Why did older versions of the AISC Manual show nominal shear stress of A325 bolts as 48 ksi? 1

[Evan1127]

This issue has definitely caused some confusion in a project I'm working on, and I haven't found any commentary on the change.

I have the 13th Edition of the AISC Steel Construction Manual. Table J3.2 shows the nominal shear stress of A325 bolts (threads not excluded) as 48 ksi. The RCSC Specification Table 5.1 has 54 ksi, as does the 15th Edition of the AISC Manual. Based on the 60% rule, 54 ksi makes sense.

It's really bugging me why mine would show 48 ksi. Does anyone have any insight?

 

[ATSE]

If I am understanding your question correctly, the AISC 15th Ed explains this for you.
Check out the footnotes in Table J3.2 between the two codes. See the 83.3% factor and the notes about connection length in the 15th Ed?
Also, the 15th Ed commentary 16.1-436 answers your question in a not so direct way.

 

[JoshPlumSE]

I think it was just newer research that made it's way into the RCSC first. However, please note that the change occurred in the between the 13th and 14th editions of the AISC manual.

https://www.aisc.org/globalassets/modern-steel/archives/2011/11/2011v11_a_comparative_look.pdf

 

[Evan1127]

Thank you, Josh. That link is exactly what I've been looking for, but couldn't find. It simply cites "improved accuracy" which I guess is good enough. I'm still a little curious how the accuracy changed, but at least I have this sanity check.

ATSE, thanks for the feedback, though those notes appear to only address differences due to connection length, which is well-explained elsewhere.

 

[ProgrammingPE]

This change was made in the 14th edition. They used to include a 0.80 factor so that end loaded connections would still work for fastener pattern lengths up to 50 inches due to unequal bolt load distribution (see the commentary after Table 5.1 in 2004 RCSC). They changed that factor to be 0.90 and decreased the fastener pattern length limit to 38 inches before Fnv requires an additional reduction factor.

Threads Excluded:
Fnv = Fu*[connection length factor]*[shear factor] = 120ksi * 0.80 * 0.62 = 60 ksi (13th edition)
Fnv = Fu*[connection length factor]*[shear factor] = 120ksi * 0.90 * 0.62 = 67 ksi (14th edition)

Thread Included
Fnv = Fu*[connection length factor]*[shear factor]*[thread factor] = 120ksi*0.80*0.62*0.8 = 48 ksi (13th edition)
Fnv = Fu*[connection length factor]*[shear factor]*[thread factor] = 120ksi*0.90*0.62*0.8 = 54 ksi (14th edition)

Commentary for 2009 RCSC now says the 0.90 factor is for "an increase in bolt force due to minor secondary effects resulting from simplifying assumptions made in the modeling of structures that are commonly accepted in practice (e.g. truss bolted connections assumed pinned in the analysis model)."

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