Distinct element modeling of the in-plane response of a steel-framed retrofit solution for URM structures

This paper presents the results of a numerical study aimed at assessing the effectiveness of an innovative steel modular retrofit system on improving the in-plane lateral capacity of unreinforced masonry (URM) load-bearing structures. The investigated retrofit solution consists of modular steel frames fastened to the external surface of masonry piers through chemical anchors. Distinct element models are built and calibrated against experimental data coming from a series of in-plane quasi-static tests performed on piers in both bare and retrofitted conditions. Two different masonry typologies are considered herein: one made up with solid clay bricks and lime mortar, assembled in a header bond pattern, and the other with typical Italian hollow clay units and cement-lime mortar, arranged in a Flemish bond pattern. A simplified microscale approach, combined with a mesh-refinement strategy, is adopted to decrease computational demand without losing numerical accuracy. All in-plane failure mechanisms, such as mortar and unit tensile failures, as well as masonry crushing for high compressive stresses, are included in the developed numerical models. Moreover, a methodology to explicitly account for the contribution of the investigated retrofit in the distinct element method (DEM) framework is presented and discussed. The proposed modeling strategy yielded a good prediction in terms of lateral stiffness, lateral strength, hysteretic response, and crack pattern for unreinforced and strengthened specimens. A novel set of metrics is proposed to quantitatively assess the retrofit performance benefits.