Structural quantity:

Figure shows the influence line diagram of a particular quantity for the beam ABCD. The ordinates are positive between B and C; and are negative between A and B, and again between C and D. Therefore, for maximum positive value of the quantity the uniformly distributed load should cover the entire portion from B to C (Figure 2 (b)). Similarly, for maximum negative value it should cover the portion AB and CD.

A value of a structural quantity because of a uniformly distributed live load is equal to the product of the loading intensity (q) and the net area of the influence line diagram under that portion.

Here, in Figure, the value of the structural quantity due to the uniformly distributed load covering portion AB only is given by q × (area of IL diagram over AB as shown shaded).