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Neutral axis

From the top fibre of a beam to the central fibre, the fibres are in compression. The compression gradually decreases from a maximum at the top of the beam until it is zero at the centre. The centre is called the neutral axis (N/A). From the neutral axis to the bottom fibre, the fibres are in tension. The tension gradually increases from zero at the centre to a maximum at the bottom fibre.

Diagram of the layers of fibre that make up a beam. For the fibres above the neutral axis the fibres are shorter due to compression. For the fibres below the neutral axis the fibres are longer due to tension. The beam is slightly curved to reflect the pressure.