Consideration of strand fatigue for load rating prestressed concrete bridges

Abstract

This dissertation is an investigation of the use of fatigue of the strand in prestressed concrete beams for the load rating of prestressed concrete bridges. The criteria for load rating prestressed concrete bridges are provided by the American Association of State Highway and Transportation Officials in the Manual for Condition Evaluation of Bridges, which does not contain provisions for load rating prestressed concrete bridges for fatigue. Rather, a serviceability criterion is provided for the control of flexural cracks in prestressed members by limiting the concrete tensile stress at the extreme fiber of the member being evaluated. The initial thrust behind the research was the apparent discrepancy between the condition of bridges observed in the field, which show no significant signs of deterioration, and the expected condition from load ratings based on the current concrete tensile stress limit. To investigate this discrepancy, a research program was developed that included diagnostic load tests of five existing bridges and a series of fatigue tests on six, one-quarter scale specimens that were designed behave similarly to the beams in bridges considered in this investigation. The diagnostic load tests provided information used to evaluate the response of the bridges to applied live loads, and the results were compared with the current code provisions and the results from finite element analyses. The results of the fatigue tests provided a link between the stress range in the prestressing strand due to applied live loads and the fatigue life of the beams. Based on the results of the load tests, fatigue tests and related analyses, recommendations for improved load rating procedures with respect to the tensile stress criterion are provided and alternative criteria for load rating prestressed concrete bridges based on the fatigue of the prestressing strand are proposed.