Abstract
Prestress losses assumed for bridge girder design and deflection analyses are dependent on the concrete modulus of elasticity (MOE). Most design specifications, such as the American Association of State Highways and Transportation Officials (AASHTO) bridge specifications, contain a constant value for the MOE based on the unit weight of concrete and the concrete compressive strength at 28 days. It has been shown in the past that that the concrete MOE varies with the age of concrete. The purpose of this study was to evaluate the effect of a time-dependent and variable MOE on the prestress losses assumed for bridge girder design. For this purpose, three different variable MOE models from the literature were investigated: Dischinger (Der Bauingenieur 47/48(20):563–572, 1939a; Der Bauingenieur 5/6(20):53–63, 1939b; Der Bauingenieur, 21/22(20):286–437, 1939c), American Concrete Institute (ACI) 209 (Tech. Rep. ACI 209R-92, 1992) and CEB-FIP (CEB-FIP Model Code, 2010). A typical bridge layout for the Dallas, Texas, USA, area was assumed herein. A prestressed concrete beam design and analysis program from the Texas Department of Transportation (TxDOT) was utilized to determine the prestress losses. The values of the time dependent MOE and also specific prestress losses from each model were compared. The MOE predictions based on the ACI and the CEB-FIP models were close to each other; in long-term, they approach the constant AASHTO value. Dischinger’s model provides for higher MOE values. The elastic shortening and the long term losses from the variable MOE models are lower than that using a constant MOE up to deck casting time. In long term, the variable MOE-based losses approach that from the constant MOE predictions. The Dischinger model would result in more conservative girder design while the ACI and the CEB-FIP models would result in designs more consistent with the AASHTO approach.
Highlights
The purpose of this study was to examine several existing time dependent moduli of elasticity (MOE) of concrete and evaluate their effect on various prestress losses in typical bridge girders
Unlike the CEB-FIP and the American Concrete Institute (ACI) 209 Method predictions, the Dischinger Method continues to show a significant increase in MOE after the deck casting
The following conclusions may be made based on the findings from this study: 1. The time dependent concrete MOE predicted by the ACI 209 and the CEB-FIP models are in close agreement for short term as well as long term situations
Summary
The purpose of this study was to examine several existing time dependent moduli of elasticity (MOE) of concrete and evaluate their effect on various prestress losses in typical bridge girders. Towards this end, a method proposed by Dischinger (1939a, b, c) for a variable MOE was considered along with a couple of other methods from the literature. Yazdani et al (2005) developed concrete MOE models based on aggregate classes in Florida and a variable concrete strength. There have been no other studies in the past in which the effects of a variable and time dependent concrete MOE on prestress losses were investigated
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