FATIGUE OF PRESTRESSED CONCRETE BEAMS WITH INCLINED STRANDS

This paper presents the results of laboratory cyclic testing of pretensioned concrete beams with hold-downs operating in the cracked or partially prestressed condition. It is shown that the fatigue life of inclined strand is low due to flexural action in strand at cracks. This results from the noncoincidence of crack-opening direction and strand direction. A pattern of regular, fine cracks is desirable. However, the usual pattern features dominant cracking at hold-downs. Based on experimental data a lower-bound fatigue-life equation is proposed. The equation 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 pretensioned or posttensioned construction. The research suggests that in situations where heavy service loadings are frequent (e.g. railway bridges on heavy haul coal and mineral lines), partially prestressed design featuring inclined strand should be avoided.