columns. A preliminary evaluation of the constitutive law in compression of FRP confined masonry is made and the best-fitting model is adopted to model masonry in compression. Sectional analysis is performed by including the tensile strength of masonry and considerations are made on the increase of ultimate moment and curvature. Finally, the effect of column slenderness is considered using a simple numerical procedure, making it possible to calculate the allowable slenderness ratios as a function of the maximum drift, taking into account both strength and stability