Properties of Influence Lines:
Here, we should state certain important properties of an influence line, to show how they can be utilized successfully in structural engineering.
(a) To obtain the maximum value of a structural quantity because of a single concentrated moving load, a load shall be placed at which point where the ordinate to the influence line is maximum. (This property is obvious from the definition.)
(b) The value of a structural quantity due to a single concentrated moving load is equal to the product of the magnitude of the load and the value of the corresponding ordinate of the influence line. Thus, in Figure (c), the ordinate of the influence line for RA at a distance of 7 m from A is (10 - 7)/10 = 0.3.
Now if a load of 20 kN is placed at this point the magnitude of reaction RA will be 20 × 0.3 = 6 kN. This can be easily verified.
(c) To search the maximum value of a structural quantity because of a uniformly distributed live load, the load should be placed over all those portions of the structure where its influence line ordinates have the same sign.