Aluminium Design and Construction John Dwight

with �. It is seen that the predicted value based on expression (8.6) can
be as much as 36% too low for the fully compact case and 39% too low
for semi-compact.
8.3 SHEAR FORCE RESISTANCE
8.3.1 Necessary checks
We now consider the resistance of the section to shear force, for which
two types of failure must be considered: (a) yielding in shear; and (b)
shear buckling of the web. Procedures are presented for determining the
calculated shear force resistance Vc corresponding to each of these cases.
The resistance of a thin web to shear buckling can be improved by
fitting transverse stiffeners, unlike the moment resistance. This makes
it difficult to provide a general rule for classifying shear webs as compact
or slender. However, for simple I, channel and box-section beams, having
unwelded webs of uniform thickness, it will be found that shear buckling
is never critical when d/t is less than 750/�p where p is in N/mm o o 2 .
Stiffened shear webs can be designed to be non-buckling at a higher d/
t than this.
8.3.2 Shear yielding of webs, method 1
Structural sections susceptible to shear failure typically contain thin
vertical webs (internal elements) to carry the shear force. Alternative
methods 1 and 2 are offered for obtaining the calculated resistance V c
of these, based on yield. In method 1, which is the simpler, V c is found
from the following expression:
V c =0.8Dt 1 P v
(8.9)
where D=overall depth of section, t 1 =critical thickness, and p v =limiting
stress for the material in shear (Section 5.2). In effect, we are assuming
that the shear force is being carried by a thin vertical rectangle of depth
D and thickness t 1 with an 80% efficiency.
For an unwelded web, t 1 is simply taken as the web thickness t or the
sum of the web thicknesses in a multi-web section. If the thickness
varies down the depth of the web, t 1 is the minimum thickness. When
there is welding on the web, t 1 =k z1 t where k z1 is the HAZ softening
factor (Section 6.4).
8.3.3 Shear yielding of webs, method 2
The alternative method 2 produces a more realistic estimate of Vc which
is generally higher than that given by method 1. It considers two possible
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