TRANSPORTATION

Curved Precast Spliced U-Girder Bridges

Overview

Spliced girder bridges are typically used for continuous structures in order to facilitate longer spans. In the past, spliced girders have been limited to straight concrete I-girder sections. New spliced U-girder technologies now allow for curved U-shaped bridge options. Several have already been successfully constructed in Colorado.

Construction of these bridges includes pre-casting either straight or curved u-shaped sections, supporting the pier and drop in sections, and splicing the sections together on temporary supports using post-tensioning. A lid slab is cast before the post-tensioning is applied to increase the torsional resistance of the section. After the section is closed and stressed, conventional forms are placed between the boxes and a full depth deck is cast.

Span lengths for these structures can be extended by providing haunched sections at the piers.

Advantages to this type of construction compared to conventional construction include:

• Lower fabrication times
• Faster construction
• Ability to span longer distances
• Increased aesthetics by providing a unified appearance



Characteristics of this type of construction include:



• Requires extensive Shoring
• Typically entails heavy girder sections and larger cranes to place.
• Requires more field and erection engineering than typical beam construction
  • Stability must be designed for and maintained until superstructure is self supporting
  • Requires monitoring of settlement and movement of temporary foundations during erection thru post-tensioning



General requirements and allowances for using spliced girders include:




• Design Guidance provided by PCI will be considered.

  
  
• Girders:
  • Use standardized sections contained in the example drawings with standardized depths and standardized core form dimensions. Complete FDOT Standards similar to the FIB will not be available due to the unique design requirements for a given site.
  • Minimum of two girders per bridge
  • Minimum 3 tendons per web
  • Cast-in-place lid slab construction; Precast lid slabs are not allowed.
  • Interior box lighting for post tensioned applications
  • 72" minimum box girder height for PT sections
  • For haunched girders, mildly reinforced field cast filled bottom flange build up is allowed.
  • Future PT is not required.
  • Mixing of curved and straight beams within a span is not allowed.
  
  
• Allowance for inspection/access:
  • Provide access into the box through the bottom slab for PT sections.
  • Minimum 3'-0" diameter access holes in diaphragms
  • Provide handles on either side of access holes for inspectors.
  • 2'-0" minimum interior width measured along the top of the bottom slab
  • Ramps up to diaphragm access holes are required.
  
  
• Slab:
  • Longitudinal mild reinforcing steel stress in the deck is limited to 24 ksi.
  • Pretensioned concrete deck panels are not allowed as a forming system.



Pier Diaphragms:

• Post-Tensioned integral piers are allowed.



Design Criteria

LRFD Bridge Design Specifications

FDOT Structures Manual

Post tensioning and box access requirements would be similar to FDOT precast segmental bridge requirements.




Specifications

FDOT Specifications where applicable

Technical Special Provisions required where standard specifications are not appropriate.




Implementation Plan

Implement as appropriate projects are identified.

Initial projects will require extensive coordination with the Structures Design Office to develop acceptable complete designs.




Contact Information



Tom Andres, P.E.
Asst. State Structures Design Engineer
Phone: (850) 414-4269
e-mail: Thomas.Andres@dot.state.fl.us



Jonathan Van Hook, P.E.
Area Structures Design Engineer
Phone: (850) 414-4641
e-mail: Jonathan.Vanhook@dot.state.fl.us