In this paper, the strength of prestressed girders with low amount of shear reinforcement or with defective anchorage is investigated by means of a test programme carried out at the Ecole Polytechnique Fédérale de Lausanne on 10 prestressed and 2 reinforced concrete girders (10 m long, 0.78 m high). The results show that if certain conditions are fulfilled, these structures can perform suitably and provide the expected strength according to plastic design approaches. For comparisons, the elastic-plastic stress fields method is used to predict the specimens’ strength leading to excellent correlations between the measured-to-predicted behaviour and strength. Furthermore this approach allows a sound understanding of the various shear-carrying mechanisms developed in the girders and of the various failure modes observed.