Date of Award
Master of Science (MS)
Nielson, Bryant G
Nielson , Bryant G
Schiff , Scott D
Pang , WeiChiang
Adjacent precast, prestressed concrete beam bridges have become a popular solution throughout the country because deck forming can be eliminated and construction is rapid. In South Carolina, adjacent beam bridges primarily consist of flat slab or hollow core sections, and they are currently only used on secondary, low-volume, short-span bridges. Durability and load sharing issues stemming from cracking, however, have caused concern with the longevity of these bridge types. Thus, the South Carolina Department of Transportation (SCDOT) has sought an alternative to the flat slab for short span bridges that can be used on high volume roads without an overlay. This research focuses on the selection of the Northeast Extreme Tee (NEXT) D beam as an alternative and later focuses on the deck design for the bridge and appropriate slab design forces for the section.
The NEXT D sections designed for larger spans in the Northeast were scaled down since shorter spans were targeted in this project than in the original concept. Preliminary prestressed design was performed to verify the new section geometry. The deck was designed using the American Association of State Highway and Transportation Officials (AASHTO) Load Resistance and Factor Design (LRFD) Specifications assuming the deck functioned as a continuous beam with infinitely rigid supports. A sensitivity study was completed which involved varying the stiffness of the beam webs and the shear keys and studying the resulting shear and moment responses in the deck in order to determine appropriate slab forces for design.
The NEXT D section proposed in the Northeast was scaled down to six-feet (NEXT D6) and eight-feet (NEXT D8) wide alternatives, both 20 inches deep, and confirmed to meet AASHTO requirements for flexure and limit stresses for a 40-ft. span bridge. Through the sensitivity study, the AASHTO equivalent strip method was found to be conservative for shear but non-conservative for moment. The design positive moment values calculated using the AASHTO equivalent strip method for a 40-ft. span bridge were found to be on-average 2.51 times less than those determined through the sensitivity study which calculated the web stiffness using classical beam theory. Therefore, in order to be conservative, the stiffness of the beam webs should be determined using classical beam theory, instead of assuming infinite rigidity, when designing the NEXT D slab.
The average ratio of positive to negative moment generated in the shear key was found to be approximately 2:1 for the NEXT D6 and 6:1 for the NEXT D8. Therefore, the headed reinforcing bars should be placed one inch below the mid-depth of the shear key in order to optimize the moment capacity of the key by providing more eccentricity for positive moment. In addition, the translational and rotational stiffness of the shear key should be assumed to be fully rigid in order to produce conservative design forces in the key; however further numerical and experimental studies should be performed to determine more appropriate design forces for the shear key.
Deery, Daniel, "Investigation of Northeast Extreme Tee (NEXT) D Beam Bridges as an Alternative to Precast Hollow Core Bridges: An Exploration of Appropriate Slab Design Forces" (2010). All Theses. 1011.