What is the difference between sustained stress and hot sustained stress? What is the necessity of it and when and in what situation we need to consider it? Normally, we are not using such load cases, please clarify.
Please use the search function in the forum to get an answer, it's discussed numorous times. It basically has to do with differences in primary stresses in cold and hot situation due to different supporting positions. This can happen when you're using non-lineair restraints.
What you want to check in your sustained case are the primary stresses in the network during operation. That's why you compare the stresses with the allowables at the design temperature. You calculate the stresses using W+P for your sustained case. Now if for example during operation a supports lift off due to thermal expansion, one of the supports is "taken away" from your setup and thus changing the distribution of primary stresses (stresses due to weight). Compare it with a steel beam on 3 supports each at a distance of 5 meters, and then taking the middle support away.
The situation where you calculate the primary (sustained) stresses using only the supports that are supporting the pipeline during operation is called the "hot-sustained".
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