Hi every body
the client has issued new revision of stress analysis specification that including changes of load cases, but i have some problem to combination of expansion load cases logically. Is it right to combine just expansion loads together as scalar?
They changed Tinstallation from 2˚C to 15˚C and then they added load cases L7 to L9 and just stress is checked by load cases L8 and L9.
In my opinion, there is contraction at load case L7 and expansion at load cases L6 and L7(hot service),contraction and expansion does not happen at same time so it is not logical to combine them together. But they say we want to consider stress between temperature 2˚C and operating temperature by summation of expansion temperatures.
the site condition is as following:
Site condition:
Tmin= 2˚C
Tmax= 45˚C
Tsunshine: 85˚C
Tinstallation: 15˚C
Load cases Load type Summation type
L1: W+P SUS
L2: W+P+T1 OPE
L3: W+P+T2 OPE
L4: W+P+T3 OPE
L5: L2-L1 EXP ALGEBRIC
L6: L3-L1 EXP ALGEBRIC
L7: L4-L1 EXP ALGEBRIC
L8: L5+L7 EXP SCALAR
L9: L6+L7 EXP SCALAR

T1: operating Temp.
T2: design Temp.
T3=Tmin = 2˚C

any answer will be appriciated.
thanks.

Your L8 & L9 might do it but the following would, in my opinion, be more "correct":
L8: L4-L2 (EXP)
L9: L4-L3 (EXP)
CAESAR II only recommends ranges from installation to an operating state. I would add these additional load cases whenever there is an operating state both above and below ambient.
There are a few words in B31.3 Appx. S that support that scalar summation: "The displacement stress range, SE, in this example is based on the temperature range from the installation to minimum metal temperature and from installation to maximum metal temperature for the thermal cycles under analysis." (S301.7)
Don't get tied up in the timing of these strains. If one day the pipe is at minimun temmperature and at some other day, whenever, the pipe is at the maximum temperature, the system still "cycled" between this entire range. You need not go directly from min to max.

Hi dear Dave
thanks for your reply and mentioned references.

A question to you Dave, regarding your proposed load cases (L8 & L9);

- what is the proper summation type to be used in L8 and L9?

I would thought that the two alternatives for L8 and L9 produce the same results (with proper summation type applied). Is this right?

With Dave's suggestion one reduces the number of loadcases necessary which is not a bad thing.

MAHDIKHANI & habo

I prefer to add load cases like.. which is following our projects.
L8: L5+L7 EXP algebric
L9: L6+L7 EXP algebric

Ram.

My L8 & L9 would require algebraic summation.
I prefer this approach because it will work with nonlinear supports (e.g. +Y and GUIDE with GAP) and with any temperature above or below ambient.
B31.3 paragraph 319.2.3(b) refers to the displacement stress range as "the algebraic difference between strains in the extreme displacement condition and the original (as-installed) condition (or any anticipated condition with greater differential effect)".
Scalar summation works in many cases but not all.