It took a bit more than 3 minutes, but an updated section properties spreadsheet, which will work with groups of any number of shapes, can be found at the link below. Shapes can be offset and rotated, and given an "elastic modulus", using either actual or relative values.
you probably could've done it in 1 minute, if you invented a near-light-speed rocket first, and did your work onboard ...
there's a thought (for a friday afternoon) ... if time dilates, you'd think slower ... wouldn't you ? you're aging slower, an experienced second is 1 minute to someone outside the rocket ship ... so i got to ... if you're aging slower, your metabolism is slower, you eat, breathe (or poop) slower, you'd think and work slower (you'd accomplish 1 second's work in 1 experienced second).
I agree with (and use) AutoCAD to calculate basic section properties. Create a closed polyline shape and use the 'region' command to define the shape and then the 'massprop' command to calculate the the principle inertias or those about the X and Y axis as positioned in model space.
Every once in a blue moon, I have a 'hollow' shape where the hole and the section do not have the same center of mass. It would be cool if AutoCAD could perform the parallel axis calculations. I am sure somebody who knows LISP has already created the routine...
I don't have AutoCAD, but if you define the hollow shape with a single polyline, as described below, it should give you the correct properties:
- Define the exterior shape with a pline, working clockwise.
- Extend the pline to any point on the interior void.
- Define the void shape, working anti-clockwise.
- Extend the pline back to the starting point on the outer shape.
As long as the two lines connecting the inner and outer shapes are exactly co-incident, and the shapes are defined in opposite directions, the result should give the correct section properties.
Probably a good idea to check the results with a spreadsheet though
Said another way, convert your hollow shape to a non-hollow shape by cutting a vanishingly narrow channel from the outer perimeter to the inner perimieter. Good idea.
I use acad as well. It works great! I draw the shape and then use the region command. PLines are not required.
Acad can also find properties of complicated shapes. To remove a hole: region both shapes, them pick modify, solid editing, subtract.
I will hatch the new region to verify I have created the correct cross section.
