Title:
Interaction Between Prestress Secondary Moments, Moment Redistribution, and Ductility--A Treatise on the Australian Concrete Codes
Author(s):
Peter J. Wyche, Judith G. Uren, and Graeme C. Reynolds
Publication:
Structural Journal
Volume:
89
Issue:
1
Appears on pages(s):
57-70
Keywords:
bridges (structures); concrete construction; ductility; prestressed concrete; moment distribution; standards; structural design; Design
DOI:
10.14359/1297
Date:
1/1/1993
Abstract:
The Australian concrete codes, NAASRA and AS3600, permit moment redistribution in prestressed concrete structures for design at the strength ultimate limit state. Provided the sections considered have sufficient ductility, the redistributed moment at any intermediate support is taken as a percentage of the elastically determined and factored negative moment, including the secondary prestress effects. The magnitudes of these allowable redistributions are directly related to the ductility of these sections. There is a logical relationship between the magnitude of these redistributions and the ultimate capacity and ductility of the relevant cross sections. Conversely, the rules in ACI 318-89 are shown to be illogical. The inclusion of prestress secondary moments with other load moments has been a subject of considerable controversy, and this paper illustrates, from a designer's point of view, that these secondary moments can be considered to be present at the strength ultimate limit state and that they are often beneficial. Simultaneously allowing the use of moment redistribution generally provides further benefits in design, especially in the case of continuous prestressed concrete bridges. Furthermore, an example is given that illustrates that the analysis could be nonconservative when the prestress secondary moments are neglected.