Page 297 - Aircraft Stuctures for Engineering Student

278 Open and closed, thin-walled beams

A bending moment M applied in any longitudinal plane parallel to the z axis may be
resolved into components M, and My by the normal rules of vectors. However, a
visual appreciation of the situation is often helpful. Referring to Fig. 9.3 we see
that a bending moment M in a plane at an angle 8 to Ox may have components of
differing sign depending on the size of 8. In both cases, for the sense of M shown
M, = Msin8

which give, for 8 < ~12, M, and My positive (Fig. 9.3(a)) and for 8 > ~12, M,
positive and My negative (Fig. 9.3(b)).

Fig. 9.3 Resolution of bending moments.