Local buckling of thin-plate elements - The Behaviour and Design of Steel Structures to EC3 - page 123

This approximation is conservative, as it assumes that the plastic shape factor for
web bending is 1.0 instead of 1.5.
While the elastic buckling of slender unstiffened webs under combined shear
and bending has been studied, the ultimate strengths of such webs have not been
fully investigated. However, it seems possible that there is only a small reserve
of strength after elastic buckling, so that the ultimate strength can be approxi-
mated conservatively by the elastic buckling stress combinations which satisfy
equation 4.39.

4.5.2.2 Stiffened plates

Theultimatestrengthofastiffenedwebundercombinedshearandbendingcanbe
discussedintermsoftheinteractiondiagramshowninFigure4.24fortheultimate
shear force V and bending moment M acting on a plate girder. When there is
no shear, the ultimate moment capacity is equal to the full plastic moment M p ,
providingtheflangesareClass1orClass2(seeSection4.7.2),whiletheultimate
shear capacity in the absence of bending moment is

V ult = dt τ ult , (4.43)

where the ultimate stress τ ult is given approximately by equation 4.31 (which
ignores any contributions made by the bending resistance of the flanges). This
shear capacity remains unchanged as the bending moment increases to the value
M fp which is sufficient to fully yield the flanges if they alone resist the moment.
Thisfactformsthebasisforthewidelyusedproportioningprocedureforwhichit
is assumed that the web resists only the shear and that the flanges are required to

1.0

0.8

0.6

Equation 4.44

0.4

Typical interaction curve

0.2

0

0 0.2 0.4 0.6 0.8 1.0

M / M p

Figure 4.24 Ultimate strengths of stiffened webs in shear and bending.