Controlled rocking of brick masonry walls under out-of-plane cyclic loading: Analytical and experimental insights

Unreinforced masonry walls are highly vulnerable to earthquake-induced out-of-plane (OOP) instability, as boundary joint cracking and subsequent wall rocking can rapidly compromise structural stability. This study analytically and experimentally evaluates a proof-of-concept controlled rocking OOP retrofit strategy, which combines elastic steel strap-tie reinforcement with intentionally weakened mortar interfaces to stabilize rocking behavior while concentrating damage at predefined interfaces. Two wall-scale specimens, an unstrapped free rocking masonry (FRM) wall and a strapped controlled rocking masonry (CRM) wall, both incorporating identical intentionally weakened boundary interfaces, were quasi-statically tested under reversed cyclic OOP loading with a constant axial force ratio of 0.12, corresponding to 151 kN. Measured strap strains showed progressive tensile engagement of the anchored vertical straps, while the diagonal straps recorded substantially lower strains. In both specimens, the weak boundaries established predefined rocking interfaces through an approximately 17% reduction in mortar contact area at the top and bottom wall boundaries. Within this common weak-boundary configuration, the CRM wall maintained positive post-rocking stiffness up to approximately 2.6% drift and exhibited slower strength and stiffness degradation, larger hysteresis loop areas, and greater cumulative energy accumulation than the unstrapped FRM reference. Measured local strap strains remained below the measured yield strain at the instrumented locations throughout the examined loading range. Overall, within the tested configuration, the CRM wall exhibited positive post-rocking stiffness over the pre-peak response range, whereas FRM reached its positive peak at a smaller drift and subsequently lost lateral resistance, with damage concentrated primarily at the predefined boundary regions.