Aluminium Design and Construction John Dwight

The effective section is obtained by replacing the area Az of the
nominal HAZ in each softened region by an effective area kzAz . The
effective section properties may then be found by one or other of the
following methods (refer also to Sections 10.2.4, 10.3.4):
1. Method 1. An effective plate thickness of kzt is generally assumed in
each nominal HAZ region, instead of the true thickness t. In thick
material, such that the nominal HAZ penetration s is less than t, the
effective thickness is taken equal to {t-s(1-kz )}. The properties of the
effective section thus obtained are then determined in the usual way.
2. Method 2. The section properties are first calculated with HAZ softening
ignored, and suitable deductions then made corresponding to a ‘lost
area’ of Az (1–kz ) at each HAZ.
Method 2 is generally preferred for members just containing small
longitudinal welds, since it only involves A z which is readily found
(equation (6.6)), without the need to know the exact location of the
softened metal.
For any given material there are in general three possible k z -values
(see Section 6.4.1). The first two of these are needed for member design
and are selected as follows:
k z1 shear force resistance in beams (Section 8.3);
local failure in tension and compression members (Section 9.3).
k z2 moment resistance in beams (Section 8.2);
general yielding in tension members (Section 9.4);
overall buckling (Sections 8.7.4, 9.5, 9.6).
In service, the strains arising in the softened zones at a partially welded
cross-section are the same as those in the adjacent unsoftened material.
With the above method of calculation therefore, using an effective section,
the HAZ material will yield prematurely—well before the calculated
resistance of the member is reached, and possibly even at working
load. Some inelastic deflection therefore takes place, leading to a slight
permanent set after the first loading. This does not matter, since the
component will behave elastically under further load applications, and
the inelastic deflection is small. (A similar state of affairs exists with
steel members, where premature yielding occurs in parts of a section
due to the very considerable locked-in stresses present in all steel
components—much worse than in aluminium.)
6.6.2 Design of joints
In determining the resistance of a load transmitting joint, two calculations
are generally needed: one for failure of the actual weld metal, and one
for failure in the fusion zone—see Section 11.3. The latter refers to a
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