Developing Prestressed Concrete Girder Cross-Sections for Longer Spans and New Materials

Precast, prestressed concrete girders are the work-horses of the bridge construction industry.  Their initial cost-effectiveness and their low maintenance requirements lead to low life-cycle costs and make them ideal for building short- to medium-span bridges, such as freeway over-crossings.  However, spans of such bridges are relentlessly increasing, due to constraints caused by environmental restrictions and urban congestion, and the consequent difficulties in locating columns.  The longer spans require deeper girders to sustain the in-service bending moments, but they also pose challenges with respect to lateral stability during handling and transportation.  At the present record span (223 ft) the cross-sections in present use (WSDOT WF sections, Florida Bulb tees, PCI bulb tees, etc.) are close to their stability limits.

To address these stability concerns, there is a pressing need to consider new cross-section shapes.  Criteria for selection will include both in-service bending and shear capacities, and lateral stability during transportation. In addition, the shipping weight of such long girders is also increasing, so there is pressure to design the girder sections to have the greatest strength/weight ratio possible, which implies the use of lightweight concrete and minimization of dimensions wherever possible.  However, lightweight concrete typically has a lower elastic modulus, which lowers the buckling load, and the benefits of using it depend on the relative magnitudes of the changes in weight and stiffness.  Furthermore, new, high-strength materials such as UHPC can contribute to the solution but, because their strengths in different modes (shear, tension, compression) do not appear in the same relative proportions as in existing concretes, the optimization of the girder cross-sections will require careful consideration of all these characteristics.

The objective of the work is to develop a family of girder cross-sections that will allow girders to be built that are longer than those in use today.  The outcome will be guidance on recommended cross-section configurations for ranges of girder length. A particular focus will be placed on characterizing torsional flexibility and its effect on lateral instability. The effect of torsional flexibility has historically been ignored in stability calculations, but, as slenderness increases, this simplified approach becomes increasingly unsafe.

The projected activities will consist of:

  • Identification of critical parameters and selection of cross-sections that are suited to various lengths
  • Narrow down selection to promising candidates and subsequent optimization
  • Verification of suitability with other disciplines and stakeholders, such as bridge owners, girder producers and trucking companies.
  • Preparation of design examples on case study structures

Research Team:
Principal Investigator:  Dr. Richard Wiebe
Co-Principal Investigator: Dr. John Stanton
Research Assistant:  Graduate Student, TBD