THE CONCRETE BRIDGE MAGAZINE

SPRING 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/957336

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CONCRETE BRIDGE TECHNOLOGY Bridge Deck Overhang Construction: A Precast Concrete Solution by Sean P. Clifton, Magnusson Klemencic Associates, and Dr. Oguzhan Bayrak, University of Texas at Austin An experimental investigation was conducted at the Ferguson Structural Engineering Laboratory at the University of Texas at Austin to evaluate conventional bridge deck overhang construction practice and develop a simpler, precast concrete solution. This article provides a summary of the precast concrete bridge deck overhang system developed by the authors.1,2 (For another precast concrete overhang construction method, see the Concrete Bridge Technology article “Precast Concrete Overhang Panels for Safer and Faster Bridge Deck Construction” in the Fall 2017 issue of ASPIRE®.) Context The overhang construction method discussed in this article has been developed and tested as part of the master’s research of the primary author, supervised by the second author. The authors do not know if the concept, or its variants, have been implemented in practice. Implementation of the concept will likely result in further refinements. It is the hope of the authors that dissemination of this concept will help its consideration by the industry. Advantages and Disadvantages To illustrate the potential benefits of a precast concrete deck overhang system, the advantages and disadvantages of the precast concrete solution and the conventional cast-in-place deck approach that is supported by overhang brackets are outlined. The conventional, overhang bracket system referred to in this article is the system of bridge deck overhang brackets and embedded hangers, as shown in Fig. 1, that support the deck forms, screed rail, and walkway. The proposed system involving a precast concrete deck overhang solution, also shown in Fig. 1, is described in detail in this article. The precast concrete deck overhang solution presented herein serves as stay-in-place formwork and is part of the deck overhang for structural purposes. Advantages of current overhang construction technique: • Contractor flexibility: The bracket system allows for last-minute changes at the bridge site to the overhang design, and the casting profile can be adjusted as needed to fit the specified profile. • Current practice: The system is in use today and has been for decades. Disadvantages of current overhang construction technique: • Time: Installation and disassembly of overhang brackets are timeconsuming. The time required to adjust the brackets and to construct the form overhang at the bridge site is significant. • Cost and safety: The system demands significant resources for formwork, crane time, and possibly a form-stripping buggy. Brackets and non-reusable embedded hangers are costly, and the labor costs of installation, forming, and disassembly are also substantial. In cases where a bridge is constructed over traffic, safety is an issue. • Reduced stiffness: Overhang brackets and formwork can undergo noticeable deflections during placement of deck concrete that may lead to an uneven finished surface, especially if the overhang brackets slip as they engage to support the deck concrete. • Screed rail movement during concrete placement: The authors of the paper are aware of cases in which the screed rail that supports the bridge paver moved due to overhang bracket slip. Advantages of precast concrete deck overhang system: • Cost: The new construction technique will likely lead to a more efficient, and ultimately cheaper, construction process and result in a more economical bridge. A similar economy was achieved when the use of partialdepth precast concrete stay-in-place deck panels was first implemented. • Stiffness: Because the precast concrete overhang is a very stiff element and will not deflect significantly during the finishing process, it will lead to a smooth riding surface. Bracket slip problems are eliminated. • Time: Eliminating the need for assembly and disassembly of overhang brackets and wooden formwork greatly reduces the onsite time required to construct the bridge deck and eliminates safety concerns when working over traffic. Disadvantages of precast concrete deck overhang system: • New concept: As with any new procedure in construct ion, the refinement and optimization of the process will involve a learning curve. • Planning time: Because a portion of the overhang may be constructed at the fabrication yard, certain characteristics of the bridge geometry must be determined with enough lead time to be applied in the fabrication yard. Overview of Proposed Procedure Figure 2 shows the recommended steps of the precast concrete solution from girder fabrication to the completed bridge. The procedure is: 1. During the casting of the fascia girder, add supplementary reinforcement (L-shaped bar shown in Fig. 2) for the precast concrete overhang during the casting of the fascia girder. 2. After prestress transfer and prior to girder erection, move girders and form a 4-in.-thick concrete overhang on the fascia girders.

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