RNS
Browse Projects > Detailed View

Extending AASHTO LRFD Article 6.10.7.1 and Appendix A6 Provisions to Skewed and/or Curved Steel I-Girder Bridges

Description:

Currently, in the AASHTO LRFD Bridge Design Specifications, the nominal flexural resistance of compact web or noncompact web sections in negative flexure in kinked (chorded) continuous I-girder bridges, horizontally curved I-girder bridges, and straight I-girder bridges with supports skewed more than 20 degrees from normal is not allowed to exceed the moment at first yield at the strength limit state; that is, the provisions of Appendix A6, which permit sections with compact or noncompact webs to exceed the moment at first yield, may not be employed. Such bridges typically experience significant differential vertical deflections at the cross-frames and larger cross-frame forces in the elastic range. As interior-pier sections yield and begin to lose stiffness and shed their load, the forces in the adjacent cross-frames will increase. There is currently no established procedure to predict the resulting increase in the forces without performing a refined nonlinear analysis. In horizontally curved bridges and severely skewed straight bridges with discontinuous cross-frames, significant lateral flange bending effects can occur. The resulting lateral bending moments and stresses are amplified in the bottom compression flange adjacent to the pier as the flange deflects laterally. There is currently no means to accurately predict these amplification effects as the flange is also yielding. Skewed supports also result in twisting of the girders, which is not recognized in plastic-design theory. The relative vertical deflections of the girders create eccentricities that are also not recognized in the theory.

Similarly, the nominal flexural resistance of composite sections in positive flexure in kinked (chorded) continuous I-girder bridges and in horizontally curved I-girder bridges, that would otherwise qualify as compact sections, is currently not allowed to exceed the moment at first yield at the strength limit state; that is, the sections must be treated as noncompact sections and the provisions of Article 6.10.7.1 for compact sections, which permit the section to exceed the moment at first yield, may not be employed. The specifications are currently silent with regard to such sections in straight I-girder bridges with supports skewed more than 20 degrees from normal; as such, the decision as to whether the section should be treated as a compact or noncompact section is left to the judgment of the Engineer.

These restrictions were put in place several years ago; the AASHTO design specifications prior to that were silent on this issue. It was felt that pending the completion of further research addressing some of the issues identified above, it was best to take a conservative approach in the specifications. As a result, some older steel bridges falling under the categories listed above that were not subject to these restrictions may experience some difficulties in load rating. Designs for new bridges falling under these restrictions may potentially be overconservative. The removal of these restrictions may potentially lead to improved economy for new designs and higher load ratings for existing designs.

Objective:

The objective of this research is to determine if the restrictions identified above can potentially be lifted in determining the nominal flexural resistance at the strength limit state of compact web or noncompact web sections in negative flexure, and composite sections in positive flexure that would otherwise qualify as compact sections, in kinked (chorded) continuous I-girder bridges, horizontally curved I-girder bridges, and straight I-girder bridges with supports skewed more than 20 degrees from normal. The research should identify any deleterious effects on cross-frames forces, flange lateral bending moments and stresses (and the amplification of those moments and stresses as applicable), and torsional displacements of the girders that may need to be considered should these restrictions be lifted. Reasonable and rational design approaches should be developed to calculate the maximum nominal flexural resistance that can be achieved for each type of section (if any adjustments to the current approaches are necessary), along with a means to consider any other related effects that may need to be considered in the design of the various components of the superstructure system.

Since the cost of experimental testing of full- or half-scale bridge systems is likely prohibitive, it is anticipated that the research would be conducted based on a parametric study of various practical and realistic I-girder bridge geometries using refined non-linear analyses. The study should be limited to an investigation of sections in negative flexure with compact or noncompact webs and sections in positive flexure that would otherwise qualify as compact sections if the current restrictions on bridge geometry were not in effect. Based on the research findings, the research team should propose revisions/additions to the AASHTO LRFD Bridge Design Specifications in the form of draft ballot items.

Benefits:

The removal of the restrictions described herein within the AASHTO LRFD Bridge Design Specifications may potentially lead to improved economy for new designs and higher load ratings for existing designs, which would benefit all State DOTs that have skewed and/or horizontally curved steel I-girder bridges in their inventory.

Related Research:

The author of this RNS is not aware of any specific research that has previously been conducted or is ongoing with regard to the particular issues identified above.

Implementation:

The results of this research would be implemented in the AASHTO LRFD Bridge Design Specifications. A potential supporter of this research would be the AASHTO T-14 Technical Committee for Structural Steel Design (Chair: Tom Macioce, P.E., Chief Bridge Engineer, PA Department of Transportation, 717-783-7615, tmacioce@pa.gov).

Sponsoring Committee:AKB20, Steel Bridges
Research Period:Longer than 36 months
Research Priority:High
RNS Developer:Michael A. Grubb, P.E.
Date Posted:04/09/2020
Date Modified:05/06/2020
Index Terms:Load and resistance factor design, Skew bridges, Curved bridges, Girder bridges, Girders, Steel bridges, Specifications,
Cosponsoring Committees: 
Subjects    
Highways
Design
Materials
Bridges and other structures

Please click here if you wish to share information or are aware of any research underway that addresses issues in this research needs statement. The information may be helpful to the sponsoring committee in keeping the statement up-to-date.