THE CONCRETE BRIDGE MAGAZINE

WINTER 2018

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/922349

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through a partial-depth diaphragm, supporting them from the top. This a p p ro a c h p ro v i d e d t h e m i n i m u m vertical clearance over traffic because no hardware projected below the bottom surface of the girders. Span 6 of the Flyover 1 bridge is one of the longest spans in the structure, measuring 228 ft 11 in. Girder shipping weights limited the maximum piece length to 100 ft for pier girders and 115 ft for drop-in girders. Because of the pier locations and site geometry, a single drop-in girder could not span between pier girders. The solution was a temporary straddle bent to support two drop-in girders at the central splice location in the span. The straddle bent spanned approximately 70 ft to provide clearance for three traffic lanes, the barrier rail, and a clear zone behind the barrier. Structure Design The foundations are composed of 24-in.-square prestressed concrete piles driven to an average depth of 100 ft below grade. Test piles were driven and monitored at each bent location to ensure that the required load-carrying capacities were achieved. A single CIP column supports each interior pier. The column geometry was controlled by the amount of space in the median between northbound and southbound I-95 at pier 3, leading to a 4 ft by 7 ft rectangular cross section with 1 5 / 8 -in.- deep vertical reveals. Interior piers use both CIP and precast concrete pier caps. CIP pier caps are located at interior piers 4 and 5. These piers were designed with slide bearings to accommodate girder movement due to post-tensioning, creep, shrinkage, and temperature fluctuations. All other piers were designed with precast concrete caps and integral diaphragms, r e l y i n g o n c o l u m n f l e x i b i l i t y t o accommodate longitudinal movement. All pier caps were designed using staged post-tensioning to provide sufficient strength and serviceability during the multiple phases of construction and in-service loadings. The superstructure of the flyover bridge is composed of spliced, precast concrete post-tensioned U-girders. It is the second structure using these curved girders in Florida, and it is the first project using these girders for FDOT. The structure is designed using time-dependent load- history analysis. Each stage of construction is included in the analysis, and the structure is aged 30 years to account for long-term creep and shrinkage effects. The girders were designed for the entire life cycle, from casting in the precast concrete plant to final service and ultimate conditions in the superstructure. When initially stripped from the formwork and stored at the precast concrete yard, the girders were mildly reinforced. Handling devices were located at approximately 0.2L locations, where L is the length of the span, to balance the positive and negative moments when the precast concrete section was handled. After stripping the girders from the formwork, post- tensioning tendons were installed and grouted at the precast concrete plant to control concrete stresses during shipping and erection. These tendons are also part of the final structure design. Next, the girders were shipped to the jobsite and erected in sequence onto the temporary falsework towers and the concrete pier caps. To erect the drop-in girders, all other girders in the superstructure unit were erected with splices cast, lid slabs poured, and partial- length continuity tendons stressed. This process required a detailed erection sequence and coordination among the contractor, precaster, and post- Pier girder supported by precast concrete cap and falsework tower during construction. Photo: Modjeski and Masters. A temporary straddle bent supports girders at the splice location near midspan over J. Turner Butler Boulevard during construction. Bridge deck overhang falsework brackets are attached to the girders after the integral cast-in-place concrete diaphragms have been cast to support work platforms and wet deck concrete during construction. Photo: Modjeski and Masters.

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