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How to Build a Structure to Resist Various Loads: If you have a limited amount of material or don’t want your structure to be too heavy you can distribute your material in certain ways to make structures that are better at resisting certain kinds of loads.

Tension: Structures designed to resist tension (cables, ropes, chains) do not benefit from special shapes other than to make sure that there are not narrowed, weaker areas in them, which then become the area where that structure breaks (“a chain is only as strong as its weakest link”).

Bending: When a structure is designed to be loaded in bending it is called a “beam.” Beams sustain tensile loads on one side (if you are standing on a beam supporting a floor, the bottom of the beam is loaded in tension) and compressive loads on the other (in the diagram the little arrows represent the tension and compression produced by the loads represented by the big arrows). The material in the center is not loaded in either tension or compression and is said to lie on the “neutral axis.” The best distribution of material for a beam loaded in single plane bending, such as a floor beam is to place most of the material on the top and the bottom and little in the “neutral” (unloaded) center, a so called “I beam” configuration. If the beam is to be loaded in two planes, each at 90 degrees to the other the best configuration is a “box beam.” A beam that is loaded in multiple different directions is best designed as a tubular structure. As you will note, bones are loaded in bending in multiple planes and are in fact tubular structures. This follows Wolff’s law, which says that bone is removed where it is not loaded and laid down where it is loaded. The center of a tubular bone is on the “neutral axis” and is not loaded, so bone there is resorbed. The edge is loaded heavily and so becomes dense cortical bone. Note that the strongest implants we have are the intramedullary nails which also follow a hollow tube design strategy.

Compression: Structures designed to be loaded in compression are called “columns.” Short, squatty columns fail by being crushed or squashed (that is how you want your column to fail). Tall thin columns fail by “buckling” (failing in bending) and are said to be “incompetent columns” or to have an insufficient radius for their length. Thus, a good column must be strong enough in multiplane bending to resist buckling and therefore their material also is frequently best distrubuted in a tubular fashion (depending on the material and the length of the column).

Torsion: Structures designed to be loaded in torsion, such as a drive shaft, also turn out to be best designed as tubular structures as material far from the center is said to have a greater “moment of inertia” to resist torsional loads. The easiest way to understand this is to imagine yourself attempting to apply torsion (torque) to a nut with a wrench. It is intuitively obvious that you don’t want to apply your load to the wrench down near the nut, at the head of the wrench—rather you apply your load to the end of the wrench handle, as far as possible from the center of the nut, where you have a greater “moment of inertia.” In fact torque is measured in “foot-pounds” denoting how far from the center of rotation you apply how many pounds of force. Thus if you want to place your material where it is able to best resist torsional loads you distribute it far from the center (you can


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