Date of Award
Doctor of Philosophy (PhD)
Scott D Schiff
Bryant G Nielson
Prasad R Rangaraju
In South Carolina, the Northeast Extreme Tee (NEXT) D beam system, proposed by the Precast/Prestressed Concrete Institute Northeast (PCINE), was identified to be a promising alternative to the cast-in-place (CIP) flat bridge that has been widely used in the state. In order to accommodate a span length from 22 ft to 40 ft, as desired by South Carolina Department of Transportation (SCDOT), this original beam cross section developed for longer span length was scaled down to a 6-ft-wide cross section (NEXT-D6) and a 8-ft-wide cross section (NEXT-D8). The modified beam has U-bars extending from the edges of the flanges of the precast beam into field cast joints. Because it is a new system there is uncertainty relative to the joint durability, as seen in other precast systems, and also appropriate design strategies. As such, both experimental work and analytical studies were carried out to examine the joint performance and provide a deeper understanding of the load distribution of this system. A final design guideline was also provided to aid in the future design.
In the experiment phase, three different joint material combinations---traditional grout extended with PVA fibers and ultra-high performance concrete (UHPC) extended with either steel fibers or PVA fibers---were examined under both static and fatigue scenarios. In each test, the joint was examined either under a high-moment or high-shear loading configuration, so that the shear key performance under a wide range of moment-to-shear ratios can be known. Shear key stiffness obtained from the static test was used to determine the fatigue demand using finite element analysis, and the subsequent fatigue load was exerted in the fatigue test. In addition to specimen tests, material tests were also conducted to study the basic material properties and bonding performances. Fatigue tests showed that all of the material combinations gave satisfactory long-term shear key performances. However, static tests and material tests showed that the UHPC combinations gave much better bond performance than the traditional grout combination in terms of strength and failure mode. Typically a substrate failure was observed for the UHPC combinations, and bond failure was observed for the standard grout combination.
In the bridge design phase, a primary analysis of demand distribution for the NEXT-D beam system was conducted using finite element analysis. In this step, the shear key stiffness matrix is the key input for demand determination, which includes many stiffness terms that were not available from the experiment. As such a shear key finite element model was built and calibrated using experimental results, from which, the remaining stiffness terms were either directly obtained by setting various boundary conditions or derived using beam analysis. This matrix was determined for each material combination and used for a demand distribution analysis. Finally a general design guideline was provided for the NEXT-D beam system with span lengths between 22 ft. and 40 ft., and beam widths between 6 ft. and 8 ft. For beam design, it is recommended to use the live load distribution factors provided by American Association of State Highway and Transportation Officials (AASHTO) for cross section I as specified in AASHTO LRFD Table 126.96.36.199.1-1. For deck design, a four-step procedure was developed to be used together with the strip method to determine the design demands on a 1 ft. strip in Strength I and Service I limit states.
Sheng, Huan, "Modified NEXT-D Beam Bridge --- Experimental and Simulation Evaluation of Shear Key Performance and Development of a Design Strategy of the Superstructure" (2014). All Dissertations. 2311.