Aluminium Design and Construction John Dwight

1. The coefficient of friction (slip-factor) is less.
2. When the connected plates are stressed in tension, the decrease in
bolt tension and hence in friction capacity is more pronounced.
3. The bolt tension falls off with decrease in temperature.
4. HSFG bolts are not suitable for use with the weaker alloys. (BS.8118
bans their use when the 0.2% proof stress of the plates is less than
230 N/mm 2 .)
High-strength friction-grip joints must be checked for the ultimate limit
state, as with non-torqued fasteners, and also for the serviceability
limit state. The latter check is needed to ensure that gross slip, and
hence sudden loss of joint rigidity, does not occur in service.
11.2.2 Bolt material
British Standard BS.8118 states that only general grade HSFG bolts
should be used with aluminium, the specified minimum properties of
which are as follows (BS.4395: Part 1):
Stress at permanent set limit 635 N/mm 2
Tensile strength (ultimate stress) 825 N/mm 2
11.2.3 Ultimate limit state (shear loading)
It is assumed for this limit state that gross slip has occurred, and that
any residual friction is not to be relied on. All the hole clearance is
taken up and the load is transmitted by dowel action, in the same way
as for conventional (non-torqued) bolts. The joint should therefore be
checked as in Section 11.1.2. In so doing, the transmitted shear force P –
per bolt arising under factored loading is found as in Section 11.1.3;
and the calculated resistance P – c as in Section 11.1.4(2). There is unlikely
to be any need to check for shearing of the bolt, because of the very
high strength of its material.
11.2.4 Serviceability limit state (shear loading)
The checking of HSFG bolts for this limit state proceeds basically in the
following manner:
1. Find the greatest transmitted shear force P – n arising in any one bolt,
when nominal (unfactored) loading acts on the structure.
2. Obtain the calculated friction capacity P – f per bolt.
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