Precast Modified Beam-in-Slab Bridge System

Abstract

As a means of extending available resources, the Black Hawk County Engineer's Office, Black Hawk County, Iowa, has developed a precast bridge system specifically for low-volume roads with spans up to approximately 40 ft (12.19 m). The design, referred to as the precast modified beam-in-slab bridge (PMBISB), consists of four precast panels, which are fabricated at the county's casting facility, transported to the bridge site, and joined with a cast-in-place concrete joint. The PMBISB design is similar to the modified beam-in-slab bridge (MBISB) system previously developed by the Iowa State University Bridge Engineering Center (ISU BEC). Two defining features of the MBISB, the transverse arched deck and the alternative shear connector (ASC), are both incorporated in the PMBISB. Allowable stress design methods were applied, and the PMBISB is designed to resist an alternative truck loading [five 20-kip (89-kN) axles spaced on 4 ft 3 in.– (1,295-mm) centers], which is representative of local vehicles. An individual panel consists of three longitudinal W-sections that are embedded in a transversely arched concrete deck. Reinforcement within the deck is limited to that required in the ASC and that needed for crack control. All construction is preformed by county forces with existing equipment. The PMBISB system saves Black Hawk County approximately $16,000, or 17% per bridge, compared with conventional designs. ISU BEC load-tested the first PMBISB to determine service-level performance and the behavior of the joined panels. The design has been further refined and five additional PMBISBs have been constructed on the basis of the results and field experience.