Engineering Principles

The Bending Moment

The single most important principle for the strong design of structures is called the bending moment. Basically, a moment in engineering parlance is the principle of the lever. If you want to tighten a bolt, you can hold a wrench close to the bolt or you could grab the wrench at the end. The end of the wrench gives you more advantage. The distance at which a force acts influences the outcome. That is the principle of the moment.

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!

Curved Surfaces -- The Shell

Curved surfaces, shells, are stronger than flat surfaces. Take three sheets of material. If one sheet were curved along one axis to make a half cylinder, like a quonset hut, the strength would be several times that of a flat roof. A heavy snow load could be resisted. If a sheet were curved along both axes to make a dome, the strength would be greater still. Surfaces curved in two dimensions can be 40 times stronger than flat surfaces! This is the strength of the curved shell. Use this principle of shells to increase the strength of your structures! Actually, the curves only increase the distance of the material from the central axis, taking advantage of the principle of the moment just discussed. An egg shell is a great example of significant strength from a tiny amount of material.

Corrugation

You've seen corrugated sheet metal. The corrugations give it greater strength in the direction along the corrugations. Again the principle of curved surfaces and the moment are put to good use. Corrugations can be deeper than the usual corrugated metal sheet roofing. The deeper the corrugations the greater the strength, because the moment is greater. Serpentine walls take advantage of this corrugation factor. Curved walls only one brick thick have stood for centuries. Many fan-type sea shells use corrugation as a strengthening principle.

Honeycombs

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.

Stress Points

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:

  1. The moment -- put the strength at the edges.
  2. Use curved surfaces, shells
  3. Use corrugations
  4. Use honeycombs to separate inside and outside surfaces for greatest strength
  5. Avoid stress points by tying whole surfaces together along a ribbon connection or create a monolithic shell.

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