The Design of Modern Steel Bridges - TEDI

Rolled Beam and Plate Girder Design 119
Figure 5.12(b) Post-buckling stiffness of plates, with different longitudinal
edge conditions.
surface. In the former case that stage signifies the ultimate state of the plate; in
the latter case, further increase in applied loading spreads the yielding
gradually over a wider surface area and through the thickness of the plate,
though with a gradually falling plate stiffness and until no increase in load can
be resisted.
In a plate subjected to shear stresses there is often a substantial reserve of
strength after the elastic critical buckling value of the shear stress is reached. In
a state of pure shear stress there are principal tensile and compressive stresses
in directions at 45 to the direction of the shear stress, as shown in Fig. 5.13.
Buckling of the plate is caused by the principal compressive stress in direction
2–2, resulting in ripples forming with their crests stretched in the direction of
principal tension, i.e. 1–1. Because of the ripples, the compressive stress
cannot increase beyond the value at the critical buckling stage, but the diagonal
tension continues to increase with applied shear. The increased diagonal tensile
stresses form what is known as a tension field. These tensile stresses have to be
resisted on the horizontal and vertical boundaries. The flexural rigidity of the
flanges resists the pulling-in effect of the tension field, while the transverse
web stiffeners act as struts to provide support to the flanges, thus forming a
truss-type system of forces. The ultimate shear capacity is reached when the
plate in the diagonal tensile band yields and also plastic hinges are formed