Fatigue Tests on Prestressed Concrete I-Beams

Six fatigue tests of prestressed concrete I-beams containing web reinforcement are reported. Each beam was initially subjected to a high overload, and then subjected to 2,000,000 cycles of a design equivalent repeated loading. The beams failed by either flexural or shear fatigue. The flexural fatigue life of the test beams was less than expected from available information on the fatigue strength of strand. The tests demonstrated that prestressed concrete beams have a remarkable shear fatigue resistance. Prestressed beams, with enough web reinforcement to develop their flexural capacity, can be subjected to overloads which cause extensive cracking without subsequent danger of a shear fatigue failure under design loads. Shear fatigue failures do not occur suddenly. There is considerable warning, as indicated by increasing deflection and increasing inclined crack width, before failure occurs. In these and other tests, shear fatigue failures did not occur when the range in inclined crack width was less than 0.006 in. under application of repeated loads.