ASD 9th vs 13th Manual Thread Dimensions

[swearingen]

OK, class, open your books to page 4-147, the "Screw Threads" table in your 9th ASD, and page 7-83, Table 7-18 in the AISC 13th manual. Same table, updated manual.

Notice that the K distance has been adjusted slightly up for each diameter. Also, notice that on the diagram, the 17/24H has changed to 5/8H and the H/6 is now H/4.

Does anyone know why these changes were made? It gives a slight bump in capacities for bolts and I'm just wondering where it came from. Tighter tolerancing on the fabrication side, maybe?

 

[JAE]

I think you can post questions to AISC on their website for extremely detailed stuff like this.

http://www.aisc.org

 

[swearingen]

I posted to AISC right after I posted here. Your idea dawned on me right after I hit Submit Post...

For grins, I went back and looked at the last 8 manuals to see how bolt strength has changed and this is what I found, FWIW:

Shear values, in kips, A325, 3/4" diameter, threads included in the shear plane, values listed as single/double shear:

13th Manual (2005): ASD - 10.6/21.2 LRFD - 15.9/31.8
LRFD 3rd (2001): ASD* - 10.6/21.2 LRFD - 15.9/31.8
LRFD 2nd (1994): ASD* - 10.6/21.2 LRFD - 15.9/31.8
LRFD 1st (1986): ASD* - 10.3/20.7 LRFD - 15.5/31.0
ASD 9th (1989): ASD - 9.3/18.6 LRFD** - 14.0/27.9
ASD 8th (1980): ASD - 9.3/18.6 LRFD** - 14.0/27.9
ASD 7th (1970): ASD - 6.63/13.25 LRFD** - 9.95/19.88
ASD 6th (1963): ASD - 6.63/13.25 LRFD** - 9.95/19.88

* LRFD values divided by 1.5 for comparison
** ASD values multiplied by 1.5 for comparison

My guess this is a combination of better manufacturing tolerances and connection research.

 

[JStephen]

It could be a change in tolerances in the referenced ANSI standards. It could be a change from min/max to average values.

 

[swearingen]

The official response:

David,

Yes, I recall being involved in updating that particular table. My 9th edition ASD Manual (Third Impression dated 3/99) contains the same 5/8H and H/4 distances in the illustration as the 13th edition AISC Manual. Your particular 9th edition ASD Manual contains 17/24H and H/6 distances, so it probably is one of the first 9th edition ASD Manuals that were released.

After so many years, we did not know why the early 9th edition Manuals used different distances. It is still a mystery today. We went back to ANSI B1.1-1982 (as well as newer versions of this thread standard) and they specify 5/8H and H/4 distances. Hence we updated the 9th edition ASD Manual to reflect the dimensions found in the Standard.

My best guess is that an AISC Engineer back in the mid-1980’s made the mistake of using ANSI B1.1-1974 distances when developing the 9th edition AISC Manual. That is, ANSI B1.1-1974 used 17/24H and H/6 distances. Those same distances were used in the 7th edition AISC Manual, which cites ANSI B1.1-1974.

To sum it up, the error was in using an older ANSI B1.1 Standard, which contained different distances. We apologize for the confusion.

Serge

Sergio Zoruba, Ph.D., P.E.
Senior Engineer
American Institute of Steel Construction, Inc.
866.ASK.AISC

 

[nutte]

Swearingen, I believe the better bolt shear values for A325-N bolts are a result of better research. I can't find confirmation of this, but I remember learning somewhere along the way that this is why they were increased in the 2nd LRFD manual.

You'll note that the shear values for A325-X bolts are the same for the green ASD manual and the new 13th edition.

 

[feadude]

I have ASD 9TH edition secon impression (1/97) and
LRFD third edition first printing (11/01) and they both show 5/8 H and H/4

 

[swearingen]

As Dr. Zoruba noted, I have an earlier edition with the carry-over figures. Yours was updated.