Most People Don’t Know

If you manage to wrap a cylinder around the Eiffel Tower, the air inside the cylinder will outweigh the weight of the tower itself!

Gustav Eiffel, the man behind the design of the monument, was an absolute genius. The Eiffel Tower's design is actually inspired by the biological structure of the femur (the human thigh bone). He was once researching the structure of the femur and realized that the connection, or the joint, between the femur and hip bone is made up of many minute fibre like material called the trabaculae, which are woven in a criss cross pattern. So basically, they involve a lot of air gaps in between them rather than the actual bone material.

In layman's terms, there's material only where you need it to be, i.e, only where the forces are actually acting. Eiffel used this concept in designing the trusses (the X shaped components of structures) used in the tower which, if you look more closely, has a lot of holes.

This significantly reduces the weight of the structure as you can leave the rest of the space empty.

Here's the math for it (it's quite rudimentary, but it'll give you a rough picture):

The weight of the tower is 7300 tonnes. Yes, the air which would be in the place of the tower has also been counted but still, there's a difference of over 2000 tonnes! And we're literally talking steel v/s air here.

The femur bone is one of the many naturally nano engineered structures from which structural engineers derive inspiration. Imagine the amount of steel which appears to be used in the construction; that still weighs less than the air surrounding it. This particular branch of structural engineering is called biomimetics.

Sources:

1. An interesting fact about the Eiffel Tower
2. What Your Bones Have in Common With the Eiffel Tower (This one gives a very lucid explanation). Also the source for the pictures.
3. Successful Modern Day Examples of Biomimetics