Greetings:
Actually, points 1 and 2 are related. Both the design strain and Eh are used in "Correction Factor for Internal Pressure", which in turn, is used in the calculation for "flexibility factor" and "SIF". Please refer to Equation (7.7) of BS 7159:1989.
1) Eh is used to calculate flexibility factor and SIF, not allowable.
2) Either design strain, or P/Eh could do above, depending upon the setting in "Configuration -> FPR Properties -> BS 7159 Pressure Stiffening"
Regards,
Lu
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Xiaojun Lu
Intergraph CAS