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Flexural Capacity of Steel I-Girders over Interior Piers

Description:

The flexural design and behavior of steel I-girders in negative moment regions over interior piers of composite continuous-span bridges is complex. Longitudinal steel bridge members are often non-prismatic in these regions due to the introduction of flange transitions (i.e., at welded shop splices or field splices) and/or variations in the depth of the member. Specifications prior to the AASHTO LRFD Specifications were essentially silent regarding the effect of changes in the member cross-section along the unbraced length on the member lateral-torsional buckling resistance. The current AASHTO LRFD Bridge Design Specifications include some simplifications to allow the use of non-prismatic (stepped) flanges in an effort to allow for more economical designs, but these simplifications were necessarily conservative since there was not sufficient research to support more realistic provisions. There is no specific guidance currently given for calculating the lateral-torsional buckling resistance of variable depth members.

Based on recent related research performed for the Metal Building Manufacturers Association (MBMA) and the American Institute of Steel Construction (AISC), an approximate approach has been developed to handle non-prismatic I-section members with a single cross-section transition that are either constant depth or contain a linear or a concave parabolic variation of the web depth within the unbraced length. The approach is applicable to members utilizing compact web, noncompact web, or slender web sections. An updated second edition of the original AISC/MBMA Design Guide (Kaehler et al. 2011) is under final review. It is anticipated that this approach will be brought forward within the next year for possible consideration by the AASHTO T-14 Technical Committee for Structural Steel Design. Along with this, there is a need to develop a comprehensive document to explain the general procedure for calculating the lateral-torsional buckling resistance of non-prismatic longitudinal steel bridge I-section members in negative-moment regions over interior piers using this approximate approach, or with analytical tools that are now available, and provide several illustrative examples illustrating the application of these procedures for different geometries.

Additional research is also needed to extend the above-mentioned procedure to unbraced lengths with more than one cross-section transition and with a convex parabolic variation of the web depth, and to potentially more accurately determine the flexural resistance of non-prismatic members with access holes or perforations. Research is also needed to study more closely the effects of transverse normal stresses in the web within unbraced lengths adjacent to interior piers containing a parabolic variation of the web depth, particularly where the bottom flange transitions from an inclined orientation to a flat orientation near the bearing support, and to potentially develop some additional design recommendations related to these stresses.

Objective:

A primary objective of this project is to develop a written document similar to AISC/MSBA Design Guide 25 (second edition) for potential publication by the industry that applies to longitudinal steel bridge members. The publication should explain the general procedure for calculating the lateral-torsional buckling resistance of non-prismatic longitudinal steel bridge members in negative-moment regions over interior piers using the approximate approach mentioned above, or with analytical tools that are now available, and contain several illustrative examples illustrating the application of these procedures for different geometries (with and without moment or flange stress gradients along the unbraced length) utilizing those tools. It is envisioned that this publication would be referenced in the AASHTO LRFD Specifications and in the Manual for Bridge Evaluation, replacing any references to the current simplified procedure.

There are two outstanding issues related to the implementation of the AISC/MBMA Design Guide 25 approach for non-prismatic bridge members:

  1. A calculation of the elastic buckling load for a general non-prismatic I-section member with moment or flange stress gradients along the length is needed; that is, the Engineer needs to calculate ge. The elastic lateral-torsional buckling resistance for a general non-prismatic unbraced length, and even in some of the simpler cases, is not easily amenable to characterization by accurate algebraic equations. However, the technology is now becoming available for the accurate calculation of ge. General lateral-torsional buckling calculation (software) tools are now becoming available to provide a rigorous and reliable elastic buckling solution for a given non-prismatic geometry.

  2. With this capability now becoming available, specific documentation of how the AISC/MBMA Design Guide 25 approach can be applied to bridge I-section members needs to be provided, including clear design examples. The primary target of AISC/MBMA Design Guide 25 is metal building frame construction, and its examples involve the sizing of metal building frame members. The documentation of the procedures within the specific context of the AASHTO LRFD Bridge Design Specifications will greatly aid in the understanding and acceptance of the methods by bridge engineers.

Another primary objective of this project would be to perform research to extend the approximate approach mentioned above to unbraced lengths with more than one cross-section transition and with a convex parabolic variation of the web depth, and to potentially more accurately determine the flexural resistance of non-prismatic members with access holes or perforations. A final objective would be to perform an in-depth analytical study of the transverse normal stresses within the web in the unbraced lengths adjacent to interior piers containing a parabolic variation of the web depth, and to develop some additional design recommendations related to these stresses, as needed.

Benefits:

The analytical studies and design guideline development proposed in this research would allow engineers to recognize more structural capacity in steel I-girders in negative moment regions over interior piers, leading to more economical girder designs and increased girder load rating factors. This will result in lower initial construction cost for new bridges, and will help avoid the need for unnecessary load posting, retrofit, or replacement of existing bridges. The proposed research satisfies the AASHTO SCOBS Strategic Plan Objective Number 4: Maintain and Enhance the AASHTO Specifications.

Related Research:

Related research performed recently for the MBMA applies primarily to members utilized in metal building frame construction, including members with a linearly varying web depth or with double tapers. This research would extend the procedures originally developed as part of that research to non-prismatic members utilized more commonly in steel-bridge construction, such as I-section members with stepped flanges containing one or more cross-section transitions within the unbraced length and members with either a concave or convex parabolic variation of the web depth, and clearly explain their application to bridge designers and evaluators.

Tasks:

  1. Literature review

  2. Extend existing procedures for design evaluation of lateral-torsional buckling resistance of single and double linearly tapered web members, and parabolic haunches subjected to negative bending, to address a more comprehensive range of variable web depth members. These extensions would include further enhancement of procedures for parabolic haunch regions, handling of fish-belly type variable web depth members, and handling of multiple cross-section transitions. In addition, further consideration of the influence of continuous deck bracing in noncomposite and composite non-prismatic girders should be considered.

  3. Investigate the influence of flange curvature and/or discrete changes in the angle of orientation of a flange on member webs, including guidance on when transverse stiffening (e.g., partial depth web stiffeners) is necessary.

  4. Extend methods to include other non-prismatic geometry, such as variation of the cross-section along the length of arch ribs, subjected to uniaxial or biaxial bending plus axial compression, and torque.

  5. Address handling of other non-prismatic geometry aspects such as perforated flange and/or web plates in box-section members, and handling of flange or web access holes, with emphasis on simple, designer-friendly procedures.

  6. Demonstrate advanced software capabilities for analysis of the above design considerations.

  7. Develop a guidelines document, similar to AISC/MBMA Design Guide 25 (second edition), Design of Frames Using Non-prismatic Members, focused on design considerations specific to steel bridge construction.

Implementation:

Regarding implementation, a guidelines document is sorely needed that can assist engineers with the analysis and design of non-prismatic cross-section bridge members. It is envisioned that such a document would have a scope comparable to various NSBA Guide Documents. This document should be referenced from the AASHTO LRFD Specifications and other AASHTO design and/or rating specifications, and changes should be implemented in these specifications that provide the key basic information and guidance and point to the broader design guidelines document for further information. In addition, regarding advanced software capabilities, demonstration software such as SABRE2 (White, 2018) should be developed that emphasizes bridge applications. Such software may provide the impetus for software providers to implement these advanced capabilities.

The AASHTO T14 Committee can be an ideal champion of this effort. The major barrier to the use of basic design capabilities discussed in the AISC/MBMA Design Guide 25 is the lack of clear published guidance in the bridge design area. The major barrier to the use of more advanced analysis capabilities is the implementation of these methods by software providers. The demonstration of advanced capabilities in the proposed project should provide a better definition of the clear benefits to software providers who implement these capabilities, and to engineers who take advantage of them.

Relevance:

Potential users of the research results would be bridge designers and evaluators

Sponsoring Committee:AFF20, Steel Bridges
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Michael A. Grubb, P.E.
Source Info:Kaehler, R., White, D.W. and Kim, Y.D. (2011). “Design of Frames Using Web-Tapered Members, AISC/MBMA Design Guide 25, AISC, Chicago, IL.

White, D.W., Jeong, W.Y. and Togay, O. (2018). http://www.white.ce.gatech.edu/sabre (January 30, 2018)
Date Posted:02/14/2017
Date Modified:03/19/2018
Index Terms:Load factor, Load and resistance factor design, Steel bridges, Bridge members, Flexural strength, Long span bridges, Buckling, Bridge piers,
Cosponsoring Committees: 
Subjects    
Highways
Construction
Design
Maintenance and Preservation
Bridges and other structures

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