Jonathan Ochshorn's Structural Elements for Architects and Builders, Third Edition
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Chapter 1 – Introduction to structural design: Statics

Finding out what the loads are that act on a structure and how these loads are supported is the prerequisite to all structural design. There are two main reasons for this. First, the fact that a structural element is supported at all means that the supporting element is being stressed in some way. To find the magnitude of the reactions of an element is thus to simultaneously find the magnitude of the loads acting on the supporting element. Each action, or load, has an equal reaction; or, as Newton said in defense of this third law: "If you press a stone with your finger, the finger is also pressed by the stone."

The second reason for finding reactions of the structural element is that doing so facilitates the further analysis or design of the element itself. That is, determining reactions is the prerequisite to the calculation of internal loads and internal stresses, values of which are central to the most fundamental questions of structural engineering: Is it strong enough? Is it safe?