Precast, prestressed girder camber variability

摘要

Precast concrete girder camber can vary significantly between the time of prestress release and the time of erection. This occurs even in cases in which two identically prestressed concrete girders are stored in the same manner and erected at the same time. The variations in camber become more significant as the use of high-strength concrete, longer spans, and more heavily prestressed concrete girders continues to increase. Camber as large as 8 in. (200 mm) with 3 in. (80 mm) variability is not uncommon. This problem may not be a safety issue, but it creates challenges for designers, owners, and contractors. This paper addresses several issues related to prediction, design, and construction to accommodate variability in prestressed concrete girder camber: 1.In design, it is impossible to precisely predict camber. However, if modern methods for calculating modulus of elasticity, creep, and prestress loss are used, the error in estimating the mean camber should be reduced and the most probable range of camber can be predicted. 2.Recommendations for determining final girder seat elevations and detailing of the composite action reinforcement can be made to accommodate the predicted camber and its variability. 3.A frequent point of contention between the owner and the contractor is the cost incurred for concrete shims over girder flanges that are thicker than designed in order to accommodate camber that is larger than predicted. This paper includes detailed design examples. The discussion is limited to conditions up to and including application of the deck slab weight.