This paper concerns the incorporation of steel fibres in singly reinforced high strength concrete beams without stirrups Tailing under the combined effect of flexure and shear. An analytical model was developed and published for predicting the relative flexural capacity of steel fibre high strength concrete beams (M-u/M-fl) i.e. the ratio of moment with shear interaction to pure flexural moment, This paper investigates the significant role of steel fibres in increasing the beam strength up to its full flexural capacity. An equation is derived for the critical shear span-to-depth ratio (a/d)(c) at which there is a maximum reduction of the flexural strength due to shear influence, An analytical approach is developed to determine the domain of shear effect, by which it becomes possible to predict whether shear compression, diagonal tension, or flexural failure will occur for a given beam, The interaction between steel fibres and longitudinal reinforcement bars is studied, and a nonlinear expression is derived for the optimum percentage of fibres by which a singly rein-forced beam without stirrups reaches its full flexural capacity and does not fail in shear regardless of shear span-to-depth ratio (a/d). (C) 1997 Elsevier Science Ltd.