Equations of equilibrium:

In all the above three examples, we see that the number of unknown reactions exceeds the number of equations of equilibrium (which in the co-planar case is three). This excess is known as the degree of redundancy of the indeterminate structure. Thus, in Example (a) the degree of redundancy is 4 - 3 = 1. In Example (b), it is 6 - 3 = 3 and in Example (c), it is 5 - 3 = 2. Therefore, in Example (a) we require one extra equation to solve the structure, while in Examples (b) and (c) three and two extra equations are to be obtained by what is called the conditions of compatibility.