Design Memorandum

TO:                  All Design Section Staff

FROM:            Bijan Khaleghi
DATE:             August 20, 2006
SUBJECT:       Design Guidance for Non-Redundant Steel Box Girder Bridges

Section 6.1.3 of the BDM states that “Three or more girder lines are considered redundant. If a non-redundant bridge is proposed, approval must be obtained from the Bridge Design Engineer.”

Where warranted, and when justified for a particular application, steel bridges with non-redundant (fracture-critical) superstructures may be designed after consultation with the Bridge Design Steel Specialist, with the approval of the Bridge Design Engineer. Listed below are additional guidelines to follow when designing non-redundant, fracture-critical superstructures:

  • Single cell steel box girder superstructures are preferred, due to their relative ease of field inspection from within, and for their simplified painting requirements over the service life of the structure.
  • Desirable minimum vehicular vertical clearance shall be 20’-0” for non-redundant superstructures.
  • The section shall provide a desirable minimum walk-through height of 6’-6”, with all deck falsework and formwork removed from inside the box section upon completion.
  • Use High Performance Steel (HPS) for main load-carrying tension elements. The increased toughness of HPS will reduce potential for crack propagation during the service life of the bridge.
  • Non-redundant, fracture-critical structures may be considered for ramp bridges up to approximately 28 feet in width. The use of non-redundant, fracture-critical structures on mainlines of interstate highways or critical state routes should be avoided, and would require an exceptional justification.
  • The limit state load modifier relating to redundancy, as specified in AASHTO LRFD Article 1.3.4, shall be used in design of non-redundant steel structures.
  • The live load distribution factors for moment and shear specified in AASHTO LRFD Article are not applicable to single cell box girders. The Lever Rule, or another appropriate refined method of analysis as specified in Article 4.4 may be used for determination of live load distribution factors.
  • The requirements of AASHTO LRFD Article for curved steel bridges may be applicable.


As WSDOT mitigates congestion at several locations, there is an increasing demand for grade-separated interchanges. Recent large design projects have demonstrated the need for cost-effective, single lane structures. These structures are being located in previously-developed, highly constrained urban settings, requiring long spans, thin superstructure depths, and ease of construction over existing traffic. Thoughtful use of non-redundant superstructures can provide efficient, cost-effective, and safe solutions for these challenging site and structural constraints.

If you have any questions regarding these issues, please contact Rich Zeldenrust at 705-7196 or Nate Brown at 705-7219

cc:   Mohammad Sheikhizadeh, Bridge Construction - 47354

       F. Posner, Bridge and Structures – 47340