Development of an Optimized Continuity Diaphragm for New PCBT Girders
Roberts-Wollmann, Carin L.
Cousins, Thomas E.
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Over the past 50 years, many states have recognized the benefits of making precast, prestressed multi-girder bridges continuous by connecting the girders with a continuity diaphragm. Although there is widespread agreement on the benefits of continuous construction, there has not been as much agreement on either the methods used for design of these systems or the details used for the continuity connections. To aid designers in choosing the most appropriate method, an analytical and experimental study was undertaken at Virginia Tech. Analyses were done to compare the differences in the predicted continuity moments for different design methods and assumptions over a range of commonly used systems of precast concrete bulb tee (PCBT) girders and cast-in-place slabs. The results of the analyses were used to develop three continuity connection details for testing during the experimental study. Three different continuity connections were tested using full-depth PCBT 45 in deep girders made continuous with a 6 ft wide slab. The bottom of the ends of the girders were made continuous with the continuity connection by extending prestressing strands for the first test and extending 180 degree bent bars for the second test. Both connections adequately resisted service, cyclic, and ultimate loads. But, the test with the extended bars remained stiffer during cyclic loading and is recommended for use. The third test was performed on a system using only a slab cast across the top of the girders without the full-depth diaphragm. Two primary cracks formed above the ends of the girders at the joint during service testing, after which no significant increase in damage took place. Results from the analytical study indicate that the predicted positive thermal restraint moments may be significant, similar in magnitude to the positive cracking moment. Results from the experimental study indicate that restraint moments develop early due to thermal expansion of the deck during curing and subsequent differential shrinkage; however, the magnitudes of the early age restraint moments are much less than conventional analyses predict. Due to the great number of uncertainties involved in the attempt to predict restraint moments, it is recommended that the diaphragms be designed for the thermal restraint moments, but not to exceed 1.2 times the cracking moment of the diaphragm-beam interface.