THE CONCRETE BRIDGE MAGAZINE

SUMMER 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/995102

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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 32 | ASPIRE Summer 2018 The Texas Department of Transportation ( T x D OT ) h a s b e e n d e v e l o p i n g a n d implementing prestressed, precast concrete bent caps (PPBC) as an efficient, durable, and safe alternative to conventionally reinforced cast-in-place (CIP) concrete bent caps. Construction Projects TxDOT first used PPBCs in 2014, on a project located in Bexar County on Texas State Highway Loop 1604. In this project, twin- bridge structures, approximately 2100 ft in length, contained 36 bent caps designed with the same dimensions and reinforcement. The original design called for CIP bents, but the contractor requested post-letting to add a precast concrete option to help reduce construction time. Because of the similarity of the bents, the fabricator could cast them rapidly. Once installation of the PPBCs began, it took only three days to install all of the caps for one of the bridges. PPBCs were also used for an emergency project to replace a bridge on Fischer Store Road in Hays County that washed out in a major flood event in 2015. For this project, PPBCs were included in the original plan set as an alternative to CIP bent caps. The contractor elected to use PPBCs to expedite construction. Reopening the bridge quickly was important because the nearest detour added nearly an hour to commuters' drives. The contractor installed all of the PPBCs in one day, and the overall bridge was completed in 80 days, opening to traffic in February 2016. Design The design philosophy for the initial PPBCs was to match or exceed the ultimate moment and shear capacities of the original CIP design. In Texas, CIP bent caps are designed with Grade 60 reinforcing steel and TxDOT's Class C concrete with a compressive strength of 3.6 ksi. PPBCs are designed with Grade 2 7 0 , l o w - r e l a x a t i o n , 0 . 6 - i n . - d i a m e t e r prestressing strands and TxDOT's Class H concrete. The compressive strength of Class H concrete can range from 5 to 8.5 ksi, depending on what is specified in the plans. The number of strands was selected based on the moment demand, and the strands were placed symmetrically in the cap so that there would be no camber. The transverse reinforcement in PPBCs was designed to match the shear reinforcement in the CIP caps and to provide sufficient resistance to bursting stresses at the cap ends. Because the compressive strength of Class H concrete is higher than that of Class C concrete, the shear capacity of the PPBCs exceeds the shear capacity of the CIP caps with the same transverse reinforcement. Once moment and shear designs were completed, stress checks were performed at different loading situations, including casting yard, transportation/lifting, and service load, to ensure that the caps will not crack. TxDOT PPBC Standard To date, each PPBC project by TxDOT has been a custom design. In an effort to increase the use of PPBCs, TxDOT published a PPBC standard 1 for use with round columns in April 2017. This standard provides PPBC designs for use as an alternative to TxDOT's standard CIP bent caps. The PPBC standard provides details to support the following TxDOT beam types: TxGirder, box beam, slab beam, decked slab beam, and spread slab beam. These standards account for nearly 2000 different bridge configurations and can be found on the TxDOT Bridge Standards web page. The design philosophy for the PPBC standard varies slightly from the previous custom designs. Instead of designing the caps to have equal or greater capacities than their CIP versions, the standard PPBC designs are based on the loading from the superstructure. The number of strands and the amount of transverse reinforcement are optimized, and a single design is provided for each different beam type. Another difference from the earlier designs is the column connection details. The earlier PPBCs were connected to the column Precast, Prestressed Concrete Bent Caps: A Faster, Safer, and More Durable Alternative by Christopher P. Miller, Texas Department of Transportation Prestressed, precast concrete bent caps at northbound loop of Texas State Highway 1604. Prior to grouting the column connection, precast concrete caps are supported on shim packs placed on the top of each column. Photo: Texas Department of Transportation.

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