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How does this structure work?

Question asked by Ramani Aswath in #Coffee Room #Mechanical #Civil & Structural on Jun 17, 2020
Ramani Aswath · Jun 17, 2020
Rank A1 - PRO

There is a type of structure wherein an assembly that cannot be stable is quite stable in apparent defiance of theory.

A closer analysis shows that there is no contradiction.

Here is an interesting example.

New video by Ramani Aswath

What makes it work?

Kaustubh Katdare · Jun 18, 2020
Rank A1 - PRO

I haven't tried this myself; but if this works; then the CG is being balanced somewhere. They're using water bottle tied with a rope - so that there's a natural tilt. I think that pushes the CG to the right position that the stick balances.

Not sure if I'm even close.

Ramani Aswath · Jun 19, 2020
Rank A1 - PRO

Shifting the CG would give stability. However, the water bottle ensures that the rope has to be vertical, which keeps the CG vertically away from the pivot point of the top tooth pick on the edge of the table.

Consider the third tooth pick that links the two horizontal ones. The top tooth picks will try to pivot at the point on the table’s edge, where it is resting. But the third tooth pick prevents this by converting this force to a compressive one along its axis and applies this load to the centre of the horizontal tooth pick bridging the thread making it a beam that resists bending. If one looks closely, this tooth pick is seen to curve slightly downward when the supporting milk bottle is removed. The two rope sides of the top triangle feel a tensile force, which is in addition to the tension caused by the hanging weight of the water bottle. This is resisted by the bridging tooth pick, which sees a compressive force along the axis in addition to the bending force at the centre mentioned before.

By converting a torque to a bending force and a tension to compression, stability seems to be achieved.

Maybe some structural engineering CEan may give us a better explanation.

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