The Sustained load case evaluates the primary (force based) stresses in the system. But, in what "state" should the evaluation be made (operating, shut down, or something else)? If you only have linear boundary conditions, this becomes a moot point because these "states" are all the same as far as primary stresses are concerned.
However, in a system with varying boundary conditions, there is a weight redistribution between states, therefore you can have different primary stress states. Hence the need for the cold and hot sustained evaluation.
Read the write up in Chapter 6 of the Technical Reference Manual for additional details.
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Regards,
Richard Ay - Consultant