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Beam shape

The stress diagrams below show that most of the stress occurs two thirds of the distance from the neutral axis, so this is where a beam requires most of the structural material. Rectangular sections are wasteful of material compared with an I beam which is very efficient.

Two diagrams. The first diagram shows a solid rectangular beam with very minimal stress occurring in the middle third of the beam. The bulk of the stress occurs at the top and bottom third of the beam. The second diagram shows an I beam with very little stress occurring at the middle third of the beam. The bulk of the stress occurs at the top and bottom third of the beam.

Generally the further the structural material is from the centroid, the more structurally efficient the beam will be. However, if it is too far from the centroid, the beam will reach a stage when the member becomes too slender and will buckle.