(Georgia Institute of Technology, 2017-01)
Subramanian, Lakshmi P.; White, Donald W.
The American Association of State Highway and Transportation Officials Load and
Resistance Factor Design Specifications (AASHTO LRFD) require the use of longitudinal
stiffeners in plate girder webs when the web slenderness D/tw is greater than 150. This
practice is intended to limit the lateral flexing of the web plate during construction and at
service conditions. AASHTO accounts for an increase in the web bend-buckling resistance
due to a longitudinal stiffener in a plate girder. However, when the theoretical bendbuckling
capacity of the stiffened web is exceeded under strength load conditions, the
Specifications do not consider any contribution from the longitudinal stiffeners to the
girder resistance. That is, the AASHTO LRFD web bend-buckling strength reduction factor
Rb applied in these cases is based on an idealization of the web neglecting the longitudinal
stiffener. This deficiency can have significant impact on girder resistance in regions of
negative flexure. This research is aimed at evaluating the improvements that may be
achieved by fully considering the contribution of the longitudinal stiffeners to the girder
flexural resistance.
Based on refined Finite Element (FE) test simulations, this research establishes that
minimum size longitudinal stiffeners, per current AASHTO LRFD requirements,
contribute significantly to the postbuckling flexural resistance of plate girders, and can
bring about as much as 60% increase in the strength of the compression flange. A straincompatibility
based cross-section Rb model is developed that can be used to calculate the
girder flexural resistance at the yield limit state. This model is based on test simulations of
straight homogenous girders subjected to uniform bending, and is tested extensively and
validated for hybrid girders and other limit states. Hybrid girders use web plates of lower
yield strengths than the compression flange plates, leading to early yielding in the web, and
potential impact on girder strength. A simplified equation has also been provided that can
be used to calculate Rb for both longitudinally stiffened I-girders that may be homogeneous
or hybrid.
In testing the Lateral Torsional Buckling (LTB) limit state, it is found that there is a
substantial deviation between the AISC/AASHTO LTB resistance equations and FE test
simulations. A comprehensive parametric study is conducted to determine the appropriate
parameters to use in FE test simulations. The recommended parameters are identified as
the ones that provide the best fit to the mean of experimental data. Based on FE simulations
on unstiffened girders using these recommended parameters, a modified LTB resistance
equation is proposed. This equation, used in conjunction with the proposed Rb model from
the yield limit state also provides an improved handling of combined web buckling and
lateral torsional buckling of longitudinally stiffened plate girders.
In the course of evaluating the above limit states, it is observed that the noncompact
web slenderness limit in the Specifications, which is an approximation based on nearly
rigid edge conditions for the buckling of the web plate in flexure is optimistic for certain
cross-sections with narrow flanges. This research shows that the degree of restraint at the
edges of the web depend largely on the relative areas of the compression flange and the
area of the web in compression. An improved equation for the noncompact web slenderness
limit is proposed which leads to a better understanding and representation of the true
behavior of these types of members.
It is found that there is negligible interaction between the Flange Local buckling (FLB)
limit state and the LTB limit states for noncompact flanges with the flange slenderness
restricted as per the AASHTO 2014 Specifications. Also, the Rb calculated from the
proposed model, used along with the current Specification FLB equations is shown to
provide a better characterization of the flange local buckling capacity of longitudinally
stiffened girders.
Tests subjected to High-Shear High-Moment, and High-Moment High Shear are
considered in order to characterize the girder shear resistances and potential moment-shear
interaction for both homogenous and hybrid girders in the context of the above
improvements.
Preliminary studies on curved homogenous girders indicate that the proposed yield
limit state model is valid for yield limit state of these types of members.