Masonry wall systems are an essential structural component of a building, providing resistance against lateral and gravity loads. Due to its economy and efficiency, most masonry walls in low- to moderate-seismic markets are partially-grouted, in which only the reinforced cells are filled with grout. Designing a partially-grouted masonry wall to resist in-plane forces is complex due to the heterogeneous nature of masonry, the distinct nonlinear behaviour of each component in the assemblage (masonry unit, mortar, grout, reinforcement, etc.), and the relatively unknown interaction existing between solid and void spaces in the wall. As a result, current North American design provisions for masonry structures (CSA S304-14 and TMS 402/602-22) are limited in their ability to predict the diagonal tension shear capacity of partially-grouted walls with consistent accuracy. Particularly concerning is the increasing number of research studies reporting that North American design provisions tend to significantly over-estimate the shear strength of partially-grouted walls and predict behaviour inconsistent with experimental studies involving partially-grouted walls. This poor performance ultimately stems from North American design provisions attempting to quantify the in-plane shear capacity of partially-grouted walls with simplified semi-empirical equations based on a pool of outdated experimental programs that focused on fully-grouted wall specimens. This research aims to (1) develop a rational methodology to estimate the in-plane shear capacity of partially-grouted masonry walls under typical roof-type loading through the creation of a mechanics-based strut-and-tie model (STM) specific to partially-grouted walls and formulated on the basis of the STM methodology currently employed for the design of reinforced concrete structures and (2) identify and quantify the influence of key design parameters on the in-plane shear strength of partially-grouted walls. To achieve these goals, the research was divided into three steps. The first step consisted of developing a detailed masonry wall micro-model within the finite element framework that considers each component of the masonry assemblage independently and accounts for the cohesive interactions existing between them through the development of innovative shear interfaces. Verified with available experimental data, insights concerning the behaviour of partially-grouted masonry walls under in-plane loads (load paths, strut geometry and magnitudes, and location of nodal zones, etc.) will be extracted from the model to facilitate the development of the STM analysis model. The derivation scheme of the masonry STM will follow a similar path as the STM development for reinforced concrete, with the additional modifications required for masonry made as needed. A parametric study was also be conducted utilizing the micro-model to determine the influence of key design parameters (grout core spacing, vertical reinforcement, horizontal reinforcement, applied axial stress, wall openings, etc.) on the in-plane shear capacity of partially-grouted masonry walls. Finally, the findings from both the experimental validation of the micro-model and the parametric study were used to develop a STM methodology specific to partially-grouted masonry walls under typical roof-type loading. The results from this study are summarized in a transparent guide to allow for a seamless transition into existing design provisions. It is the hope of the author that this study allows for a more complete understanding of the shear strength of partially-grouted masonry walls, ultimately resulting in safer, more economical masonry solutions.