moment of inertia, section modulus

[darkhbk]

Hi,
Want to calculate Section modulus for the attached cross section. I drew the section in autoCAD and applied massprop command.
1.But I am not sure that the moment of inertia value I am getting is correct so clarification/confirmation is highly appreciated
2.From the results I wanted to know which moment of inertia to use to find the section modulus for this particular section.
3.Since the section modulus is I/Y , Can I take from the centroid to the bottom most as "Y"?

Please pardon my ignorance if this is not the right place to ask this question.

 

[rb1957]

1 it's easy enough to calc I by hand to check things.
2 the problem I see is that the CAD isn't telling you the centroid's coordinates. this is important 'cause "y" is the extreme fiber distance, so it can be to the top of the bttm of your sketch (I suspect the top, but how far ?).
3 usually you need to worry about tension and compression stresses and treat them differently, and you'll only know which side is in tension when you know the bending on the section. Of course, you can be conservative and use the bigger y for both tension and compression.


another day in paradise, or is paradise one day closer ?

 

[Denial]

Maybe AutoCAD got confused between radius and diameter?

 

[IDS]

The AutoCAD numbers are wrong for the shape as shown, but not by so much as to be due to using diameter instead of radius:

- The area, Ix and Iy are all about 90% of the correct value, but all by slightly different factors.
- The bounding box is 1 mm too small in the X direction and about 0.5 mm too small in the Y direction
- Based on the bounding box Y values, the centroid Y value is off by about 0.17 mm or -0.26 mm, depending which boundary you use.

All of which suggest that the dimensions as shown are different to those actually used in the drawing.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[IDS]

Using 4.5 mm for the wall thickness, and 228.5 for the outer radius, I get near exact agreement with the AutoCAD figures for Area, Ix and Iy.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[darkhbk]

Hello everyone.
Firstly, Sorry for the long delay in reply. Thanks for all you replies and suggestions.
1.The moment of inertia taken from autoCAD is indeed correct (cross checked by drawing various shapes and compared with Ansys section properties).
2.Since the object is not symmetrical in Y direction, two different Y values taken from the centroid. centroid to the bottom most fiber is taken in to account when calculating stress at the bottom and centroid to the top most fiber is taken when finding stress on top. So two different section modulas is used.
Once again thanks everyone for spending your valuable time to answer my question