Determination of design stresses in EN 13480

[XL83NL]

I'm currently developing pipe specs for 800H (1.4958) material designed to EN 13480. I'm having difficulties getting the design stresses right.

Acc. EN13480-3 Ch 5.1, the design stress is the lesser of the time-dependent and time-independent stress values. Determining the
- time-independent stress values is done using EN 13480-3 eq 5.2.2-1 and values from EN 10216-5 table 7 and 9. For example, at T = 550 C, I get a design stress f = 63.3 MPa. This is the result of R_p1.0 = 95 MPa divided by 1.5. This is the 'highest stress I could find', which is for 550 C.
- time-dependent stress values is done using EN 13480-3 eq 5.3.2-1 and values from EN 10216-5 Annex A, table A.1. For example, at T = 560 C, I get a design stress f_cr = 138.7 MPa. This is the result of S_RTt = 95 MPa divided by Sf_cr = 1.5.

Im not sure where EN 13480 defines the onset of creep (i.e. time-dependent stress values). This topic is closely related to this topic

I'm obviously missing the point here somewhere, as design stress values just cant go up like that, esp in the creep regime.
Someone can point what I'm doing wrong?

 

[DSB123]

Hi,
If I read your post correctly you say for the time dependant stress you have SRTt divided by 1.5 and get 138.7 ??? Do the math again!!!

 

[XL83NL]

Haha, correct, big typo error there . It should have read;

For example, at T = 560 C, I get a design stress f_cr = 138.7 MPa. This is the result of S_RTt = 208 MPa divided by Sf_cr = 1.5

PS: for my calculations I use the 10.000 hr creep rupture strength values from EN 10216-5, as we have a reduced plant design life.