Aluminium Design and Construction John Dwight

2. Material factor (� m ). In the checking of members, BS.8118 adopts a constant
value for this factor, namely � m =1.2. For connections, the recommended
value lies in the range 1.2–1.6, depending on the joint type and the
standard of workmanship. Table 5.1 includes suggested values. The
lower value 1.3 given for welded joints should only be used if it can
be ensured that the standard of fabrication will satisfy BS.8118: Part
2. Failing this, a higher value must be taken, possibly up to 1.6.
It is emphasized that the Table 5.1 �-values need not be binding. For
example, if a particular imposed load is known to be very unpredictable,
the designer would take � f higher than the normal value, or if there is
concern that the quality of fabrication might not be held to the highest
standard, � m ought to be increased.
Often a component is subjected to more than one type of action-effect
at the same time, as when a critical cross-section of a beam has to carry
simultaneous moment and shear force. Possible interaction between the
different effects must then be allowed for. For some situations, the best
procedure is to check the main action-effect (say, the moment in a beam)
using a modified value for the resistance to allow for the presence of the
other effect (the shear force), In other cases, it is more convenient to
employ interaction equations. Obviously, a component must be checked
for all the possible combinations of action-effect that may arise,
corresponding to alternative patterns of service loading on the structure.
After checking a component for static strength, a designer will be
interested in the actual degree of safety achieved. This can be measured
in terms of a quantity LFC (load factor against collapse) defined as follows:
(5.3)
where CR and NA are as defined in Section 5.1.2. For a component
which is just acceptable in terms of static strength (FR=FA), the LFC
would be given by:
(5.4)
where is the ratio of the action-effect under factored loading to that
arising under nominal loading (i.e. a weighted average of � f for the
various loads on the structure). Thus for example a typical member
might have and � m equal to 1.3 and 1.2 respectively, giving a minimum
LFC of 1.56. This implies that the member could just withstand a static
overload of 56% before collapsing. The aim of limit state design is to
produce designs having a consistent value of LFC.
Different results are obtained in checking static strength, depending
on whether the Elastic or Limit State method is used. The two procedures
may be summarized as follows (Figure 5.1):
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