Formula for "J" in AISC manual 3

[tnteng]

Does anyone know what formula is used to calculate the value of "J" that is listed in the structural tubing tables in the AISC manual of steel construction?

Thanks in advance for the help.

Tony Billeaud
Mechanical Engineer
Franks Casing Crew

 

[trainguy]

Usually, for round tubes, J = Ix + Iy, but please verify this yourself, I don't have this manual.

tg

 

[tnteng]

I was refering to the "J" values listed for rectangular tubing. The value listed does not equal Ix+Iy.

Thanks,

Tony Billeaud
Mechanical Engineer
Franks Casing Crew

 

[cvg]

AISC refers to the following publication for additional information and design aids: "Torsional Analysis of Steel Members".

 

[trainguy]

OK,

for a rectangular tube,

J = 2*t*b^2*d^2 / (b + d)

where b and d are measured to the mid-thickness,

i.e. for 4 x 2 x 1/4 tube,

d = 4-0.25 = 3.75
b = 2-0.25 = 1.75
t = 0.25

I have obtained this from Blodgett - Design of Welded Structures, section 2.10, Table 2.

Blodgett call it R ( torsional resistance ), we call it J (torsional constant) in Canada. This should be what you're looking for.

tg

 

[JedClampett]

Try (4 times the Area defined by a midline of the wall squared divided by the interior length of the perimeter) all times the thickness:
For a TS 8 x 6 x 1/4 (4*(7.875*5.875)^2/(2(7.75+5.75))*.25= 79.3 in^4
The difference between the AISC number (74.9)and my number is probably because of the corner radii. I got this from Steel Structures by Gaylord and Gaylord page 234.

 

[tnteng]

I believe that the Blodgett equations and the Gaylord and Gaylord equations are equal to each other.

Tony Billeaud
Mechanical Engineer
Franks Casing Crew

 

[JedClampett]

By the time I typed in the formula and submitted it, Trainguy beat me to it.
Give him the star.

 

[aesoton]

Does anyone have the formula for J for a T section beam?
Thanks
Andrew299

 

[boo1]

see:

http://www.efunda.com/math/areas/SquareTbeam.cfm

 

[Denial]

Any open section made up entirely of thin sections can have its torsion constant calculated to an adequate accuracy as the sum of the Lt^3/3 values for all its constituent parts.

The T section mentioned above probably qualifies as being both open and thin. The rectangular hollow section mentioned further above does not qualify because it is closed rather than open. A non-hollow rectangular section would probably not qualify because it would not be thin enough.

HTH

 

[prex]

andrew299
Look in the site below, under Beams -> Cross sections -> I beams -> Cut : by clicking onto 'Formulas' you'll see a more to the point formula than the general (correct) one given by Denial.
And please start a new thread for a new question.


prex

http://www.xcalcs.com

Online tools for structural design