The Design of Modern Steel Bridges - TEDI

Chapter 6
Stiffened Compression Flanges of
Box and Plate Girders
6.1 General features
Stiffened compression flanges of box and plate girders consist of a flange plate
stiffened longitudinally by either open (i.e. flat, bulb-flat, angle or tee) or closed
(i.e. trough, vee) types of stiffeners spanning between transverse stiffeners
which are supported by the girder webs or web stiffeners. Such compression
flanges may be subjected to the following stresses:
(1) Longitudinal stresses due to the bending moment (and axial force) on the
main girder; these stresses may vary across the width of the flange due to
shear lag, and along the length due to the variation in the bending moment;
additional longitudinal stresses may be caused by restrained warping of
a box girder.
(2) In-plane shear stresses in the flange plate due to any shear force on the
girder and/or torsion in the case of a box girder.
(3) Flexural stresses in the flange stiffeners due to any loading applied locally
on the flange, e.g. wheel loading on a bridge deck.
(4) In-plane transverse stresses in the flange plate due to bending of transverse
flange stiffeners, and in the case of box girders due to distortion of
the box cross-section and in the vicinity of internal diaphragms over box
girder supports.
Typical stiffened flange details are shown in Fig. 6.1.
A stiffened compression flange comprises several parallel struts each continuous
over and supported at many transverse stiffener locations; it can be
idealised as a series of pin-ended struts supported at transverse stiffeners. Apart
from the complex stress field mentioned above, the following geometrical
complexities need investigation:
(1) Longitudinal continuity over transverse stiffeners.
(2) Transverse continuity between parallel stiffeners.
(3) Four separate buckling modes, namely local buckling of flange plating
between longitudinal stiffeners, local buckling of stiffener components,
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