Development of System Fracture Analysis Methods for Fracture Critical Steel Bridges

The development of high performance steel (HPS) has prompted the use of fewer beams in a typical steel bridge superstructure. As the number of beams decreases, the redundancy of the system is reduced, thereby increasing the possibility of system-wide failures. This trend has brought about renewed discussion regarding the traditional definition of fracture critical bridges or members. Designers and owners have differing opinions on the definition of fracture critical bridges, and many owners will not allow new fracture critical bridges in their jurisdictions even though they are allowed by the AASHTO code (with the appropriate design, construction, and inspection procedures). Interestingly, bridges that would be traditionally classified as fracture critical are often some of the most economical structural configurations. The AASHTO code does not give guidance on the appropriate methods for system wide fracture analysis. Individual designers have developed methods for this issue; however there is no consensus on the loadings, the approach, and what constitutes failure. Some of the most common debates about fracture critical redundancy are with respect to continuous-span twin trapezoidal box girder ramp bridges, and continuous span two and three girder bridges. These structure types have been used in the past; however, most owners avoid them due to concerns about redundancy. Questions do arise with respect to redundancy or nonredundancy of existing and even newer bridges which were built using superior steels (especially using any type of HPS), subjected to advanced NDT techniques, and fabricated using higher quality welding procedures than in the past. In addition, those fabricated using the fracture control plan (FCP) are also supposed to be of superior quality than their cousins built prior to the introduction of the FCP. Modern steel bridges are also built with a composite deck slab and are inherently more capable of carrying redistributed loads through alternate paths. Two-girder curved bridges, especially those built since the early 1980s, almost always contain heavy transverse cross-frames capable of carrying significant load from one girder to another. The proposed system analysis methods will provide answers to system redundancy or nonredundancy quantitatively. The proposed research will focus on the development of defined fracture analysis methods for steel bridge systems. The research will involve the following tasks: literature research of past studies on system-wide redundancy and fracture; study of the effects of energy release at the time of fracture, how this is can be accounted for in the analysis, and development of realistic definitions of failure that are tied to the use of the bridge; determination of the appropriate live load models that need to be applied to the bridge at the time of fracture; determination of the appropriate live load models that should be applied to the bridge after the fracture; determination of the appropriate load factors that should be employed; and development of AASHTO specifications for system-wide fracture analysis for new and existing bridges, including an updated fracture critical member (FCM) definition in context to the findings of this study. For new bridges, the efficient use of new high-performance steels is being hampered by the concerns of owners and designers about the fracture critical designs. In many cases, more beams are added to spans to improve the redundancy, which make the bridge less efficient and therefore more costly. In several states, multi-span continuous trapezoidal box girder ramp bridges are avoided due to concerns about fracture redundancy. Existing bridges that are categorized as fracture critical require additional inspections that are costly to owners. Removal of a fracture-critical designation on existing bridges will result in fewer inspections over the lifetime of the bridge. If a system-wide analysis method is developed, many new bridges and many more existing bridges can be classified as non-fracture critical. This will save owners significant monies in design, construction, and inspection.

Language

  • English

Project

  • Status: Proposed
  • Funding: $400000.00
  • Contract Numbers:

    Project 12-87

  • Sponsor Organizations:

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001
  • Project Managers:

    Dekelbab, Waseem

  • Start Date: 20100615
  • Expected Completion Date: 0
  • Actual Completion Date: 0
  • Source Data: RiP Project 26492

Subject/Index Terms

Filing Info

  • Accession Number: 01463916
  • Record Type: Research project
  • Source Agency: National Cooperative Highway Research Program
  • Contract Numbers: Project 12-87
  • Files: TRB, RiP, USDOT
  • Created Date: Jan 3 2013 2:32PM