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Design Recommendations for the Optimized Continuity Diaphragm for Prestressed Concrete Bulb-T Beams
Koch, Stephanie Ann.
Roberts-Wollmann, Carin L.
Michael C. Brown
Year: 2008
VTRC No.: 09-CR1
Abstract: This research focused on prestressed concrete bulb-T (PCBT) beams made composite with a cast-in-place concrete deck and continuous over several spans through the use of continuity diaphragms. The current design procedure in AASHTO states that a continuity diaphragm is considered to be fully effective if a compressive stress is present in the bottom of the diaphragm when the superimposed permanent load, settlement, creep, shrinkage, 50% live load, and temperature gradient are summed, or if the beams are stored at least 90 days when continuity is established. It is more economical to store beams for fewer days, so it is important to know the minimum number of days that beams must be stored to satisfy AASHTO requirements. In addition, if the beams are stored for 90 days before erection, the positive moment detail must have a factored nominal strength (fMn) greater than 1.2 times the cracking moment (Mcr). In 2005, Newhouse tested the positive moment diaphragm reinforcement detail that is currently being adopted by VDOT. The first objective of this research was to determine if the detail was adequate if beams are stored for 90 days. The second objective was to determine if, based on AASHTO requirements, beams could be stored for fewer than 90 days. After the analysis of all PCBT beam sizes and a wide variety of span lengths and beam spacings, it can be concluded that Newhouse's detail, four No. 6 bars bent 180° and extended into the diaphragm, is adequate for all beams except for the PCBT-77, PCBT-85, and the PCBT-93 when the beams are stored for a minimum of 90 days. For these three beam sizes, three possible solutions are presented: one with two additional bent strands extended into the continuity diaphragm, one with an additional hairpin bar extended into the diaphragm, and one with L-shaped mild reinforcing bars extended into the diaphragm. To determine the minimum number of storage days required to satisfy AASHTO's requirement for compression at the bottom of the diaphragm, a parametric study was performed. The PCA Method was used in this analysis with the updated AASHTO LRFD creep, shrinkage, and prestress loss models. The parametric study included all sizes of PCBT beams, with two beam spacings, three span lengths and two beam concrete strengths for each size. Both two-span and three-span cases were analyzed. It was concluded that about half of the cases result in a significant reduction in the minimum number of storage days if the designer is willing to perform a detailed analysis. The other half of the cases must be stored for 90 days because the total moment in the diaphragm will never become negative and satisfy the AASHTO requirement. In general, narrower beam spacing and higher concrete compressive strength results in shorter required storage duration. A recommended quick check is to sum the thermal, composite dead load, and half of the live load restraint moments. The beam must be stored 90 days if that sum is positive, and a more detailed time-dependent analysis will indicate a shorter than 90 day storage period if that sum is negative.