Aluminium Design and Construction John Dwight

3. The operating temperature is significantly lower than when the bolts
were tightened (differential thermal contraction).
4. There is poor fit-up because the plates are initially warped.
In situation (1) it is essential to make a suitable calculation and obtain
a modified value for R – . This may be done using the following expression:
R – =To-kT1 (11.10)
in which T1 is the applied external tensile force that arises under nominal
(unfactored) loading per bolt. The factor k is typically taken as 0.9 in
steel design. In aluminium construction, because of the lower modulus
of the aluminium plates, a larger proportion of T1 is used up in increasing
the bolt tension, and the drop in R – is correspondingly less. Despite this,
BS.8118 still takes k=0.9 in equation (11.10). We would recommend k=0.8
as a reasonable and more favourable design value.
The Poisson’s ratio effect when the connected plates are stressed in
tension (situation (2)) is more of a factor with aluminium than it is with
steel, because of the lower modulus E and the higher Poisson’s ratio. In
steel, the effect is usually ignored. In aluminium, the reduction in R – is
greater, possibly reaching 10 or 20% of To under the worst conditions.
The designer must therefore decide whether or not to make an arbitrary
adjustment to R – , depending on the level of the tensile stress in the
plates. British Standard BS.8118 suggests that no allowance is necessary
until the plate stress under nominal loading reaches 60% of the proof
stress.
The temperature effect (3) does not arise in an all-steel joint. In
aluminium, if the ambient temperature falls to 30°C below that at the
time of torquing, one would expect a decrease in R – of about 10% of To due to this effect. Again it is up to the designer to decide whether any
allowance is needed, depending on the environment.
Poor fit-up (4) only becomes a factor in massive joints between thick
plates containing a lot of bolts. It can be minimized by employing a
careful torquing sequence, and is likely to be less serious in aluminium
anyway because of the lower modulus.
11.2.6 Slip factor
The value to be taken for the slip factor in equation (11.9) depends
critically on the preparation of the plate surfaces before assembly. British
Standard BS.8118: Part 1 states that if a standard procedure is used,
which involves blasting with G38 grit (refer to BS.2451), the value =0.33
may be taken, compared with a typical figure of 0.45 used in steel
design. Note however that there seems to be inconsistency between
Parts 1 and 2 of BS.8118 in this respect.
Copyright 1999 by Taylor & Francis Group. All Rights Reserved.