Hi all:
I have been looking into jointing steel with welds and their respective fatigue strength, in this case, it's jointing cicular tubes and square tubes. I have a few questions, or seeing what's you guy's view on interpretion (I'm currently using AWS D1.1:2008):
Here is something I knew ( please point me out if I"m wrong):
Result Graph of Table 2.5 (Fatigue design parameter Category) is located in Figure 2.11 (nonTubular connections)
Result Graph of Table 2.7 (Fatigue Category for Circular Sections) is located in Figure 2.13 (Tubular Structure)
Category B, C, D is mainly for CJP.
Category E, F is mainly for PJP or Fillet Welds
T is for Tubular
Questions:
1) Difference between E (E, or ET) and F (F, FT)?
I saw Category F has "loaded in tension or bending" under column "situation", but the column "Kinds of Stress" has "Shear in Weld". However, in Category E, the Column "Kinds of Stress" has TCBR (Tension Compression Bending Reverse). Originally I though if we evaluate TCBR, we use Category E, and if we evaluate Shear, we use F. However, the wording under the Situation column in Category F has "Loading in Tension or Bending". So if I'm loaded in Tension, which category should I use, E or F?
2) The Title said it's for circular sections, then is it for both solid and tubular circular sections right? If it is for solid, what is the difference between 2.5, and 2.7? What would Square tube classified under (it's not circular but it's tubular)?
3) Category X_2 has the following description under situation:
Intersecting members at simple T-, Y-, and K- connections; any connection whose adequacy is determined by testing an accurately scaled model or by theorectical analysis (e.g. finite element)
The Fatigue limit for Category X_2 is 48MPa which is actually pretty decent. My interpretation is: As long as I'm modeling the parts as accurately as possible, I can use this 48MPa Fatigue limit as my design parameter. Am I correct? CJP is pretty easy to model in FEA, Fillet is a bit harder, and PJP is really tough, but as long as I"m able to model these in there, I can use this X_2 Fatigue Limit?
If anyone can help me answer any of these 3 questions, It will be greatly appreciated. Thanks for reading!
Kevin
Fatigue categories B, C, and D are primarily concerned with the changes in geometry. Notice the sketches included in the right column show copes, blocked flanges, access holes, lugs, etc. The sketches also show the most likely location of crack initiation in the case of fillets, PJP and CJP groove welds.
Cf is a constant that changes from one stress category to another. The Cf used is dependent on the fatigue category. Cf is used to determine the allowable stress range and is dependent on the expected number of load fluctuations and of course the fatigue category of the connection.
If the Fsr is below the value Fth, the load is below the endurance limit such that fatigue should not be an issue. If Fsr is greater than Fth, there is a finite number of load cycles at which fatigue cracks can be expected.
For tubular connections, the "values" of C, D, E, ET, F, and FT are found in clause 2.21.6.7 and addresses the size of the connecting weld or member. The tubular connections may be built-up members or HSS. When T, K, or Y connections are used, the values for ET or Ft come into play.
Tubular connection subject to fatigue utilize the S-N curve that is separate from those used for nontubula. In the case of AWS D1.1:2010, Figure 2.13 is used for tubular connections subject to fatigue. The stress categories for tubular connections is selected from Table 2.7.
I hope this helps.
Al
Hey Al:
Thank you for replying. I check that 2008 vs 2010 is the same regarding to the table and figure number.
Table 2.7 => Fig 2.13:
I understand "when T, K, or Y connections are used, the values for ET or Ft come into play". But my question is: When to use ET vs FT? As they have significant fatigue limit differences 1.1ksi vs 3.9ksi (350% differences). In the similar way, fatigue limit for E vs F is 2.9 ksi vs 7.0 ksi ( around 330% differences). It's a difference between a design is having a factor of safety of 3.0 vs a factor of safety of 0.9, it's pretty significant, so I really want to know when to use each case.
Table 2.7 stated that it is for circular section only, However, Figure 2.13 refers to it and Figure 2.13 said it's for Tubular structure. *** Circular section vs Tubular !!? So what is it, is it circular or tubular??. Since Table 2.7 has cases specifically for tubular joint (DT, ET, FT), so which means the others are not tubular joint. So I come down with a conclusion that the wording of Figure 2.13 is wrong and it should be for circular section only instead of tubular. In this case, I don't know what to put if I have a square tubular joint, in which it's not circular, so Table 2.7 doesn't apply. So what does square tubular classified under? Can I still use Table 2.7 and Figure 2.13?
The third thing is what qualify me to use X_2 as the category? A fully modeled weld with details down to CJP, PJP and fillet in every single joint would qualify?
Thank you so much Al and others for reading, would really appreciate for the help.
ET and FT only come into play when a T, Y, or K connection is used. T, Y, or K connections can be used when the members are round, not round, or consist of built up members (box sections).
At issue is the stiffness of the flat portions of nonround members. The flat portion of the square or rectangular sections are very flexible, thus the transfer of stress is not uniformly distributed through the entire cross section. The highest unit stress is at the corners of the cross section. That forces the welds to yield before the rest of the section. As a result, the unit stress of the entire cross section is reduced to mitigate the probability of a situation where the corners are over stress an extends into the plastic range rather than the elastic range.
Al
hmm, let's say I have a Y joint, with pjp groove weld, in Tension, Compression, Bending stress (TCBR), is it a ET stress category, or FT stress category?
Check each condition separately and select for the worst case would be my first approach. Like all tables in AWS codes, make sure you check all the applicable foot notes. A multi step process is involved that requires the designer to take into consideration a number of factors. I would have an engineer that was well versed in tubular connections check the design. Tubular connections aren't for the novice.
Best regards - Al
I see. So I guess it's an engineering call. Thank you, Al, once again!
What about square tube Y joints, can I still use Table 2.7 and figure 2.13 for stress category and stress limit? I'm afraid of the circular vs tubular part nature of the Table 2.7 and Figure 2.13.
Thanks for reading,
Kevin C
Are you asking as an inspector or design engineer?
Al
That's what I have been wondering through this whole thing.
He Is In Control, Have a Great Day, Brent
Ask for the mailing address so you can send the bill to for services rendered, or ask for a PO at this point!
It's amazing how you answer the questions asked only to be asked another one immediately afterwards as if you were working in the same company and it's normal for any one of us to design the entire project for them!!! And for free no less! SSSHHHEEEEEEEEESSSHHHH!
Respectfully,
Henry
My advice is worth exactly what I charge. In this case, nothing.
Happy 4th of July all!
Best regards - Al
