Title:
Experimental and Analytical Investigations of Doubly-Symmetric Built-Up I-Girders Subjected to Large Moment Gradient
Experimental and Analytical Investigations of Doubly-Symmetric Built-Up I-Girders Subjected to Large Moment Gradient
Author(s)
Phillips, Matthew L.
Slein, Ryan
Kamath, Ajit M.
Sherman, Ryan J.
White, Donald W.
Slein, Ryan
Kamath, Ajit M.
Sherman, Ryan J.
White, Donald W.
Advisor(s)
Editor(s)
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Supplementary to
Abstract
Recent analytical studies have indicated that the current American Institute of Steel
Construction (AISC) 360-16 Specification overpredicts the flexural resistance of certain built-up
I-girders. The largest overpredictions are observed in I-girders subjected to a high moment gradient
(i.e., high shear-to-moment ratios) having unstiffened webs with a large web slenderness ratio
(i.e., height-to-depth ratio, h/tw). These recent analytical studies have consisted of elastic shell
finite element analysis (FEA) buckling solutions and full-nonlinear shell FEA solutions. The
elastic buckling solutions have targeted a wide range of web slenderness ratios and moment
gradients, and the full-nonlinear solutions have targeted members with high web slenderness ratios
and moment gradients. However, there is a lack of experimental data to confirm the analytical
findings that these I-girders are susceptible to strength overpredictions by the AISC 360-16
Specification. Additionally, the analytical studies have focused exclusively on either purely elastic
material idealizations or on high web slenderness ratios and moment gradients; therefore, a lack
of analytical data exists to determine the combined influence of inelastic material effects (i.e., web
postbuckling and/or the onset of yielding) and web slenderness for specimens subject to inelastic
lateral torsional buckling limit states. The objectives of the current research are to:
1) Provide experimental validation of the strength overpredictions through large-scale
experimental testing,
2) Validate the accuracy of full-nonlinear shell FEA solutions, and
3) Investigate the influence of web slenderness on the behavior of built-up I-girders subjected to
large moment gradient.
The current study is comprised of two main thrusts: a large-scale experimental effort and
complementary FEA studies. The experimental effort consists of six large-scale tests targeting I-girders with an unbraced length near the intersection of the scaled AISC 360 inelastic/elastic LTB strengths with the strength plateau. Recent full-nonlinear analytical studies suggest that
specimens at this unbraced length have the largest overpredictions by the AISC 360 Specification.
The large-scale tests consist of three unique cross sections and include both single curvature (three point bending) and reverse curvature loading configurations. The single curvature configuration
corresponds to a moment gradient factor, Cb, of 1.74, and the reverse curvature configuration
corresponds to a Cb value of 2.31, calculated using common design equations. One suite of full
nonlinear FEA simulations is conducted to evaluate the correlation with the experimental tests
(using measured dimensions, material properties, geometric imperfections, and an assumed
residual stress pattern), and a second suite of full nonlinear FEA simulations parametrically
extends the experimental studies over a range of unbraced lengths (using nominal dimensions,
material properties, geometric imperfections, and an assumed residual stress pattern). The results
from these simulations are compared to predicted strengths from recommended AISC 360
Specification provisions and from the first-generation Eurocode 3 standard. A third suite of
parametric FEA simulations further explores the influence of continuity across brace points, the
effects of material nonlinearity on FEA solutions, and the use of common design approximations
versus rigorous calculations for Cb.
The results from the current study show:
1) The experimental specimens exhibited strengths significantly smaller than strengths predicted
by recommended AISC 360 Ch. F provisions (up to 16 % and 32 % smaller for the single
curvature and reverse curvature specimens, corresponding to professional factors of 0.84 and
0.68, respectively), providing validation of the analytical strength overpredictions.
2) The full-nonlinear shell FEA modeling approach implemented in the current research provides
accurate simulations of the experimental test results.
3) Web slenderness directly influences the strength overprediction of the members (i.e., the
members with smaller web slenderness values had larger normalized LTB resistances and
smaller strength overpredictions by the AISC 360 procedures than the members with larger
web slenderness values).
4) The strength overpredictions of the built-up I-girders by the AISC 360 design provisions is
primarily a consequence of two factors:
a. Web distortion effects that are exacerbated by web shear stresses and are not adequately
accounted for, and
b. The direct scaling of the AISC 360 uniform bending LTB strength curve by the elasticallyderived
moment gradient factor, Cb, to levels of major-axis bending moment where
significant yielding effects are encountered (other than the AISC LTB strength curves
being capped by the plateau strength, additional yielding effects associated with the
strength increases from the application of Cb are not accounted for when using the direct
scaling methodology employed in AISC 360).
5) Built-up I-girders exhibiting early web shear buckling have significant web postbuckling
strength. As such, full-nonlinear analyses must be used to accurately predict the strength of
these members via FEA simulations. However, it was observed that the increase in strength
from postbuckling action of the web is generally negligible when the theoretical web shear
buckling strength is larger than the LTB strength. In addition, webs that exhibit early web local
buckling in flexure exhibit substantial postbuckling strength, requiring full-nonlinear shell
FEA to accurately predict the strength by FEA simulation.
Sponsor
Metal Building Manufacturers Association (MBMA); American Institute of Steel Construction (AISC); American Iron and Steel Institute (AISI)
Date Issued
2022-08
Extent
Resource Type
Text
Resource Subtype
Technical Report
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