# G = Shear modulus

I have a question in an assignment and am using the Θ = LT/JG equation to find the angle of twist in an I section beam to be used in a monorail. I have all other figures to put in the equation but am not sure where i get G value from

Help )

Thank you

Astronuc
Staff Emeritus
This might help -

http://www.diracdelta.co.uk/science/source/s/h/shear modulus/source.html

http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/elastic_constants_G_K.cfm

http://en.wikipedia.org/wiki/Shear_modulus (don't use these values for actual engineering design work - they are examples and should only be used for educational purposes). G is material dependent. Best to use values from materials strength testing or some certified source, such as the supplier of the material or structural component.

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Thanks for your help. This is all new to me as i've been out of the learning chain for many years now and am finding it difficult to get my head around so many new formule at the same time. Its a steel monorail that we're using as an example so it would be made of steel but not sure which steel. Some friends have said that we have used 80GPa earlier on in equations but under a different heading. I have struggled to find any values but did find one page that gave a range of 79GPa-84GPa, so maybe this 80 is correct. Would that sound about right.

Thanks again.........Hopefully i will get the hang of this soon )

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Astronuc
Staff Emeritus
Well, 80 GPa is probably a good estimate.

For a typical bridge steel -
http://www.matweb.com/search/datasheetText.aspx?bassnum=MS514L

See this reference - http://www.aisc.org/Template.cfm?Section=Bookstore&Template=/Ecommerce/ProductDisplay.cfm&Productid=2283 [Broken]

BUT, one should know what steel is being used and obtain the appropriate properties, unless this is a homework problem, and not a safety-related design matter.

When doing actual engineering design, one cannot leave anything to chance. An engineer must know his/her material, the intended service, and intended environment.

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FredGarvin
If you are using an isotropic material, you can calculate G from the usual knowns, E and $$\nu$$:
$$G =\frac{E}{2(1+\nu)}$$