When loading is applied to a trussed rafter (from tiles, ceiling construction snow etc), two main kinds of force are generated in the members:
Bending moment causes neighbouring sections of timber to tend to rotate relative to each other (see figure 21a).
Axial force may be either tensile, i.e. pulling adjoining sections of timber away from each other, or compressive, i.e. crushing adjoining sections of timber into each other (see figure 21b and 21c).