FATIGUE OF PRESTRESSED CONCRETE BEAMS WITH INCLINED STRANDS

This paper presents the results of laboratory cyclic testing of preten sioned concrete beams with hold-downs operating in the cracked or part ially prestressed condition. It is shown that the fatigue life of incl ined strand is low due to flexural action in strand at cracks. This re sults from the noncoincidence of crack-opening direction and strand di rection. A pattern of regular, fine cracks is desirable. However, the usual pattern features dominant cracking at hold-downs. Based on exper imental data a lower-bound fatigue-life equation is proposed. The equa tion indicates that for a fatigue life of 2,000,000 cycles the stress range in inclined strand should be restricted to 70 MPa for either pre tensioned or posttensioned construction. The research suggests that in situations where heavy service loadings are frequent (e.g. railway br idges on heavy haul coal and mineral lines), partially prestressed des ign featuring inclined strand should be avoided.