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Two Round Tube Assembly, Section Properties - Moments of inertia of the area

Question asked by Dan Smith on Aug 6, 2020
Latest reply on Aug 14, 2020 by Hemantkumar Patel


I am looking at an assembly of Two Round Hollow Tubes running parallel next to each other. The tubes are 2.5in Diameter and 0.125in Thick.

I would like to see how the separation between the tubes impacts the bending and torsional rigidity of the system. Clicking on a single tube's cross section surface and looking at the section properties, the Area moment of inertia in the y-direction is 0.66in^4 and the Polar Moment of inertia is 1.32in^4, as expected when looking at the formula for moment of inertia of a hollow tube:


Separating the tubes by 100mm and looking at the surface properties of both tubes gives a area moment of inertia in the y-direction of 8.55in^4 and a Polar Moment of Inertia of 9.87in^4:


Increasing the separation of the tubes to 200mm drastically increases the area moment of inertia in the y-direction to 30.23in^4 and the Polar Moment of Inertia to 31.55in^4:

Can these results be trusted? I am having a hard time understanding how the bending resistance in the y-direction could increase by increasing the distance between the two members?