Title:
Built-Up I-Section Member Flexural Resistance: Inelastic Cb Effects from Web Shear Post-Buckling and Early Tension Yielding
Built-Up I-Section Member Flexural Resistance: Inelastic Cb Effects from Web Shear Post-Buckling and Early Tension Yielding
Author(s)
Deshpande, Ajinkya M.
Kamath, Ajit M.
Slein, Ryan
Sherman, Ryan J.
White, Donald W.
Kamath, Ajit M.
Slein, Ryan
Sherman, Ryan J.
White, Donald W.
Advisor(s)
Editor(s)
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Supplementary to
Abstract
To address the influence of nonuniform bending on the lateral-torsional buckling (LTB)
capacity of steel I-section members, the AISC 360 Specification directly scales the calculated
uniform bending resistance by the moment gradient modification factor, Cb. Various Cb factors are
recommended in the Specification and its Commentary. Most of these factors are derived from
elastic LTB solutions using thin-walled open-section (TWOS) beam theory.
When the LTB resistance is scaled to certain moment levels, additional flexural yielding occurs
in the physical member. The corresponding reductions in member stiffness tend to limit the
buckling strength. This behavior may be referred to as an “inelastic Cb effect.” The present AISC
Cb calculations do not account for this effect. The resulting over-estimate of the strength tends to
be relatively small in many situations; however, this effect can be significant in certain problems.
For instance, significant reductions in flexural strength can occur due to web post-buckling
distortion in thin-web members subjected to high shear demands. These reductions may be
considered as a moment-shear interaction problem; however, they can be described more directly
as an inelastic Cb effect. The more commonly recognized inelastic Cb effect is often influenced
substantially by yielding induced by significant second-order compression flange lateral bending
that occurs as the strength limit is approached; web shear post-buckling deformations exacerbate
these effects. Several specific recent advances in I-section member design – advances that capture
the substantial shear post-buckling strength of unstiffened webs, as well as improvements that
recognize significant inelastic reserve strength in sections exhibiting early tension flange yielding
– potentially can lead to larger inelastic Cb effects.
This research aims to investigate the accuracy of recommended improvements to the AISC
360-16 Section F4 and F5 provisions for the design of general built-up I-section members, with a
primary focus on addressing inelastic Cb effects in cases where they become important. The
research evaluates the strength behavior and ultimate load capacity of a number of specific sets of
I-section members having geometries particularly sensitive to these effects. Refined shell finite
element analysis (FEA) test simulations are implemented to investigate the detailed influence of
web shear post-buckling distortions as well as flexural yielding effects including early tension
flange yielding. The results from the simulations are compared to “manual” calculations using the
iv
Specification Sections F4 and F5, as well as recently recommended improvements to these
provisions. Refined TWOS inelastic buckling solutions using stiffness reduction factors based on
the recommended equations are also considered.
The research studies show that web transverse stiffening based on a rule of thumb originally
recommended by Basler is effective to limit some of the largest reductions in strength due to web
shear post-buckling distortion effects. In addition, it is found that the traditional application of Cb
solely in the calculation of the elastic LTB stress, FeLTB, followed by the use of the ratio Fyc/FeLTB
in a form of the recommended resistance equations, provides an accurate to conservative
calculation of the flexural resistance in cases where simple scaling of the uniform bending
resistance significantly over-estimates the capacity. These alternative calculations are the same as
employed for general nonprismatic I-section members in the AISC/MBMA Design Guide 25, and
are akin to the use of Fy /Fe in the AISC column strength equations. Guidelines are provided
defining when these alternative calculations are needed, and when the more common scaling of
the uniform bending resistance is sufficient.
Sponsor
Metal Building Manufacturers Association (MBMA); American Institute of Steel Construction (AISC); American Iron and Steel Institute (AISI)
Metal Building Manufacturers Association (MBMA)
Metal Building Manufacturers Association (MBMA)
Date Issued
2021-03
Extent
Resource Type
Text
Resource Subtype
Technical Report
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