Aluminium Design and Construction John Dwight

3. The design is satisfactory if for any one bolt
where � s is the serviceability factor (Section 11.2.7).
(11.8)
The shear load arising per bolt (P – n ) is determined using the same kind
of calculation as that employed with non-torqued fasteners (Section
11.1.3), but taking nominal instead of factored loading. This implies
that there is microscopic slip at the various bolts, as redistribution of
load takes place. Such microslip or ‘creaking’ under service conditions
must not be confused with the gross slip that occurs when the friction
finally breaks down.
The calculated friction capacity (P – f ) depends on the reaction force R–
between the plate surfaces (per bolt) and on their condition. It is given
by the expression:
P – f =nµR–
(11.9)
where n=number of friction interfaces and, µ=slip-factor (Section
11.2.6).
11.2.5 Bolt tension and reaction force
A fabricator installing HSFG bolts is required to use a torquing procedure
which ensures that the initial tension in the as-torqued condition is not
less than the specified proof load To for the size being used. Here To is
the tension corresponding to a stress, calculated on the stress-area A2 of
the bolt, that is slightly below the minimum permanent set value of the
bolt material. For general grade HSFG bolts to BS.4395: Part 1 this
stress is 590 N/mm2 , and Table 11.5 gives the resulting To-values for a
range of bolt sizes.
Ideally, a designer would hope to take the reaction force R – in equation
(11.9) equal to the bolt proof load To . In practice, there are four possible
situations in which R – becomes reduced:
1. The joint has to transmit an external tensile load, acting in the axial
direction of the bolts, as well as the shear load.
2. The connected plates are under in-plane tension (Poisson’s ratio effect).
Table 11.5 Proof load T o for HSFG bolts
Note. The quoted values apply to general grade HSFG bolts (BS.4395: Part 1) and are based on a stress
of 590 N/mm 2 acting on the stress area.
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