Turning moment is a force that produces motion around a point or an axis. Non concurrent forces create turning moments. Non concurrent forces may create deformation, or raking in timbers when the beam itself is presented with compressive and tension forces. The strength of materials and the forces that buildings are typically designed to resist are described in modern code compliance books. Joints are the connections made between different parts of a structure.
Most joints lock two building parts tightly together.
However, in some structures in which a high degree of expansion is unavoidable, expansion joints are constructed. Lap joints are just that, two timbers lapping each other. This joint requires a fastener, just as a nail or wooden peg to hold the parts together. The joint is only as strong as the fastener. A scarf joint is a lap joint joining two timbers at 180 degrees.