A modified strut-and-tie model for deep beams was developed. Several significant improvements were made and gave rise to better prediction performance of deep beam shear strength, evaluated by 233 test results. No empirical stress limit was required and the concrete softening effect was embedded in the model. The proposed model was further extended to calculate continuous deep beams with rigid and elastic supports, which was validated by 54 beams from literature as well as the author’s experimental programme on continuous deep beam subjected to differential support settlement. The effects of support settlement were studied and discussed through crack patterns, failure loads, steel strains and load-deflection responses. The author further developed the model into a generalised form, including asymmetrical loading conditions and the capability of predicting failure modes. Eight beams were tested and conclusions were drawn on the effects of unsymmetrical loadings on the beam behaviour. The strut-and-tie model was also developed to account for size effect in deep beams, followed by a test programme of eleven geometrically-similar specimens with varied sizes. The proposed model outperformed several other methods and is a promising tool for engineers.