STRUT-AND-TIE MODEL FOR EXTERNALLY PRESTRESSED CONCRETE BEAMS

A model based on the strut-and-tie method is presented to predict the strength of simply supported externally prestressed concrete beams sub jected to a midspan concentrated load. The model defines a safe domain within which the beam would riot collapse under the applied load. It predicts four possible modes of failure, which may be broadly classifi ed into shear-type failure and flexural-type failure. Shear-type failu re of the beam is due to the crushing of the diagonal compressive conc rete strut or the yielding of the web reinforcement, while flexural-ty pe failure is due to the yielding of the internal longitudinal reinfor cement or the yielding of the external prestressing tendons. The actua l mode of failure depends on the shear span-to-depth ratio (a/h) of th e beam the ratio of the loading platen width to the beam depth (w/h), the longitudinal and transverse reinforcing indexes (omega(l), omega(v )), the ratios of effective depths of reinforcements to beam depth (d( pe)/h, d(e)/h), and the loading path relating the stress in the extern al prestressing tendons to the applied load. The model is able to pred ict the main events leading to the failure of externally prestressed c oncrete beams. The predictions of the model are compared with availabl e test results of prestressed concrete beams with and without external prestressing tendons. Good agreement between the predicted and observ ed values is obtained.