THE CONCRETE BRIDGE MAGAZINE

Summer 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

Issue link: http://www.aspiremagazinebyengineers.com/i/1134112

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C O N C R E T E B R I D G E T E C H N O L O G Y 34 | ASPIRE Summer 2019 by Robert Hass and Arielle Ehrlich, Minnesota Department of Transportation Minnesota's MH Shape: The Development of Efcient Shallow- Depth Prestressed Concrete Beams T h e M i n n e s o t a D e p a r t m e n t o f Transpor tation (MnDOT) has been designing and building bridges using precast, prestressed concrete beams since the late 1950s. Currently, these types of structures make up 70% to 80% of the state's new bridges annually. MnDOT has worked with local fabricators to continue improving the quality and efficiency of these beams. As new two-span bridges are scoped to replace existing four-span bridges with side piers, project leaders often choose between including a grade raise of up to 18 in. or constructing the bridge with steel girders to accommodate vertical clearance requirements. Shallow concrete beams could be a more cost-effective solution. An analysis of MnDOT-owned bridges designed since 2001 and state-funded local highway bridges designed between 2009 and 2016 showed that a significant portion of bridges spanned 75 to 105 ft, with beam depths between 27 and 45 in. Through this analysis, MnDOT identified efficiency gaps in the shorter spans, and that finding led to the development of new 30-, 35-, and 40-in.-deep "MH" girders. Shape Geometry The study began by analyzing MnDOT's 36-in.-high prestressed concrete beam against those standardized by other states. 1 For example, Ohio uses a WF36-49 beam. 2 Combined with a typical MnDOT strand configuration, this type of beam would span 12 ft farther than MnDOT's 36M shape. The next step was to understand which features allowed the WF36-49 beam to span so much farther than MnDOT's same- height shape. After investigating several combinations of top- and bottom-flange shapes, MnDOT selected a beam shape with the following attributes: • Top flange: A tip depth of 5 in. was chosen to facilitate deck replacement. A 34-in. width was determined to be the best option to resist stresses at transfer. • Web: A width of 6½ in. provided ample shear capacity and the ability to place shear reinforcement. • Bottom flange: A width of 39 in. was selected to match the bed width of current precast producers. This option also minimized the flange area where strands could not practically be used and flattened the slope of the top face to reduce weight. The new MH cross section also provides softened flange-to-web radius transitions to enhance form release and increase aesthetic appeal. For comparable depths, the MH shape provides maximum span lengths within 2% of the modified Ohio beam and is 30% lighter per foot. MNDOT chose the depths of the MH shape to fill gaps between existing beam shapes and provide the minimum depth required for typical railroad crossings. Design Method Full design criteria can be found in the MnDOT LRFD Bridge Design Manual. 1 For the selection table, maximum span length was determined at 5, 7, 9, 11, and 13 ft beam spacing. At each spacing, the MH shape was the most efficient, either by spanning farther or providing comparable span lengths with a lighter section. The MH and MN/OH shapes span farther than other shapes at the same depth. The MH shape was chosen because it weighs 6% less than the MN/ OH section. Iteration of cross-section geometry before finalizing the MH shape. Girder cross sections considered during development of the MH girder. All Figures: Minnesota Department of Transportation.

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