Section III NF Bolting Stress Limits 1

[KiddT]

I'm a little thrown by what I'm reading in Section III, Division 1, subsection NF for supports in regards to bolting stresses. I was originally under the impression that the limiting stress for all parts/bolts etc would come from Section II Part D Tables, but it seems like that may not be the case for bolting.

For design by analysis, per subsection NF, both plate and sheet type as well as linear support type analysis it seems like they point to 3324.6 for bolting analysis. (Table 3131(a)-1 all points to 3324 for bolting, or 3225 which in turn points to 3324 if I'm following correctly). In section 3324.6 (1) it states that for tensile stress only the tensile stress shall be such that Su/2. This is much higher than the values in Table 3 or 4 from Section II for allowable bolting stresses. Have I missed something along the way here or are the allowables for bolting in subsection NF really that much higher?

 

[TGS4]

There's a bit of confusion regarding the term "allowable" when it comes to bolting. The Sa values in Section II, Part D are to be used for the design of flanges. That is not to say that in actual usage that the stress int he bolts can't go higher.

As an example (in the non-nuclear world), Section VIII, Division 1 and 2 have essentially the same rules as Section III, Division 1 for the design of flanges. However, ASME PCC-1 suggests that the target bolt assembly stress value should be at least 50ksi for bolts with an allowable of 20-25ksi. This 50ksi is still < Su/2.

So, the limit in 3324.6(1) is the correct one - just don't go designing a flange with that value...

 

[KiddT]

So if you're making a non Flange (a standard support per NF-1214 in this case), you use the Su/2 as the maximum allowable? I was just surprised because if you were to use a SA - 307 Gr.A Carbon Steel bolt (per code case N-249) then Su/2 is 30 ksi which is pretty close to the yield value (36 ksi I believe). I know in 3324.6 it limits to not going above yield in the different service conditions..again I'm just surprised and wanted to make sure that's all.

 

[TGS4]

I've seen bolting stresses go to about 90% of yield during operation, so if you follow those rules, you'll be OK.