Enhanced Characterization of the Flexural Resistance of Built-Up I-Section Members
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Abstract
The AISC 360 Specification Chapter F I-section member flexural resistance provisions are a
central part of structural steel design in the United States. The “unified” procedures of Sections F4
and F5 address general singly and doubly symmetric I-section members. Analytical studies and
experimental tests subsequent to the implementation of these provisions within the 2005 AISC
Specification suggest that the corresponding inelastic Lateral-Torsional Buckling (LTB) and
Tension Flange Yielding (TFY) resistance calculations can be improved. Sixteen new large-scale
experimental tests on thirteen specimens are targeted in this research to further investigate these
predictions. In addition, extensive shell finite element analysis (FEA) test simulation studies are
performed correlating with and parametrically extending the experimental results. The broad
objective is to provide additional supporting data for improvements to the AISC Specification
Section F3 to F5 provisions for general built-up I-section members. These improvements provide:
(1) reductions as well as increases in calculated capacities via changes to the anchor points
(Lp, Mmax) and (Lr, ML) in the LTB resistance equations, (2) increases in calculated capacities
recognizing inelastic reserve strength in members experiencing early yielding in flexural tension,
via a number of advancements, and (3) substantial shortening and streamlining of the Specification
provisions by eliminating all TFY resistance checks and addressing the corresponding behavior in
the primary limit states calculations. This report discusses the design and execution of the 16
experimental tests and hundreds of test simulations, including the details of how the test fixtures
and bracing systems are configured to minimize incidental restraint in the physical tests, as well
as the direct modeling of residual stresses and geometric imperfections in the test simulations.
Updated professional factors, Mtest /Mn, obtained from the new tests and test simulations,
considered in conjunction with recommended Chapter F provisions, show significant
improvements relative to values obtained using the current Specification rules. The updated
professional factors exhibit mean values close to 1.0, with relatively small dispersion, across the
entire range of the design space. The results from the current research, combined with an updated
assessment of historical test strengths versus predictions from the recommended procedures,
shows close to a uniform reliability index, β, of 2.6 (for building design and a live-to-dead load
ratio of 3.0) across the design space evaluated at the completion of this research.
Sponsor
Metal Building Manufacturers Association (MBMA); American Institute of Steel Construction (AISC); American Iron and Steel Institute (AISI)
Date
2022-07
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Technical Report
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