We have one typical problem.
Our system has two case
Case 1: Ope+Weight+Pressure considered as sustained case
case 2: Ope+weight+pressure+long term settlement

My question is whether the second case shall be considered as sustained case ? Actually the system is failing in sustained case. However system is withing alloable limit for operating case ( Analysis code is B 31.8)

Use the 'search' facility. There has been many discussions on this subject. In general, settlement is not considered a sustained stress since it is practically a one-off load. It is really a secondary stress and thus can be allowed to cause limited yielding, hence many engineers will allow a limit of 3Sh.

Hi MoverZ

I agree with you that settlement can be considered as secondary load & one time load so limited yielding is acceptable though someone may argue as it a primary load.
However I will not go until 3Sh value for sustain. We will almost reach material allowable limit by that & with some added thermal load eventually exceed limit.

Regards

Habib

shr,

I did not say that settlement is a sustained stress. Clearly a sustained stress that truly exceeds yield will cause rapid failure.

For sure, do it your way, but you would probably need more pipe, more restraints and springs .... It is common practice to do as I've explained above. The point is, settlement is a one-off load which is similar to an expansion (secondary) stress, which is self limiting, and can thus be allowed to exceed local yield stress.

Also note that in my experience, expected settlement figures quoted in basis of design documents are often grossly exaggerated.

Hi MoverZ

Thanks for your clarification.
Yes additional bend or spring need to introduce to overcome that. Even sometimes bellow or hose recommended if settlement is related to some equipment nozzle ( nozzle load is deciding factor rather than stress).

Regards

Habib

shr,

When we talk of settlement being one-off, it usually refers to differential settlement between piled and slab formed foundations such as a vessel (piled) and a pump base for instance. There are circumstances such as sttlemnt of large storage tanks during filling / emptying which is not one-off, but cyclic due to the massive load change and the underlying soil spring rate. In addition there may be tank bulging. A double tied bellows expansion joint is in my opinion an elegant solution here. That said, I would not use flex joints or springs for one-time settlement.

Another point ... 'Design' settlement of for instance, a slab foundation, may well occur almost fully before any piping is hooked up.