Seismic strengthening of low-strength masonry walls using internal steel strips and polymer mortar: experimental and parametric numerical investigations

In response to the increasing demand for strengthening low-strength brick masonry structures with varying service ages, this study proposes a composite strengthening method using internal steel strips and polymer mortar. A total of twelve full-scale masonry walls were constructed and tested under quasi-static loading to compare the seismic performance of unreinforced masonry (URM) walls, orthogonal composite method (OCM) reinforced walls, and diagonal composite method (DCM) reinforced walls. Key seismic indicators—including hysteretic behavior, load-bearing capacity, ductility, and stiffness degradation—were systematically evaluated. The results show that the ultimate load-bearing capacity and ductility of the OCM walls were more than 1.9 and 2.0 times those of the URM walls, respectively, and their skeleton curves exhibited a distinct plastic stage. The DCM walls demonstrated superior shear resistance compared to some OCM walls. Finite element simulations accurately reproduced the observed failure modes and hysteretic responses. Parametric analyses further revealed that OCM is effective in dissipating energy and delaying crack propagation, whereas DCM contributes more significantly to enhancing shear capacity. Based on these findings, and in conjunction with the Chinese Code for Seismic Strengthening Design of Masonry Structures, the shear capacity formula for masonry was modified to develop a calculation method suitable for this composite strengthening system.