Bridges, Tunnels and other Transportation Structures
Throughout the nation, solutions to handle the increased traffic flow and tight right of way have required innovative interchange configurations. These interchanges have posed many challenges for the development of structural solutions. This paper discusses the structural and construction solutions for the replacement of the I-75 twin concrete bridges over the SR50 simple diamond interchange in Brooksville, FL with new twin steel bridges over a new Single Point Urban Interchange (SPUI).
The design-build project widens I-75 with an in-line roadway alignment to three lanes in each direction and provisions for future expansion to five lanes. The SR 50 SPUI accommodates three through lanes, two left turn lanes and a bicycle lane in each direction, while providing operational movement efficiency with a reduced number of signal cycles. The geometric criteria and traffic movements required a large unrestricted area which resulted in 298’-6” twin steel plate girder simple span bridges, making them the longest of their kind in the State of Florida.
Long spans present many analysis and design challenges, and since the 298’-6” span is beyond the limits of the AASHTO live load distribution formulae, understanding of the bridge behavior was of paramount concern. Finite element models were performed to capture the bridge behavior, natural frequencies under wind and live loads, and served as calibration tool for live load influence surfaces. In addition, for this type of structure, FDOT requires that both of AASHTO’s span to depth and deflection criteria are satisfied. The design resulted in 10’-11” deep plate girders with a total dead load camber of approximately 25” and an estimated 3¼” live load deflection at the bridge midspan.
The in-line roadway alignment utilized the existing embankment footprint reducing the need to haul fill and reduced right of way encroachment. An innovative construction sequence enabled the traffic control plans to include an interim widening of the existing concrete southbound bridge to carry two lanes in each direction. The construction sequence strategically located the girder splices to repurpose the existing bridge piers as temporary supports. Embankment stability was maintained with two phases of temporary critical walls and phased construction of the permanent MSE walls.
This paper will provide discussion of innovative structural and construction solutions in a design-build environment.