Time -dependent effects in post -tensioned concrete -steel composite girder bridges

Asnee Pochanart, Purdue University

Abstract

In continuous concrete-steel composite girder bridges, the concrete contribution to the composite action at interior supports is impaired because the concrete deck is in tension. Post-tensioning introduces compression to the structure and reinstates the contribution of concrete to the composite action. Janssen and Spaans (1994) proposed a post-tensioning scheme, which involved using concrete end diaphragms as anchorage and concrete intermediate diaphragms as draped points for steel tendons. In the short-term, this scheme produces structurally and economically efficient structures. Long-term efficiency, however, can only be assessed once the time dependent effects caused by creep, shrinkage, and relaxation are considered. In this study, finite element models were used to study the time dependent effects in post-tensioned composite girder bridges. How post-tensioning induced stresses, internal forces, and deflection change with time were investigated. Parametric studies were conducted to study how changes in these quantities were affected by bridge geometry, material properties and surrounding conditions. Finally, a simplified design approach to account for time dependent effects was proposed. It was found that time dependent effects in terms of change in stresses and internal forces are considerable. While shrinkage has the most profound effect in terms of prestress loss, creep plays a major role in terms of stress changes and moment redistribution. Change in deflection is small due to the cancellation effects of creep and shrinkage. Despite the time dependent change in the post-tensioning effect, substantial material saving can still be realized in this type of bridge. Post-tensioned composite girder bridges still represent structurally and economically efficient structures.

Degree

Ph.D.

Advisors

Ramirez, Purdue University.

Subject Area

Civil engineering

COinS