Dear all,

How do you model miter joints (formed by butt welding trimmed ends of 2 pipes) in CII? To my understanding it's not a bend nor closely/widely spaced miter bend- as it contains 0 segments.

A similar question is, can i change the direction of the pipe between 2 runs without specifying "Bend"? What would be the SIF in that case? Is it considered as a normal butt weld joint or CII computes for special SIF for that?

Call it a bend and then specify miter points = 1. With miter=1, the bend radius is not used in the calculation.

That's a widely-spaced miter. CAESAR II will apply the appropriate SIF and flexibility factor.

You do not need to indicate anything special at a change in direction. If you do not specify the bend or (single) miter, the flexibility factor and SIF will both be 1.0

Dave,

I have questions too regarding bends. If the designer uses a bend other than 90 or 45 say 38 degree as an example and he wants it fabricated by cuttimg a segment from a standard 90 bend/elbow. Does CII automaticaly calculates the bend stiffness according to this angle?

Art

Art,

Bend flexibility is defined by the piping code. B31.3 does not include bend angle in setting flexibility. But smaller bend angles produce stiffer components because they have shorter arc lengths.

Try this...
Build a bend with no attached straight pipe. Anchor one end and put (an in-plane) rotation on the other (say, -0.1 degree about Z on an X-Y bend). Use the bend scratchpad to display the bend flexibility factor. Calculate the moment on the anchor due to that displacement set. You will get the same moment on a straight pipe whose length equals the arc length of the bend times the bend flexibility factor.

Shorter arcs would give shorter cantilevers; they are stiffer.

Aside... I'm not sure if B31.3 would allow you to wack off more than 6 degrees (total) on a "listed component" (like your 90 or 45 degree bend) and still assume it is a listed (pressure)component. But that's not a stiffness/analysis issue.

Dave

Hi Dave,

I tried your suggestion of modeling the single-miter bend as bend with miter=1 (and i even experimented with 2 bend angles of 5 and 15 deg. each), but SIF always stayed equal to 1. By the way, the program cannon ignore the bend radius as may have suggested. I computed for new radius using the R equation from Appendix D of B-31.1 (R=r[1+cot(theta)]/2), but it still gave me SIF of 1.
The weird thing is that i even tried increasing the number of miters (2,3.. etc), and still SIF was 1. Can you please explain what is wrong with what i'm doing?
Regarding Art's question, so do you recommend to model trimmed 90/45 deg elbow as bend which the program -in return- will install an SIF that matches the 90/45 deg elbow SIF? won't that be under-estimating the SIF at that specific fitting?

Thank you Dave of your previous reply... and you coming ones!

Ahmad,

I can drop in a single miter 90 degree joint on a 6" std pipe in B31.1 and I get an SIF listed in the expansion stress report as 4.540. What are you doing that's different? (You may have to send me your input if this goes much further.)

On your other point - the SIF for the bend is not based on the angle (while it does play a role on the closely-spaced miters). The SIF will be the same for a 45, 38 and 90 degree bend. The bend angle will change the overall stiffness of the component through the changing arc length as stated earlier.

A revelation (to me) - I never worried about resetting the program's long radius default for single miter joints because the miter radius (R) doesn't appear in the Appendix D calculations (of h, k & i). But today, because of this thread, I ran a single cut with the default (long) radius and compared it to a single cut with the Appx. D calculated radius of r/2 (actually, CAESAR II will not allow me to run a radius less than OD/2 - I'll ask to get that fixed here). The SIFs were the same but the stresses were different. It turns out that, here too, the stiffness of the component is a function of the arc length. With different arc lengths you get different loads and stresses.

And still more... I focus on B31.3 but looking closer at Appx. D in B31.1 I see a few more complications. B31.1 limits the half-angle of the change in direction (theta) to 22.5 degrees. That means these data apply to single miters that sweep up to 45 degrees. Of course I see no guidance for greater angles. CAESAR II does not monitor this limit.