Time-Dependent Deflection of In-Span Hinges in Prestressed Concrete Box Girder Bridges
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Post-tensioned multi-cell reinforced concrete bridges with in-span hinges in California have been experiencing undesirable and unexpected differential movements at expansion joints during and after construction. The deformation of in-span hinges in cast-in-place (CIP) prestressed concrete (PS) box girder bridges is referred to as “hinge curl” and is due to post tensioning forces. The difference between the elevations of the two sides of the hinge creates a bump on the road and presents a road hazard with risk to the travelling public safety. Accurate prediction of instantaneous and time-dependent deformation of superstructure in-span hinges is important to avoid mismatch at the intermediate expansion joints of bridges. A method to estimate hinge curl was developed by the California Department of Transportation (Caltrans) through Memo to Designers (MTD) No. 11-34 and has been used in design. However, this method often leads to estimate of deformations that are different from those in the field. Hence, grinding of the superstructure at the hinge and other remedial measures are often necessary, and this results in extra cost and delay. The principal aims of this study was to evaluate the MTD method based on field measurements and analytical studies, identify the extent and sources of discrepancy between the estimated and actual “hinge curl” deflections, and propose a new method to more accurately estimate short-term and long-term hinge curl. The research presented in this dissertation consisted of six parts: (1) field measurement of hinge movements in five bridges, (2) analysis of data and comparison with the estimated movements using the current method, (3) analytical studies of the five bridges using relative simple models utilizing software package SAP2000, (4) analytical studies of the five bridges using detailed finite element models utilizing ABAQUS, (5) analytical parametric studies of the effect of superstructure skew at abutments and horizontal curvature on the hinge curl, and (6) development of a new, practical method to improve on estimation of hinge curl.Deflections of superstructures were measured and monitored for five bridges in the state of California during construction until bridges were opened to traffic. Temperature and relative humidity data were also collected during field measurements. The field data were analyzed and the correlation with the current method for estimating hinge curl was investigated. Hinge curls were estimated according to Caltrans MTD 11-34 with the aid of computer models developed using CTBridge software. Substantial differences between the field data and estimated hinge curls were noted due to the inaccurate boundary condition assumption and other issues determined in the current design equations. Analytical studies were conducted using two modelling approaches, stick model and finite element model, to further investigate the deformation behavior of the bridges. SAP2000 was utilized for the first modelling approach and a more sophisticated program, ABAQUS, was utilized for the second approach to capture the three-dimensional deformation behavior. Construction sequence and material time-dependent effects were modelled in both approaches. Parametric studies of the effect of skewed abutments and horizontal curvature on hinge curl were performed using the finite element approach on ABAQUS. A new method was developed for estimating the immediate hinge curl. Modifications of the time-dependent deflection multipliers were proposed to improve prediction of the long term hinge curl. Hinge curls were calculated according to the proposed method and compared to those measured for verification. The study validated the applicability of the proposed method for hinge curl prediction. The new method and the modifications were summarized in addition to a design example in a new proposed document with MTD format to facilitate adoption of the new method by Caltrans.