February 15, 2011

Walking on a rope

And so... how many invisible men were you able to count on our last visual test? The answer, below:

Congratulations! I guess you were for sure able to tell that there were three invisible men walking on the rope. But perhaps it wasn't that clear for the plank.

What's the difference? It's on the way they stand the loads applied to them, and on the way they deform. While the rope clearly changes its shape according  to the people walking on it, the plank doesn't do it in such a clear manner. That's why you could count three invisible men, but perhaps were not so sure about the plank.

Cables (like the rope) are so flexible that they must deform in order to carry loads. They can only resist tension, and those pulls straighten them. So, in the end, they become straight segments between the hanging loads. If a new load is added, the cable changes its shape accordingly.

Structures based on principles similar to cables are called form-active. Because they shape themselves in harmony with the loads, they're very efficient when it comes to the amount of required material. Consequently, they can be incredibly slender. But you should realize that because of that, they ask for more depth than equivalent beams, and for a deep understanding of their structural behavior. There are several beautiful and remarkable built examples based on form-active structures, like arches, and membranes (we will see some of them in future posts).

Beams (that's what the plank is) are more rigid than a cable. They're usually so rigid that you don't even notice they deform. But of course, they deform (remember this?) though in a rather different way: they curve. That's because a beam does not only stand tension, but also compression and bending moments, as well.

Most of our everyday structures are made of beams, which are section-active. Somehow, they're the opposite to form-active structures. Instead of shaping themselves according to the loads they're submitted to, they rigidly force the forces to follow their shape: there's no way out of them. There's a clear advantage to that: the same shape is able to stand a diversity of loads.

Two different points of view, two ways of achieving the same mean.

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