RNS
Browse Projects > Detailed View

Impact of Tension Flange Holes on the Strength and Ductility of Composite Steel Girders

Description:

Eq. 6.10.1.8-1 in the current AASHTO LRFD Bridge Design Specifications provides a limit on the maximum major-axis bending stress permitted on the gross section of a steel girder, neglecting the loss of area due to holes in the tension flange, when checking flexural members at the strength limit state or for constructability. This equation is used in lieu of the 15 percent rule that had existed in older AASHTO design specifications, which allowed holes with an area less than or equal to 15 percent of the gross area of the flange to be neglected. For higher-strength steels, with a higher yield-to-ultimate tensile strength ratio (Y/T ratio) than Grade 36 steel, which was the most commonly used grade of structural steel back when the 15 percent rule was initially applicable, the 15 percent rule is not valid; such steels are better handled using Eq. 6.10.1.8-1. However, because of concerns about potential early rupture of the critical net section in tension flanges of girders composed of steels with higher Y/T ratios, and/or in tension flanges of composite girders in regions of positive flexure where the neutral axis is relatively high in the cross-section and the inelastic deformation demands on the tension flange may be larger, an upper limit of Fyt (where Fyt is the specified minimum yield strength of the tension flange) is currently conservatively enforced in Eq. 6.10.1.8-1.

At compact composite sections in positive flexure in straight I-girder bridges and at sections in negative flexure with compact or noncompact webs in straight I-girder bridges designed according to the optional provisions of Appendix A6, the factored flexural resistance of the section is permitted to exceed the moment at first yield at the strength limit state. However, it is conservatively required that Eq. 6.10.1.8-1 also be satisfied at the strength limit state at any such cross-sections containing holes in the tension flange. As a result, Eq. 6.10.1.8-1 will prevent a bolted splice from being located at a section where the factored flexural resistance of the section at the strength limit state exceeds the moment at first yield, unless the factored major-axis bending stress in the tension flange at that section is limited to the value given by the equation. With the use of longer spans and higher-strength steels, Eq. 6.10.8.1-1 can become more critical and may lead to staggering of flange-splice bolts even when the splice is located at lower-moment locations in order to satisfy the requirements of the equation. Eq. 6.10.8.1-1 can also control where lateral bracing members or lateral connection plates are bolted directly to the tension flange, which is a recommended detail to provide improved fatigue performance.

A potential relaxation of the current restrictions related to the impact of tension flange holes on composite steel flexural members would lead to improved flexibility in the location of bolted splices, the location of the connections for lateral bracing members, and improved economy in the member design, along with higher load ratings, at locations where holes must be placed in the tension flange, particularly for girders with longer spans where higher-strength steels are employed.

Objective:

The objective of this research is to further study the impact of tension flange holes on the strength and ductility of composite steel girders, focusing primarily on the impact on girders utilizing steels with higher Y/T ratios and on the tension flanges of composite girders in regions of positive flexure where the neutral axis is relatively high in the cross-section and the inelastic deformation demands on the tension flange are typically larger. The research should determine if modifications can be made to the existing AASHTO LRFD Eq. 6.10.1.8-1 to ensure that adequate strength and ductility can be achieved in such cases, while achieving a targeted level of reliability, prior to net section rupture of the tension flange under the larger inelastic deformation demands as the section exceeds the moment at first yield and approaches the plastic-moment resistance at the strength limit state. The effect of various parameters such as the Y/T ratio and the dimensional characteristics of the cross-section should be considered. The research should focus on Grade 50 and Grade HPS70W steels as a minimum, and perhaps include Grade HPS100W steels if the budget permits. It is anticipated that the research would consist of some level of experimental testing combined with analytical investigations. Further, the research should confirm that bolted splices designed according to the procedures given in the 9th Edition AASHTO LRFD Bridge Design Specifications are adequate to support the larger factored flexural resistance that may be permitted in the member at the splice location at the strength limit state. 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 composite steel-girder bridges in their inventory.

Related Research:

The following references related to this particular topic have been identified:

  • Dexter, R.J., S.A. Altstadt, and C.A. Gardner. 2002. “Strength and Ductility of HPS70W Tension Members and Tension Flanges with Holes,” Report to FHWA, AISI, PDM Bridge and Bethlehem Steel, University of Minnesota, Minneapolis, MN, March, 92 pp.
  • Dexter, R.J. and S.A. Altstadt. 2003. “Strength and Ductility of Tension Flanges in Girders,” Recent Developments in Bridge Engineering, Khaled M. Mahmoud (Ed.), Proceedings of the Second New York City Bridge Conference, Balkema, pp. 67-81.
  • Geschwindner, L.F. 2010. “Notes on the Impact of Hole Reduction on the Flexural Strength of Rolled Beams,” Engineering Journal, American Institute of Steel Construction, Chicago, IL, First Quarter.
  • Sivakumarin, K.S. and P. Arasaratnam. 2012. “Impact of Flange Holes on the Strength and Stability of Steel Beams,” Behavior of Structures in Seismic Area, STESSA 2012, F. Mazzolani and R. Herrera (Ed.), CRC Press, pp. 555-561.
  • The author of this RNS is not aware of any other ongoing research related to the particular issues identified above.
Implementation:

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.

Sponsoring Committee:AKB20, Steel Bridges
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Michael A. Grubb, P.E.
Date Posted:04/09/2020
Date Modified:05/06/2020
Index Terms:Flanges, Strength of materials, Ductility, Composite materials, Girders, Steel bridges, Flexure,
Cosponsoring Committees: 
Subjects    
Highways
Design
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.