Read the article at http://www.nachi.org/structural-design-concepts-home-inspector.htm
More inspection articles at http://www.nachi.org/articles.htm
What does this mean?
"In general, system performance includes two basic concepts known as load-sharing and composite action. Load-sharing is found in repetitive member systems (including wood framing) and reflects the ability of the load on one member to be shared by another, or, in the case of a uniform load, the ability of some of the load on a weaker member to be carried by adjacent members. Composite action is found in assemblies of components that, when connected to one another, form a “composite member" with greater capacity and stiffness than the sum of the component parts. However, the amount of composite action in a system depends on the manner in which the various system elements are connected. The aim is to achieve a higher effective section modulus component than members taken separately.”