113Stiffening of platform chassis Figs 14 and

To appreciate the stresses imposed on and the resisting stiffness offered by sheet steel when it is subjected to bending, a small segment of a beam greatly magnified will now be considered (Fig. 1.4(a)). As the beam deforms, the top fibres contract and the bottom fibres elongate. The neutral plane or axis of the beam is defined as the plane whose length remains unchanged during deformation and is normally situated in the centre of a uniform section (Fig. 1.4(a and b)).

The stress distribution from top to bottom within the beam varies from zero along the neutral axis (NA), where there is no change in the length of the fibres, to a maximum compressive stress on the outer top layer and a maximum tensile stress on the outer bottom layer, the distortion of the fibres being greatest at their extremes as shown in Fig. 1.4(b).

It has been found that bending resistance increases roughly with the cube of its distance from the neutral axis (Fig. 1.5(a)). Therefore, bending resistance of a given section can be greatly improved for a given weight of metal by taking metal away from the neutral axis where the metal fibres do not contribute very much to resisting distortion and placing it as far out as possible where the distortion is greatest. Bending resistance may be improved by using longitudinal or cross-member deep box-sections (Fig. 1.5(b)) and tunnel sections (Fig. 1.5(c)) to restrain the platform chassis from buckling and to stiffen the flat horizontal floor seat and boot pans. So that vibration and drumming may be reduced, many swaged ribs are pressed into these sheets (Fig. 1.5(d)).

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