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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C O N C R E T E B R I D G E T E C H N O L O G Y 34 | ASPIRE Winter 2018 In September 2017, the Southbound Interstate 95 (I-95) to Eastbound State Road 202 (SR 202; also known as J. Turner Butler Boulevard) Flyover Bridge was opened to traffic in Jacksonville, Fla. The structure allows for a continuous flow of traffic through the interchange and provides a significant upgrade to the previous configuration of an off ramp with a traffic light, which led to traffic backups of a mile or more on I-95 during peak travel times. Among the many challenges faced during the design-build process were the constrained jobsite conditions due to the heavy traffic volume on each roadway at the interchange. In just seven spans, this bridge has three major traffic crossings in an interchange that carries more than 200,000 vehicles per day. The Florida Department of Transportation (FDOT) required that all lanes remain open for the duration of construction and permitted only nighttime traffic closures, which limited construction operations to a maximum of 8 hours per day. A Project Profile article, "Southbound Interstate 95 to Eastbound State Road 202 (J. Turner Bu tler Boulevard) Flyover," appears in this issue of ASPIRE ® on page 26; this article focuses on the precast concrete pier caps. Precast Concrete Pier Caps Provide Construction Solution With all lanes open to traffic during construction, four of the six interior piers were adjacent to active traffic as the bridge was built. This situation eliminated the possibility of the typical cast-in-place (CIP) pier caps because the shoring would interfere with traffic. Additionally, any solution needed to provide at least 16 ft 6 in. of vertical clearance. Precast concrete pier caps ultimately provided the best solution. The contractor chose to fabricate the pier caps on site, but away from traffic. The minimum clearance was easily maintained, with the bottom of the precast concrete cap at least 17 ft 6 in. above the roadway. Additionally, the precast concrete caps were used to support girders during construction, minimizing the amount of temporary falsework required for the project. Precast Concrete Pier Cap Design The d esign of the precas t concrete pier caps focused on a time-dependent, staged- cons truc tion analysis. This approach allowed the designer to ensure that the caps would p e r fo r m a s i n t e n d e d a n d meet all FDOT and American Association of State Highway and Transportation Officials' design criteria through each stage of construction and the full life of the structure. Loadings at each stage were considered, and time-dependent effects over the life of the structure were analyzed and included in the design. Th e p r e c a s t c o n c r e t e c a p s w e r e designed to be handled and erected as mildly reinforced components prior to the application of any post-tensioning. Designing the caps this way allowed the contractor flexibility in scheduling the erection of the caps and their phased post-tensioning. Column-to-Cap Connection The column-to-cap connection is one of the most important and interesting aspects of the design. The connection needed to transmit out-of-balance loadings during girder erection and work in concer t with the CIP diaphragms to transfer all composite loads to the column. Each interior pier was designed with a single 7-ft by 4-ft column. The precast concrete cap was constructed with a full-depth vertical blockout that is 1 in. larger than the column cross-section dimensions. All the reinforcement from the column projected through the blockout and was developed into the CIP diaphragm. Within the blockout, four rows of steel channels (C6) were installed to create four continuous shear keys around the perimeter of the blockou t. The pier cap was erected by threading it over the column reinforcement and bearing on temporary support brackets. Once it was in place, 10-in.-long shear studs were welded to the C6 channels and the blockout was cast with concrete that was the same class as the column concrete. The governing design condition during construction was the out-of-balance loading that occurred when the first girder was erected onto the pier cap. Multiphase Post-Tensioning The staged post-tensioning is another important aspect of the design. Each precast concrete cap was designed with Precast Concrete Pier Caps Aid Construction of Jacksonville Flyover Bridge with Tight Site Conditions by Andrew Mish, Summit Engineering Group (a Modjeski and Masters company) Steel channels with welded studs form shear keys as part of the column-to-cap connection. Photo: Modjeski and Masters.

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