Bolts - Relationship between Bolt Thread Pitch and Tensile Area

[Angsi]

I have got another question. Will someone be able to clarify the relationship between the pitch and the tensile area of a bolt in terms of a formula?

Otherwise how is the tensile area availble for a bolt of a certain thread system (UNC for exmaple), pitch and shank/nominal diameter calculated?

Thanks.

 

[codeeng]

Tensile stress is based on the bolt core diameter/area. As long as you have enough thread engagement the thread pitch does not come into play.

 

[jte]

Angsi-

From the AISC's Manual of Steel Construction, Allowable Stress Design, 9th edition ("the green structural ASD book") page4-147:

Tensile Stress Area = 0.7854[D-(0.9743/n)]^2 where D is the basic major diameter in inches and n is the threads per inch.

Whether you use the gross area or tensile stress area or min root area depends on your situation. I do not think that VIII specifies which area to use (corrections humbly accepted!). When calculating exchanger bolt torques, I use the tensile stress area. But then, torque calcs and torquing are such an inexact science that there's plenty of wiggle room.

jt

 

[jamesl]

jte:
If you use section VIII allowable stress, you have to use min. root area. The code itself uses the term "cross sectional area of bolts", Ab. There is a code interpretation indicated that Ab should be calculated based on root diameter.

 

[jte]

jamesl-

Got that interpretation number by any chance? I'm not doubting you, just like to make a note of it in my code book. In any case, when calc'ing torques for bolting up exchangers, the code allowable stresses are tossed. Typical target stress values for torque are on the order of 50% to 80% yield. In this situation, I feel that the bolt tensile area is a more realistic means of determining the force than a conservative area based on the root diameter. By forcing the root diameter to be used, the code is in effect providing beefier flanges, and I'll agree with that. See Appendix S for a write up on bolt stresses. A few excerpts:

"The maximum allowable stress values... are design values... However, a distinction must be kept carefully in mind between the design value and the bolt stress that might acutally exist."

"An initial bolt stress of some magnitude greater than the design value therefore must be provided."

"...it is evident that an initial bolt stress higher than the design value may and, in some cases, must be developed in the tightening operation, and it is the intent of this Division that such a practice is permissible..."

jt

 

[jamesl]

jt,
The interpretation number is VIII-1-86-216.

The root diameter is used to size the bolts in flange design. I agree with you that code allowable stress is ignored when calculating bolt torque.

 

[jte]

jamesl-

Thanks! I went to mark up my code book and... looks like they already took care of the question in the "Notation" section (2-3) of Appendix 2.

jt

 

[2minto]

ANSI B1.1-1982 is the version I have at my desk in Appendix B there are formulas for thread tensile stress area and thread shear area for all UNC/UNF sizes listed.
Caution when using shear allowable stress B31.1-2001 102.3.1(B) only allows 80% allowable in shear.

 

[chicopee]

USE THE CROSS SECTIONAL AREA BASED ON THE MINOR DIAMETER OF THE THREAD TO DETERMINE TENSILE STRENGTH OF BOLT.

 

[cloche]

chicopee,
this is a good rule for safety, but does not necessarily correspond to Norms. European Norms state that, as soon as you have sufficient thread engagement, the threads themselves participate to the resistance and thus the resistant area is defined as the area of the circle having diameter equal to the mean between the core diameter and the nominal diameter (VDI, EN, UNI-ISO). Based on this, and on the fact that nominal diam, core diam and pitch are related, one can very easily calculate the formula for getting from pitch to Resistant Area (or Tensile Area)

 

[chicopee]

You are correct cloche and this design practice that you are mentioning is well discussed in any machine design textbook.