Moment of Inertia – Section Modulus of Tubes

I sometimes forget that not everybody knows what I and SM are.  They are the basic language of the strength and stiffness of a beam or tube of any kind.  Any beam should satisfy deflection, bending strength and shear strength.  In a very long span, deflection will govern.  Medium length, bending will govern, and with something short, like a chainplate, shear will govern.

When I specify an extrusion like a bow tube, I typically call out the moment of inertia it must have.  The “I”.  That is a language that any mast builder understands.  I see that Richard Woods has called me lazy for doing that.  I guess in lieu of declaring an actual section size.  I prefer to call out moments instead so people can have some freedom in picking a section.  Many might actually do the job.

With a round tube, this information in an Excel spreadsheet might do the trick.  When I have more time I will post the spreadsheet.  I can only do that from the laptop.

enter O.D. in inches 3.50 or mm.
enter I.D in inches 3.00 or mm.
thickness in inches 0.25
SM= 1.937065 inches to third power
I= 3.389588 inches to fourth power
area 2.7489 square inches

Ok, now for SM paste in =(0.098168*((E3^4)-(E4^4)))/E3

with the sizes in the E column.  Substitute these equations  into the bold above.

For I, paste in, =0.04908*((E3^4)-(E4^4))

For area, for shear, paste, =(6.2832)*(E3/2)*(E5)

I have used these for so many years that I forget where I got them.

One thought on “Moment of Inertia – Section Modulus of Tubes”

  1. I can see it now. KH dies and goes to hell. Satan opens the door and says Hello mr Hughes. We’ve been expecting you” there in the background is RW holding his slide rule. DK is over in the corner, by the fire, mixing up a batch of polyester….hahahahahahahahaha

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