LONG TERM DEFORMATION OF POST TENSIONED CONCRETE BRIDGES

Abstract

Accurate camber prediction in prestressed concrete bridge beams is important to all parties involved in bridge design and construction. many current prestressed loss prediction models, were developed many years ago necessary for proper camber calculation, and predicated on assumptions that may no longer be valid as higher strength concrete, wider beam spacing, and longer span lengths become more commonplace. This throws into question which models are appropriate for use in camber calculation by the bridge engineers and contractors of today. Eccentricity of the prestressing strands in a prestressed concrete beam and some small initial prestress losses result in the initial camber of the beam. Elastic shortening and anchorage losses compose the initial prestress losses. Long term camber growth is affected by several inter-related time-dependent factors. creep, shrinkage, and relaxation of the prestressing strands contribute to a gradual reduction of the effective prestressing force in the strands, which creates prestress losses. Shrinkage, which is a volumetric change, results in a shortening of the beam length. Creep occurs as the concrete deforms under the application of a sustained effective upward load (due to prestressing force eccentricity). Meanwhile, the prestressing strand is itself being subjected to relaxation, which is a loss in prestressing force due to elongation of the stressed strand held at constant length. As the force in the strand decreases due to relaxation, shrinkage also occurs, which further decreases the force in the strand as the beam length shortens. As the force in the strand decreases, the creep-inducing upward force component (camber) also decreases and the process continues at a decreasing rate as the beam ages. Other factors also affect camber growth, including modulus of elasticity, aggregate type, ambient and curing conditions, and age at release. These factors, along with creep, shrinkage, and relaxation, will be discussed individually along with the findings of previous researchers. The main aim of Thesis is to analyse a simply supported bridge girder and studies the results with consideration of different factors which contribute to affect the Long term deformation and formation of a generalised mathematical tool in form of FORTRAN computer program..