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# Torsional elastic modulus

## Torsional elastic modulus

(OP)
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:

J=k*b*t^3

where:

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.

prex
motori@xcalcs.com
http://www.xcalcs.com
Online tools for structural design

### RE: Torsional elastic modulus

(OP)
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

(OP)
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.

Hariharan

### RE: Torsional elastic modulus

Rowland,

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.

Riz

### RE: Torsional elastic modulus

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

### RE: Torsional elastic modulus

Rowland,
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
Riz

### RE: Torsional elastic modulus

(OP)
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

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