The model and the selection. The age-old way to make sheet steel stiffer is to corrugate it, giving it a roughly sinusoidal profile. The corrugations increase the second moment of area of the sheet about an axis normal to the corruga- tions themselves. The resistance to bending in one direction is thereby increased, but in the cross-direction it is not changed at all.Corrugations are the clue, but — to be useful — they must stiffen the sheet in all directions, not just one. A hexagonal grid of dimples (Figure 12.7) achieves
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