Likewise, the strength of a structure is not just a function of the kind of material it is made of, but how it is shaped -- the distances involved. Take three 1/8 inch thick boards 2 ft long and 3 inches wide. If you were to place them flat on top of each other and support them on the ends only, you could easily snap them by stepping on them with one foot. However, if you were to construct a triangular beam out of them, they would probably support your whole weight. Do you think a sheet of paper will support the weight of a book? If you form the sheet into a cylinder and stand it on end, you could easily support a small book.
So the moment is the combination of force and distance -- the force times the distance from the axis that the force is applied or resisted. The bridge truss, the box beam, and the I beam all take advantage of this by putting the strongest material at the outside edges -- as far as possible from the central axis. The material at the center takes no force at all. The material at the edges takes all the force and maximizes the strength.
Do you think that you could make an airplane wing out of styrofoam? Remember, you can snap styrofoam with your hands. Even so, most homebuilt airplanes have styrofoam wings. How do they do it? The styrofoam is shaped into the perfect shape for a wing using sandpaper, then the outside edges are covered with fiberglass. The wing of styrofoam is now strong enough to support an airplane where neither the foam nor the fiberglass used alone would have any significant strength.
Remember the principle of the moment: It does not matter much what the inside is made of, if the outside edges are strong, the structure will be strong. You can make a house out of material as light and fragile as styrofoam and it would be strong enough to fly if the surfaces are coated with a strong material. Use this principle in the design of all your structures!
If you've ever seen a paper wasp nest or a bee hive, you've seen a honeycomb. Honeycombs are used to maximize the use of materials. A honeycomb with a shell on each side is one of the strongest structural engineering designs. We use this idea for dome shells, walls, and virtually every other construction.
The reason most structures fail is not just because of the weakness of the material, but because of its connections to other materials. Connections such as bolts, nails, and screws cause localized stress points near the connections which fail long before the material itself would fail. It is these localized stress points that are the weakest link in conventional structures. It would be better to avoid localized stresses altogether if possible. This can be done by not using localized stressors such as nails, bolts, and screws. Instead, connections should be continuous, like ribbons. Ribbon connections continuously tie two surfaces together and prevent localization of stress at a point. Even better, make a monolithic shell without the need for connections.
Now, with an understanding of the basic engineering principles you can begin to create structures which have great integrity, but which are light and inexpensive. Remember the principles: