Standard Design for Nebraska County Bridges

Many of the Nebraska county bridges are in need of replacement due to structural deficiency or functional obsolescence. Most of the bridges needing replacement are in the 30 to 60 ft range. This span range appears to be lacking 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 forming, 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. It is proposed to expand the precast plank units to cover spans to 60 ft. Both conventionally reinforced and post-tensioned precast units will be investigated. Criteria for the system will be developed through meetings with Nebraska Department of Roads (NDOR) and several counties. After the cross section shape is finalized, full-scale two-girder units will be constructed and tested to examine the structural behavior and production efficiency. An important companion to this effort will be the development of an economical mixture design that meets the requirements of early concrete strength for prestress release (if prestress is used), mechanical properties requirements, and durability. Local aggregates and cements will be used in optimizing the concrete mixture design. In the option when the beam is conventionally reinforced, effort will be made to use a mix very similar to the deck mix currently used for NDOR projects.