Before explaining this concept again, I need few question to be answer, just few algebraic expressions to solve to get a unique answer. Here it is,
Q1. Determine the value of x from the following equation
5 x + 15 = 45
Q2. Determine the value of x and y from the following equation
2 x + y = 5
That's it.
You can confidently say that the value of x in question Q1 is 6 with a great confidence but when it comes the Q2, you can not have a unique answer, one can say for its x = 2 and y = 1 while second one can say that its x = 2.5 and y = 1 and third one can say its x = 0 and y = 5 and so on. In other words, you obtain a unique answer for Q1 and infinite number of answers for Q2.
Now, let me modify the Q2 and name it as Q3. Here it is,
Q3. Determine the value of x and y from the following equation
2 x + y = 5
x + y = 3
Now, for Q3 you can say that the answer is x = 2 and y = 1 with the same confidence as for Q1.
The idea for obtaining degree of indeterminacy for structure is same. For example when you evaluates static degree of indeterminacy for a structure you make a judgement will you certainly obtain the values of support reactions by using equilibrium equations for provided loading conditions.
