1. FORCES AND LOAD TERMINOLOGY
Structures are subjected to many kinds of natural forces. The most basic force is gravity which is always at work and usually acts upon buildings in a simple, downward direction. Sideways or lateral forces can be produced by wind and earthquakes. Wind passing over a roof can also create suction which is an uplift force. Lateral forces vary in intensity based on the building's location on our planet, whereas gravity acts similarly on all buildings. Other forces include impact loads, temporary loads such as construction materials stockpiled while the building is being constructed, and moving loads caused by automobiles or construction equipment. The term force is used interchangeably with load and sometimes weight. This booklet deals with the vertical forces created by gravity. Lateral and moving loads require special analysis and are separate subjects.
The goal of the whole design process is to achieve an equilibrium of the forces acting upon a structure. Without equilibrium the building will move and that is not good! Equilibrium must be accomplished for the building as a whole and for all the parts or smaller assemblies within the building as well. For all of the forces acting downward due to gravity, an equal, opposite force called a reaction must be pushing up. In other words, as the loads travel down load paths through the structure, each element such as beams and posts, must be capable of supporting or reacting to the loads above it. All of the loads acting on a structure will ultimately accumulate in the foundation and must be met with an equivalent reaction from the earth below.