Stress Distribution of Prestressed Concrete Structures as Influenced by Time and Temperature

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Title: Stress Distribution of Prestressed Concrete Structures as Influenced by Time and Temperature

Author(s): N.C.M. Tsang and G.L. England

Publication: Special Publication

Volume: 246

Issue:

Appears on pages(s): 1-20

Keywords: bridge; relaxation; steady state; thermal creep; thermal stress

Date: 9/1/2007

Abstract:
It is well known that creep and shrinkage of concrete cause long term deflections of continuous prestressed concrete bridges. A large amount of research has been conducted in this area. While the bridges continue to deform with time, the moment distributions of the bridges remain essentially unchanged under the influence of creep if the bridge temperature is uniform. The change of stresses is caused by prestress losses due to the creep, shrinkage of concrete and the relaxation of steel. However, rate of creep of concrete increases with temperature. The seasonal and daily temperature variations of a prestressed concrete deck not only induce thermal stresses but also significant stress redistribution due to non-homogeneous creep rates in the concrete. The paper presents some recent findings from Steady State analyses. The provision in the latest European code EN1992 (EC2) for thermal creep is discussed. The amount of moment redistribution will be quantified against a range of thermal creep rates. The results show that stresses could be significantly underestimated if temperature and thermal creep are not considered in design. The advancement of EC2 as well as its limitation in modelling the long term behaviour of prestressed concrete bridges are also discussed. The paper also addresses the problems that can be created when the structural continuity is created for the first time due to the remedial works after the bridge has been in service many years. Under these circumstances creep can have a major effect on both serviceability and safety.