Hello,

I'm a relatively new piping engineer and have been doing some reading on SIF's, how they are calculated, etc. I was taking a look at the SIF's shown in the stresses report in CAESAR and noticed that they don't match the values I see in the 3D Plot view. Has anyone noticed this?

In my case I have an 8" Sch.40 elbow and in the stresses report the SIF values are 2.365 In Plane and 1.971 Out Plane. The same node when viewed in the 3D plot has SIF values of 2.44 and 2.03. I know in this case it's unlikely to be significant due to how close these values are but can anyone explain why they are different?

PS

I decided to check another model of mine and found a larger variance in SIF values between the report and 3D plot.

Stresses Report - 1.710 and 1.425 (IP/OP)
3D Plot - 2.33 and 1.94 (IP/OP

Regards,

Mark

I'm unsure of your reference to SIF values shown in the "3D Plot view".

And I will assume you are using B31.3.

These stress intensification factors (SIFs) have been in the Code for many years - Markl's team introduced them in the late 1940's. They were bending moment multipliers intended to produce an effective stress used in evaluating the effects of fatigue. These numbers are calculated in accordance with (B31.3) Appendix D. [And updated, now, in B31J.]

Appx. D Note 6 has a pressure adjustment for (thin wall, large diameter) bend SIFs. This will vary the SIF from one load case to the next based on the pressure included in the load case. If the change you see is rather small, this is the most-likely cause. Note that the Miscellaneous Report holding SIFs will show a range of SIFs because of the changing pressure and, perhaps, Young's Modulus.

That SIF is also now used to set the "sustained stress index" or SSI. If you are viewing a sustained or occasional load case, you might be seeing this newer (B31.3) multiplier. In many cases SSI=0.75(SIF). Maybe that is what you are seeing.

Hello Dave,

Thank you for your response.

The 3D Plot view I'm describing is an optional view in the static output processor. Once in the 3D Plot view I can review the model and have a visualization of displacements, stresses, etc. In this view there is an element viewer which allows the review of stresses, loads, displacement and SIFs at specific nodes for any of the load cases. When I compare the SIFs in this element viewer in the 3d Plot view to a stresses report in the static output processor I get the differences I described in my original message. This isn't an isolated incident, it appears to be the case for all of my models and load cases.

I don't know if a pressure adjustment is being made because I don't believe this would be considered large diameter thin walled pipe would it?

Regardless, I'm assuming that the SIFs should be identical for a given node/load case regardless of where in the software they are viewed from. This doesn't appear to be the case unless I'm missing something. I guess I can determine which SIF is being used in the actual calculation by back calculating it from the bending/torsional and code stresses? Still leaves the question of why they aren't identical in the different static output processor views.

Regards,

Mark

Dear Mark,

Can you send your file and some screen shots to technical support. I see you are located in Canada, we have a local representative there. Do you have there contact information. Go to http://www.codecad.com/ and click on support.

We would like to investigate this and if it is an issue add to our development backlog.

For B31.3 analysis, the stress intensifiers (SIFs and SSIs) displayed in the stress report found in the output plot window are the basic SIFs found in Appendix D.
The standard B31.3 stress reports from the output processor show the bend SIFs (expansion) and bend SSIs (sustained & occasional) that include the bend adjustments for pressure and elastic modulus specified for that load case. (You can select which P & which E is used for each load case.)
The Miscellaneous Output report holding bend SIFs shows the P & E variations by listing the range of SIFs applied in the model.

I agree that the SIFs listed in the stress report associated with the plot should be consistent - they should be the SIF or SSI used to calculate the Code stress listed on that line.

Hello Dave,

Thank you it all makes sense now where these numbers are coming from. Like you said, the SIFs in the 3D Plot Element Viewer are the basic Appendix D SIFs with no adjustment for pressure or modulus for elasticity. The SIF values in the standard output processor stresses report is 0.75i or 1 (whichever is greater) and also includes the adjustment for P and E (Table D300 - Note 6 - ASME B31.3).

Mandeep,

I will send in a support request to CodeCAD about this as I don't believe this difference in SIFs displayed is intended. As we have discussed I believe the SIFs should be displayed the same in the stresses report and 3D plot element viewer and if we wish to see the basic Appendix D SIF it should be clearly defined as that or shown elsewhere.

Thank you both Dave and Mandeep.

Mark