Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Torsional elastic modulus

Torsional elastic modulus

Torsional elastic modulus

Could anyone give me the fomula for finding the torsional elastic modulus of a rectangular bar.  Thanks

RE: Torsional elastic modulus

Do you mean the torsional constant, J?  If so, the equation is:



b = width of section
t = thickness of section
k = constant depending on b/t ratio, as follows:

b/t        k
1          0.141
1.2        0.166
1.5        0.196
2          0.229
2.5        0.249
3          0.263
4          0.281
5          0.291
infinity   0.333

RE: Torsional elastic modulus

Go to the site below and follow Library -> Beams -> Cross sections -> Rectangle : you'll find what you are looking for. There's a button to display the formulae too.

Online tools for structural design

RE: Torsional elastic modulus

I am looking at the Australian Code 1288-1994, Appendix H., it's not the tortional moment of inertia.  Maybe an Engineer from Aus can help?

RE: Torsional elastic modulus

I can't find AS1288 in my design references. Can you be more specific re. the Code Title name plus the exact text extract you're looking at. I'm OZ trained and am in OZ currently so I might be able to assist.

Anthony Tugwell
Project Director & Consulting Engineer - just relocated to Australia

RE: Torsional elastic modulus

ADT thanks for responding, my reference is Australian Code AS 1288-1944, Appendix H, pg 98.  It is concerning the design of glass fins for a curtain wall.  I just have an extract of three pages so can not be any more specific.
The exact text extract is "The value of torsional elastic modulus (G) may be taken as 28.3 GPa for glass fins"

RE: Torsional elastic modulus

Is it likely that you may be referring to the shear modulus (G)?
The shear modulus is used in computing torsional deformation
(twist) of a member.

G = E / 2 / (1 + mu)

E = young's modulus
mu = Poisson's ratio.

Check any book on structural mechanics for the relationships
between torsional moment, torsional constant, G and twist.


RE: Torsional elastic modulus


Modulus of elasticity, G, also known as Modulus of Rigidity.

G= fV/Y
fv= torsional shearing stress(Pa)
Y= gamma, strain in the material

If u r designing glass than 28.3 GPa is correct, upto 30 is fine.

I would suggest u use the G values directly rather than compute from first principles.


RE: Torsional elastic modulus

If u use formula given by Hariharan than the parameters are as follows:
E = 70KPa
mu= 0.22
Thus G = 28.7GPa

RE: Torsional elastic modulus

If u use formula given by Hariharan than the parameters are as follows:
E = 70KPa
mu= 0.22 ( for stress parallel to grain )
Thus G = 28.7GPa

RE: Torsional elastic modulus

Many thanks for your help, I think I can put this one to bed now.

RE: Torsional elastic modulus

Hariharan & Riz are right!

Anthony Tugwell
Project Director & Consulting Engineer - just relocated to Australia

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


White Paper - Considerations for choosing a 3D printing technology
The adoption of 3D printing into major companies’ product development life cycles is a testament to the technology’s incredible benefits to consumers, designers, engineers and manufacturers. While traditional production methods have limitations in manufacturability, 3D printing provides unparalleled design freedom due to the additive method of building parts layer by layer. Download Now
White Paper - Are all 3D printed metals the same?
With so many different 3D printing metal systems and processes today, how can you know exactly what you’re getting? Today, there are several different methods for manufacturing 3D printed metal components with their own strengths and limits. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close