This thesis presents a novel simplified approach for the practical assessment and retrofitting of multistorey reinforced concrete (RC) buildings located in seismic hazard regions using shear walls, with emphasis on the buildings of Lebanon. In view of this, the retrofitting of existing buildings is currently a process that relies heavily on engineering judgment and layout constraints, which can be an onerous task. Even when retrofitted, such structures would likely respond inelastically under a major earthquake, where the response is most realistically represented by computationally expensive nonlinear time history analysis (NLTHA). Towards reducing such computational demands, a simplified modelling approach is developed based on the capacity spectrum method (CSM), which utilises nonlinear static pushover analysis in conjunction with the dynamic characteristics of a structure to establish whether the seismic capacity of the structure meets the demands of seismic ground motion. The reliability of the CSM requires satisfying the main assumption that the maximum lateral storey drifts are governed by deformations of the fundamental mode of the originally elastic system. This research presents an approach that reduces the torsional effects in the fundamental mode which consequently enforces the predominance of the latter in the seismic response, and this is accomplished by reducing the average Euclidean eccentricity at all storeys. Furthermore, it encompasses a simplified numerical approach that determines the dynamic characteristics of buildings in a simplified manner that avoids the need for assembling the mass and stiffness matrices that is required by conventional eigenvalue analysis. The work is also complemented with a simplified procedure that estimates the pushover response of RC frame buildings reinforced with shear walls through direct application of static analysis. Last, the aforementioned findings are assembled in the context of the CSM in order to build a solid retrofitting strategy that facilitates and guides the strengthening of existing substandard RC frame buildings. Finally, the proposed work is demonstrated using a few case studies of existing RC buildings, and the results are verified against a complete NLTHA performed on ETABS. Therefore, the proposed work offers a new context on structural assessment and promises to bring immediate impact in an attempt to enhance the safety of multi-storey RC buildings under seismic action.