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In terms of Australia I believe the permissible/allowable stress codes were
replaced by limit state codes because:

1) Stress considered unreliable indicator of elastic instabilities, even
though the permissible stress code transformed column buckling load into a
stress. The authors considered it preferable to work directly with the
buckling loads, since buckling has to be prevented before yield of the
material can be initiated.

2) The permissible stress codes applied a single design-factor to the
strength, the result was that capacity tables for structural sections were
partly dependent on the loading code. That is they did not reflect the
resistance of the member. The limit states code split the design factor into
capacity reduction factor and partial load factors. The partial load factors
can also vary dependent on application, to assess stability. For example
0.9G should reflect the 5 percentile estimate of dead load, whilst 1.2G
should reflect the 95 percentile estimate of dead load. The appropriate
value being selected to assess instability of a structural system.

3) The idea is to move to a more probabilistic risk based assessment of
applied loads and available resistance. Capacity reduction factors can be
pushed closer to 1, if improve control over manufacturing. Whilst partial
load factors are determined probabilistically.

Since most of the theory and research concerns elastic instabilities, it is
easier to present the codes in terms of forces rather than transform into
stresses. (Plus the researchers believe the stresses are misleading, and
therefore from their professional viewpoint don't wish to present in terms
of stresses.)

But as I say as a designer, you can use what ever method you wish to find a
design-solution. Once a design-solution has been found, then need to confirm
against the methods of the code. That may mean doing the calculations twice,
but then design is iterative. Alternatively if really don't like the code,
get someone else to code check the design-solution.

Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
South Australia

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