From this point on, all of the methods in this book will be for the analysis of indeterminate systems exactly. We will not have to make any assumptions in addition to those general assumptions which underlie all of the analyses that we are doing (i.e. the Bernoulli-Euler beam assumptions - see Chapter 5). The approximate methods described in Chapter 6 relied on a group of reasonably good assumptions to convert an indeterminate frame into a determinate frame that we can solve using equilibrium equations; however, the methods to come will not work this way. They will instead use both equilibrium and compatibility to solve indeterminate structures (see Chapter 1).
This chapter will introduce the force method which is also often called the method of consistent deformations. This method relies on the compatibility of the structure to give us extra information that we need for the analysis. This extra information will make up for the fact that we cannot solve indeterminate systems using equilibrium alone. It will also make heavy use of the concept of superposition that was introduced in Chapter 5.