Aluminium Design and Construction John Dwight

11.1.5 Joints in shear, member failure
A third possible mode of failure in a shear-type joint is tensile failure
of the connected member at the minimum net section. This will normally
have been covered already under member design. In some situations,
however, there is a possibility of joint failure by ‘block shear’. The
check for this is well covered in Eurocode 3 for steel design, the same
principles being applicable to aluminium.
11.1.6 Joints in tension, fastener force arising
Here we consider tension joints made with ordinary bolts or other nontorqued
threaded fasteners. For design purposes, any initial tension is
ignored, and the joint is analysed as if the bolts were initially done up
finger tight. In many joints, the tension P – arising per bolt under factored
loading can be taken as the external force P divided by the number of
bolts in the group. In other situations, P – may be calculated making the
same assumptions as in steel.
A problem arises when a connected flange is thin and the bolts are
so located as to cause ‘prying’ action to occur, with a significant increase
in the bolt tension. Rules for dealing with this appear in steel codes, but
their validity for use with aluminium is not clear.
Although the static design of bolts in tension ignores the initial bolt
tension, this does not mean that the tightening of the bolt is unimportant.
In all construction it is essential to do bolts up tight: (a) to improve the
stiffness of the joint; (b) to prevent fatigue failure of the bolts; and (c)
to stop them working loose in service.
11.1.7 Joints in tension, fastener resistance
Here we just consider bolts and other threaded fasteners, rivets being
unsuitable for tensile loading. The calculated resistance per fastener
may be found from the expression:
P – c =p t A2 (11.7)
where: p t =limiting stress in tension (table 11.1),
=0.45f u for aluminium bolt,
=0.55f u for steel or stainless steel bolt,
f u =minimum ultimate stress of bolt material,
A 2 =‘stress-area’ (table 11.2).
The reason for taking a seemingly more conservative p t -value for aluminium
bolts is their lower toughness. And the justification for using the stress
area A 2 , which is greater than the core area A 3 , lies in the redistribution
of stress that occurs after initial yielding at the thread root.
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