Date of this Version
Sim, C., Hu, J., Tadros, M., Bleyhl, A. T., Gee, D. M. (2019) Standard Design for Nebraska County Bridges. NDOT Research Report SPR-1(17) M064.
Many county-owned bridges in Nebraska need replacement due to their structural deficiency. Most of the bridges needing replacement are in the 40 to 60 ft range. This span range lacks a standard design that fits Nebraska county practices in terms of speed and simplicity of construction. The current systems being used are (a) Precast 1 by 2 ft planks which can span up to 30 ft, (b) Cast-in-place slab bridges which can span up to 50 ft but require extensive field formwork, concrete placing, and curing, and are best when constructed in three-span units, and (c) Inverted tees which can span 40 to 80 ft, but require cast-in-place decks. The objective of this research project is to develop and evaluate a cross section that can be easily configured for optimal structural efficiency across a range of spans from 40 to 60 feet, while reducing the number of shear keys, and retaining the ease of construction presented by the plank design. To achieve this objective, four phases of research were conducted. The first phase included evaluating various sections for spans up to 60 ft. This phase was completed through an extensive literature review and a, new type of cross- section was proposed in this study. The second phase of the research evaluated a new type of transverse connection to connect adjacent units of the proposed cross section for the proposed state county bridge system through small-scale testing on ten slab specimens. The third phase of the research includes testing five sets of full-scale bridge specimens to evaluate the system behavior, including the performance of the proposed transverse connection that included the new type of mechanical connection, staggered rebar splice joints with a commercial high-performance concrete used for the shear key, and full-scale specimen with the staggered splice joint filled with three different types of high performance. Finally, the last phase of the research conducted a design review of various proposed sections and generated span charts that could be implemented for Nebraska County bridge design.
Test results indicated that the new type of mechanical joint system (transverse connection of adjacent precast beam bridges) can resist an experimental joint moment of 38 ft-kip on average, provided that the maximum spacing between mechanical joints along the bridge span does not exceed 4 ft. It was also noted that the high-performance concrete can carry a joint moment of 17.5 kip-ft per foot length which is 2.5 times larger than the equivalent moment carried by the mechanical joint system with self-consolidating concrete grout. Other possible designs that were not tested through the small scale or full-scale experimental program were proposed by a local engineering firm in Omaha and reviewed in this research. The final standard design and design span charts are proposed for Nebraska County Bridges based on literature review, small scale testing, full-scale testing, and engineering calculations.