"J" For Lateral Torsional Buckling equations

[Joseph R. Grill]

I have a small “T” section that will be a fabricated section.  I’m a bit embarrassed, but I can’t seem to remember how to calculate the “J” for a section (always been able to pull the value from the tables).  It is needed for Lateral Torsional buckling equations for a tee section in the 13^th edition of the AISC.  Can anyone out there help me out.

 

Thanks,

Joe

 

 

[Conrad Harrison]

Assuming J is the Torsion Constant. Then for open sections approximately

 

J = sum( bt^3/3)

 

Where b = length of element

           T = thickness

 

For special case of  circular section J equals the polar second moment of area.

 

Regards

Conrad Harrison

B.Tech (mfg & mech), MIIE, gradTIEAust

mailto:sch.te...@bigpond.com

Adelaide

South Australia

 

 

[Liaquat Ally Akhand]

Dear,

"J" is the polar moment of inertia.

It is the square root of summation of square of moment of inertia
about "x-axis" and "y-axis".

You can find it in any standard Handbook.

Regards,

******* ****** ******* ******** ******* ******* ******* ***
* Read list FAQ at: http://www.seaint.org/list_FAQ.asp
*
* This email was sent to you via Structural Engineers
* Association of Southern California (SEAOSC) server. To
* subscribe (no fee) or UnSubscribe, please go to:
*
* http://www.seaint.org/sealist1.asp
*
* Questions to seai...@seaint.org. Remember, any email you
* send to the list is public domain and may be re-posted
* without your permission. Make sure you visit our web
* site at: http://www.seaint.org
******* ****** ****** ****** ******* ****** ****** ********

[Joseph R. Grill]

I'm sorry, I was not clear, but as Conrad pointed out it is the J as used as
the torsional constant and the approximate equation he sent is what I was
looking for and what I also found after some searching through my storage in
the garage. It was also in my older Salmon and Johnson text.
Thanks
Joe

[Christopher Wright]

On Nov 23, 2009, at 6:29 PM, Liaquat Ally Akhand wrote:

> "J" is the polar moment of inertia.

For structural purposes J is the torsional stiffness. The polar
moment of inertia is the torsional stiffness only for the special
case of circular sections

>
> It is the square root of summation of square of moment of inertia
> about "x-axis" and "y-axis".

The polar moment of inertia is the sum of the cross-sectional moments
of inertia, not the root-sum-square

> You can find it in any standard Handbook.

Blodgett is your best resource. It explains everything

Just so you'll know.

Christopher Wright P.E. |"They couldn't hit an elephant at
chr...@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania
1864)
http://www.skypoint.com/members/chrisw/