The National Academies

NCHRP 20-07/Task 415 [Final]

Proposed AASHTO Guidelines for Bottom Flange Limits of Steel Box Girders
[ NCHRP 20-07 (Research for AASHTO Standing Committee on Highways) ]

  Project Data
Funds: $80,000
Research Agency: Georgia Tech Research Corporation
Principal Investigator: Dr. Donald W. White
Effective Date: 8/9/2018
Completion Date: 7/8/2019

Steel box girder bridges are being used on a more and more widespread basis as they represent an alternative to steel I-girder bridges. Designers can be tempted to specify very thin/slender bottom flanges for steel box girders in positive moment regions (where the bottom flange is in tension). However, specifying too thin or too slender of a bottom flange could result in problems during fabrication, transportation, erection, or future inspection/maintenance, and potential strength and fatigue issues. 
Currently there is no consensus on minimum thickness and maximum slenderness limits for steel box girder bottom flanges. Industry recommendations on minimum bottom flange thicknesses vary, but are generally in the range of ¾” to 1”. Industry recommendations for bottom flange width to thickness (b/t) slenderness ratios also vary, generally over a range from 80 to 135. Since there are currently no limits in the AASHTO LRFD Bridge Design specifications, designs with even higher slenderness ratios are not precluded.
The objective of this project was to propose AASHTO guidelines for the minimum thickness and maximum slenderness limits for steel box girder bottom flanges to (1) minimize issues during fabrication, handling, and erection; and (2) enhance the long-term performance of steel box girders.
STATUS: Agency's final report is available at:

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