Limited information about the existing bridge was given to my team of 5 via hand-drawn plans (see right) with a hidden identity due to the railroad company's information release policies. However, the existing bridge was a 66-foot multi-span concrete slab bridge over a small creek. The following was requested:

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• Bridge designs must meet AREMA specifications

• Bridge deck must be ballasted

• At least a 50-year lifespan

• Hydraulic opening must meet or exceed that of the existing bridge

• No elements of the existing bridge may be reused

• 24-hour maximum track outage for construction

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In the proposal/schematic design phase my team explored many options for the replacement, but we decided to focus on single-span steel girder and prestressed concrete girder superstructure alternatives. One strategy to meet the accelerated construction schedule was to decrease the span length of the bridge to 60 feet to allow for the new abutments to be constructed beneath the existing bridge without interrupting rail traffic.

Using LEAP Bridge Concrete, my focus was designing the prestressed concrete superstructure. I started by using standard and wide flange I-shapes to meet the strength requirements but decided to focus on prestressed box beams due to the shorter beam depth that met strength requirements and the minimum hydraulic opening constraint. Once I selected the ODOT B33-36 (36" wide, 33" deep) as optimal, I moved on to prestressed strand/tendon design. Without any prior experience with prestressed strands, this process was iterative and assisted by our professional mentor.

Next, I moved onto vertical and horizontal shear reinforcement design with #6 and #7 rebars, and I performed several design checks within LEAP to verify that the strength and serviceability requirements were met by my design. Though the design software has AASHTO design checks built in, we had to get familiar with AREMA requirements and input custom loading to ensure that the software's checks satisfy our railroad bridge design requirements. I also performed a cost estimate for the design based on ODOT historical bid data for relevant components.

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After performing a decision analysis between our design alternatives, the group decided the steel superstructure alternative would best fit the client's needs, and the next semester we moved forward with finalizing and detailing the proposed steel design, with my primary focus being shear studs to form a composite section and deck reinforcement.

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There are 3 shear studs transversely across the flange of each W36X160 beam, each row with a 9" pitch across the length of the beam. For this bridge deck, #4 rebar was selected for transverse and longitudinal reinforcement. For longitudinal reinforcement, however, the center of the cross section beneath the track utilizes #7 rebar, also spaced at 12” on center, to accommodate vehicular braking force and the induced longitudinal tensile stress in the concrete deck due to Load Combination Service II. Per AASHTO LRFD Bridge Design Specifications Section 6.10.1.7, at least 1% of the deck cross-sectional area must be longitudinal reinforcement, which led to the selection of #7 rebar for the tributary areas associated with this specific service load case.

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