This paper reports the effect of pre-existing cracks on the web crushing strength of reinforced concrete beams. Two experimental series were conducted. The first series involved the test of four slender beams with large amounts of stirrups. The second series involved the test of five deep beams without transverse reinforcements. In both series, vertical pre-cracks were created in the shear span of the specimens by means of reversed flexural loading. Then, the beams were loaded in shear to fail by web crushing. The width of pre-existing cracks was a main parameter in the test. Both experimental series demonstrated that the pre-existing cracks reduced the shear strength of the beams. The reduction in shear capacity varied with the width of pre-existing cracks. The strength degradation mechanism was proposed to comprise (a) the reduction in effective contact area and (b) the apparent strength degradation owing to micro-cracking damages. Based on these two physical mechanisms, a simple strength degradation model has been formulated. The first mechanism was represented by an effective contact area factor adopted from previous researches. The second mechanism was represented by a damage parameter which was derived from the test data. The model was then used to calculate the degraded compressive strength of concrete for a state of cracking damage defined by the width and spacing of pre-existing cracks. A strut-and-tie model was also constructed to analyse the specimens in both series using the degraded compressive strength obtained from the model. The strut-and-tie prediction closely matched experimental shear capacities of specimens in both series with the difference from the test less than around 4-5%. The research highlights the significance of incorporating degraded concrete compressive strength into the structural analysis and investigation of concrete members. © 2010 Thomas Telford Ltd.