validity limit for elastic deflection equation due to simple bending

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Hi all,

I would like to know if the elastic deflection equation for transverse bending loadings : dy^2/dx^2=Mf(x)/EI is valid for any beam dimensions.
The Beer Handbook Mechanics of Materials says it is valid for any cross section. However, is it valid for thin-walled beams? Dimensions like L=1 m , h=0.3 m, b=0.2 m and t=0.002 loaded with a concentrated load at midspan??? I could not find this information anywhere. I looked for it on a lot of books I have.

Thanks,
Best regards,
Mark

 

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my guess is that the cross section being 300x200 means that a 1000mm long beam doesn't fit that well for use of beam theory as it is too short (not slender enough)

http://en.wikipedia.org/wiki/Bending