Aluminium Design and Construction John Dwight

act on the structure. However, BS.8118 gives a designer the option to
increase f r by multiplying the nominal stress range by a factor � mf (>1).
This might be felt advisable if: (a) the structure will have to operate in
a very corrosive environment: or (b) failure at the position considered
would result in total collapse, i.e. there is no alternative load path. In
practise, it is fairly unusual to take � mf > 1.
British Standard BS.8118 allows a relaxation when f ranges from f t
tensile to f c compressive, in which case the compressive component
may be reduced by 40%. In other words, we then take f r =f t +0.6f c .
12.3.5 Stress-range spectrum
With variable amplitude loading, an essential step is to obtain the different
stress ranges (f r1 , f r2 , etc.) in each cycle, and one possible procedure for
so doing is the ‘reservoir’ method described in BS.8118. Referring to
Figure 12.2, the graph showing the variation of f during the cycle is
regarded as a reservoir, in which the greatest depth of water gives the
value f r1. The reservoir is then drained from its lowest point, the deepest
remaining pocket (or pockets) giving the value f r2 . The process is repeated
until all the water has been drained, thus obtaining f r3 , f r4 , etc. This
enables a stress-range spectrum to be plotted, as shown in Figure 12.3.
This method is suitable when there is a sequence of loading events
repeated many times. An alternative procedure is the ‘rain-flow’ method
described in BS.5400: Part 10 (Steel, Concrete and Composite Bridges),
which is more convenient when long and variable stress histories have
to be analysed.
Figure 12.2 Variable amplitude loading, ‘reservoir’ method.
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