I have a case that :
The individual stress of W and P1 is smaller,but the combination of W+P1 is larger than allowable code.
Pls. find the results attached below.
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• From Code Allow
• Node Stress Stress
• (-- KPa --) (-- KPa --)
• CASE 1 (OPE) W+T1+P1 30 203582. 0.
• CASE 2 (SUS) W 30 19281. 130311.
• CASE 3 (EXP) T1 30 7. 173748.
• CASE 4 (OPE) P1 30 22540. 0.
• CASE 5 (SUS) W+P1 30 136715. 130311.
• CASE 6 (EXP) L6=L1-L5 30 68352. 173748.

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Can anybody tell me the reason?

Superposition doesn't hold for piping systems, especially if you have non-linear restraints in the model.