Amorn Pimanmas

pp. 205 - 216

Abstract

This paper addresses the effect of long-term creep and prestressing on moment redistribution for large prestressed concrete segmental bridge constructed by the balanced cantilever method. The Pathum Thani bridge across the Chao Praya River is investigated in this paper as a case study. Following the typical
practice of bridge design, the bridge superstructure is modeled as an assemblage of three-dimensional linear beam elements where each element represents a cast segment of the bridge. The partial creep factors are calculated based on the Norwegian code. The creep strain is calculated for each element and applied on the element as restrained deformation. The result shows that the creep can increase the magnitude of negative moment, rather than decreases it as widely understood. A simplified method commonly known to practising designers is to estimate the creep effect from the fraction of dead load moment at completion and that in the continuous state. The finite element analysis shows that this simplified treatment may lead to a considerable error in creep estimation in case of prestressed concrete bridge. Sensitivity study demonstrates that the top cantilever prestressing has a noticeable effect on creep redistribution while the bottom continuity prestressing has little relative impact. Since cantilever prestressing counteracts the gravity, a higher level of top prestressing results in a smaller decrease in long-term negative moment. If the prestress is beyond a threshold value relative to the bridge dead weight, the creep may increase the magnitude of negative moment. The simplified
formula for estimating the long-term moment could not predict the increase in negative moment magnitude, and is therefore not always valid for prestressed concrete bridge. The rational analysis should trace the construction sequence of segmental casting and prestressing order.