Aluminium Design and Construction John Dwight

Figure 8.12 Tension-field action.
is valid when 2.5 > a/d > 0.5. However, if the presence of visible buckles
at working load is unacceptable, V c must be found in the usual way as
in Section 8.3.5, with tension-field action ignored.
In any given panel between transverse stiffeners, it is only possible
for the tension field to develop if it is properly anchored at either end,
i.e. if there is something for it to pull against. In an internal panel, such
as I in the figure, anchorage is automatically provided by the adjacent
panels. But in an end panel (E) there is generally nothing substantial
for a tension field to pull on and the tension field cannot develop. Only
if a proper ‘end-post’ (EP) is provided, designed as in Section 8.6.4, can
effective tension-field action in an end-panel be assumed.
The value of V c based on tension-field action is found as follows for
a plain web (Figure 8.10(a)):
V c =dt{p v1 +k(v 2 +mv 3 )p v } (8.16)
where d, t=web depth and thickness; p v =limiting stress in shear; p v1 =initial
buckling stress (Section 8.3.5); m=lesser of m 1 and m 2 ; k=k z1 for welded
webs or 1.0 for other webs;
S f is plastic modulus of effective flange section about its own horizontal
equal area axis, taken as the lower value when the flanges differ. In a
hybrid beam the expression for m 2 should be multiplied by the squareroot
of the ratio of p o for the flange material to p o for the web.
For a web with properly designed tongue-plates V c may be found
using equation (8.15), with V cw now taken as the value of V c that would
be obtained from expression (8.16) putting d equal to the web plate
depth between tongues. When determining S f it is permissible to include
the tongue plate as an integral part of the flange.
In a multi-web beam, V c is again taken as the sum of the values for
the individual webs. In finding m 2 , the quantity S f should be shared
between the webs.
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