Experimental and Analytical Investigation of Continuity Diaphragms in Continuous Precast Prestressed Concrete Bridges

Abstract

Continuous bridges provide several structural and operational advantages over simply supported bridges. On the other hand, designing continuous concrete bridges can be complex due to the construction sequence and the time-dependent nature of materials. These challenges arise because the statical system changes from a simply-supported to a continuous bridge while concrete continues to undergo creep and shrinkage, causing restrained deformations and redistribution of internal forces and moments over time. If continuity diaphragms are not properly detailed, extensive cracking due to time-dependent effects and temperature gradient can compromise the degree of continuity. This dissertation aims to reduce uncertainty in the design and behavior of precast, prestressed concrete bridges made continuous for live load through experimental and analytical investigations. Since the mid-2000s, the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications has allowed a simple design approach for continuity diaphragms. If girders are allowed to age for at least 90 days before being made continuous, time-dependent restraint moments do not need to be calculated because most of the creep can be assumed to have occurred prior to continuity establishment. The positive moment reinforcing can be designed such that the design moment strength (ϕMn) of the connection is not less than 1.2 times the cracking moment (Mcr). In 2005, Newhouse developed a positive moment connection detail for Precast Concrete Bulb-Tee (PCBT) girders consisting of interlocking hairpin bars which was adopted by Virginia Department of Transportation (VDOT). In 2021, a new approach to the design of continuity diaphragms was proposed and approved for inclusion in the 10th edition of the AASHTO Specifications (2024) to allow for accelerated bridge construction. The new method requires that time-dependent restraint moments be calculated for any diaphragm, regardless of the girder age at continuity establishment. However, no calculation method is prescribed, and only recommendations are available in the commentary. In addition, there is lack of experimental and analytical research on continuity diaphragms with irregular configurations, such as those in skewed and chorded bridges. To provide design guidance for continuity diaphragms, existing methods in the literature were investigated and compared to experimental data of half-scale bridge tests. Modifications were proposed and a simple modified method based on the work of Tadros et al. (2018) was recommended. This method showed close agreement with the test data while maintaining simplicity and consistency with the current AASHTO method for time-dependent prestress losses. A finite element (FE) modeling approach with beam elements was developed to capture time-dependent restraint moments using calibrated viscoelastic Kevin chains. The behavior of the current detail of continuity diaphragms was investigated with different diaphragm configurations, skewed and chorded diaphragms. Three 45-in-deep PCBT skewed and chorded stub specimens were constructed and tested under service, cyclic, and ultimate loading. The results were compared to those of the control specimen tested by Newhouse. High-fidelity FE models with 3D solid elements were developed for the stub specimens and validated using the experimental results. The results showed that the current positive moment detail performed adequately, within the range of bridge skewness and curvature investigated in this research. The modeling techniques were extrapolated to investigate the behavior of a full-scale multi-girder bridge with different diaphragm configurations. The results indicated that interior girders develop wider cracks and higher continuity bar stresses than exterior girders due to the transverse effect of volume changes. In addition, skewness and curvature in bridges, within the range considered in this investigation, have no significant influence on crack widths and stresses in continuity reinforcement.