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.

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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 24 | ASPIRE Summer 2019 by Jerry Pfuntner, FINLEY Engineering Group Interstate 49 Inner City Connector Interchange—Simplifying Complexity Th e 3 6 - mil e I n t er s t a t e 4 9 ( I - 4 9 ) Corridor is a four-lane highway, with a 4-ft-wide inside shoulder and a 10-ft-wide outside shoulder, that stretches from Interstate 220 (I-220) in Shreveport, La., to the Arkansas state line. Located in Caddo Parrish, La., Segment K of the project is a new interchange with four ramps connecting I-220 and I-49.The I-49/I-220 interchange ramps are the first post-tensioned, precast concrete segmental box-girder bridges constructed in Louisiana. The three segmental bridge ramps consist of 700 precast concrete segments and have 271,000 ft 2 of deck area. The three ramps present complex geometry for rural interchange, with the ramps having precast box-girder widths between 31.5 and 50.83 ft, straddle piers, cantilever piers, and horizontal curves with a minimum 550 ft radius. This ar ticle describes the measures taken to simplify the precast segmental concrete design to maximize construction efficiency, ensuring that the proposed p ro j e c t c o u ld c o m p e t e a ga in s t a n alternative steel box-girder design. First Steps B e fo r e t h e fi r s t c a l c u l a t io n w a s performed, the design team defined the precast concrete project in terms of constructability. Variables such as access, maintenance of traffic, and the number of segments were assessed to determine the most cost-effective solution to construct the three segmental bridge ramps. It soon became apparent that balanced- cantilever erection with ground-based cranes would be the most economical solu tion for this projec t. With the construction method set, a conceptual d esign was generated to maximize design efficiency and streamline the project details. The designers decided on the concept of external continuity post-tensioning (PT) with diabolos and a combination of linearly haunched and constant-depth segments, which met the project's aesthetic goals. Varying Ramp Geometry The project design used a total of 700 precast concrete segments. To make this design economical, a cross section was developed that would require only one box-girder core form. If the project had required multiple segment cores, casting would have been less efficient and the costs of precast concrete segment fabrication would have been higher. The geometric requirements for each ramp were distinctive. Ramp EN is 3070 ft long with a horizontal radius o f 3 5 0 0 f t . Th e b ox - gi r d er w id t h varies between the typical width of 42.5 ft and a maximum of 50.83 ft within a gore transition span. Ramp SE is 3300 ft long, with a box-girder width of 35.5 ft and a horizontal curve radius of 840 ft. Ramp WN is 700 ft long, with a box-girder width of 31.5 ft and a minimum horizontal curve radius of 550 ft. To develop a box-girder cross section that would envelop such a wide range of widths and span lengths, a key part of the conceptual design involved balancing the PT details, transverse reinforcement bar sets, torsional requirements, and cantilever wing design to create a one- size-fits-all box-girder core section. Additionally, the use of external continuity tendons eliminated the integration of the blisters and internal tendons, which allowed for a smaller box core to accommodate the smaller bridge widths. The box girders have a single core form with a linear haunch over the piers to increase the segment depth and maximize the span lengths while providing an aesthetically pleasing appearance. External Tendons—the Right Solution for the Project The use of external tendons for continuity Cross section of precast box girder showing how width and depth variations were accommodated with a single core. All Photos and Figures: FINLEY Engineering Group.

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