Aluminium Design and Construction John Dwight

k in order to give a fair comparison, two different geometries being
thus covered.
Our suggested treatment is broadly tailored to be slightly conservative
when compared with what has been proposed for the European (EU)
draft, when due account is taken of the different load factors used (�
values). We differ from the EU draft in our approach to bearing. First,
we only require a designer to check bearing on the ply, as in USA and
Canada, and ignore bearing on the fastener. Secondly, our expression
for p p includes both the proof and the ultimate stress of the ply material,
whereas the draft Eurocode relates it only to the ultimate. Our justification
for this is that ply-bearing concerns the gradual stretching of the hole,
which must be a function of the proof as well as the ultimate.
It is seen that the BS.8118 values for aluminium bolts in shear and
in tension are very low when compared with our suggested treatment.
For bearing on the ply, it is remarkable that the British Standard value
is 50% higher than that for bearing on an aluminium bolt made of the
same material.
11.1.10 Joints made with proprietary fasteners
When joints are made using special rivets of ‘proprietary’ design (Section
3.2.3), designers will probably rely on the manufacturer of these for
strength data. Alternatively, they may conduct their own tests to establish
the resistance. In either case it is necessary to consider carefully the
value to take for � m , and a value higher than the usual 1.2 might be
appropriate.
11.2 MECHANICAL JOINTS (FRICTION-GRIP)
11.2.1 General description
High-strength friction-grip (HSFG) bolts, made of high tensile steel, are
employed for joints loaded in shear when joint stiffness at working
load is the prime requirement. They are used in clearance holes and,
until slip occurs, transmit the load purely by friction between the plate
surfaces. Under service loading, they thus provide a rock-solid connection,
much stiffer than when close-fitting conventional fasteners are used
(non-torqued). The bolts are made of high-strength steel and are torqued
up to a high initial tension, so as to generate enough friction to stop
slip occurring at working load. The control of tightening and the
preparation of the plate surfaces is critical (Section 3.2.2).
High-strength friction-grip bolts were developed in USA in the 1940s
for use on steel, following pre-war work by Professor C. Batho at Birmingham
University in Britain. They are less attractive for use with aluminium because:
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