As a further alternative,

where M' is the component of the applied moment about the N.A., In.a. is determined either from the momental ellipse or from the Mohr or Land constructions, and n is the perpendicular distance from the point in question to the N.A.

Deflections of unsymmetrical members are found by applying standard deflection formulae to bending about either the principal axes or the N.A. taking care to use the correct component of load and the correct second moment of area value.


It has been shown in Chapter 4 of Mechanics of Materials 1 ^ that the simple bending theory applies when bending takes place about an axis which is perpendicular to a plane of symmetry. If such an axis is drawn through the centroid of a section, and another mutually perpendicular to it also through the centroid, then these axes are principal axes. Thus a plane of symmetry is automatically a principal axis. Second moments of area of a cross-section about its principal axes are found to be maximum and minimum values, while the product second moment of area, / xydA, is found to be zero. All plane sections, whether they have an axis of symmetry or not, have two perpendicular axes about which the product second moment of area is zero. Principal axes are thus defined as the axes about which the product second moment of area is zero. Simple bending can then be taken as bending which takes place about a principal axis, moments being applied in a plane parallel to one such axis.

In general, however, moments are applied about a convenient axis in the cross-section; the plane containing the applied moment may not then be parallel to a principal axis. Such cases are termed "unsymmetrical" or "asymmetrical" bending.

The most simple type of unsymmetrical bending problem is that of "skew" loading of sections containing at least one axis of symmetry, as in Fig. 1.1. This axis and the axis

+ EJ. Heam, Mechanics of Materials 1, Butterworth-Heinemann, 1997.

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