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.
Description
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