Investigation of the Use of Steel Intermediate Diaphragms and Temporary Bracing Alternatives for Prestressed Concrete Girder Bridges
Type of Degreethesis
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Common in bridge construction today, diaphragms are placed transverse to the girders, connecting adjacent girders to provide stability and transmit loads. Diaphragms are defined as either end diaphragms – used at the ends of girders in simply supported spans and over the supports for continuous spans – or intermediate diaphragms – used at any number of points with the span. The focus of this research is on the use of intermediate diaphragms in simple-span prestressed concrete girder bridges, specifically those with I-beams and Bulb-tees. Intermediate diaphragms are used in precast concrete girder bridges for three primary reasons: 1) to prevent torsional girder rotations during girder erection and deck placement operations, 2) to increase the vertical load distribution between girders, and 3) to transfer and spread an impact load from an overheight vehicle to adjacent girders. Typical practice includes the design of end diaphragms, but there is significant variation in the practice of specifying or requiring intermediate diaphragms between state transportation agencies. Wide variations exist in the acceptance of steel alternates to traditional cast-in-place concrete. In addition to the lack of cohesion in material choice, there is also significant variation in the type and geometry of steel intermediate diaphragms, spacing within the span, and alignment relative to the girder. The importance of intermediate bracing in a span during construction is widely accepted as essential; however, its contribution to a bridge in service, after the bridge deck has gained strength, is considered by some to be very minimal. As part of this research, a detailed survey of design practices by individual state bridge design agencies throughout the United States (U.S.) was conducted for all 50 states. This is the first effort to successfully profile the use of intermediate diaphragms in all 50 states. This thesis provides details on the usage of steel intermediate diaphragms by numerous states. When this research began in January 2013, the Alabama Department of Transportation (ALDOT) was interested in re-evaluating their practice of requiring reinforced concrete intermediate diaphragms in precast girder bridges. Initially they wanted to look into steel intermediate diaphragm alternates based on the interest from contractors. Later, in August 2013, it was decided that ALDOT would no longer specify intermediate diaphragm details and the contractors would assume responsibility for designing an adequate bracing scheme for bridges during construction. The objective of this research was to investigate temporary bracing schemes used for prestressed girder bridges across the U.S. and provide recommendations to ensure stability and safety of girders during construction by temporary bracing.