Some years ago, I started a blog called Harmonic Structures, to explain structural concepts in a funny and intriguing way. This is one of the posts I published back then, on 11/10/2009
Let’s answer the previously made question. Or… let’s do it in a different way: instead of giving the answer, I’ll give you a clue.
An old fable tells about a proud and enormous oak who made fun at a fragile and small reed. Then a strong wind blew. The tree tried to stand but was uprooted. The reed, instead of keeping still, moved with the wind, and… he wasn’t pulled out! He was able to deform with the wind, and this guaranteed his survival.
We usually think that our houses and the structures which support them, are immovable. Luckily, it’s not that way. Structures do move (and I don’t mean because of earthquakes!). Actually, they stand up because they deform. What usually happens is that these deformations are so small that we usually don’t see them.
Let’s think on a spring or a rubber band: if we stretch it (we apply a force), it deforms (it’s longer): the bigger the force, the bigger the deformation. It’s not only that the spring (the structure) deforms itself when the force is applied. It is also that it necessarily must deform to stand the applied force.
So if it’s deformed, a force is applied to it.
No force, no deformation. No deformation, no force.
Now, look again at the image, Who would you say now that’s really pulling?