Concept of Dead Load Live Load and Influence Line :
The load on the structure may be concentrated or distributed. The distributed loads are usually the weight of the structure itself or the weight of something that rests directly on the structure. The loads on the structure may or may not in a fixed pattern for an indefinite period of time. The load which rest on the structure such as any equipment may hang on different locations at different times. Wind and snow may come at different times. People or goods may move.
The load that is permanently there (on the structure) is called dead load. For example, the weight of the structure itself and perhaps the weight of the fixed wall partition or some similar fixtures resting on the structure constitutes the dead load.
The load that may or may not be there (on the structure) is called live load, which may include snow, wind, people or stored goods or traffic such as heavy trucks on a highway bridge. In such cases, a certain percentage of the live load is added on as impact load as well.
Generally when the strength of structure is considered it is assumed that loading is concentrated or distributed. That is not the case always, the loading may involve impact or suddenly applied loads. When loads are applied suddenly then resulting stresses induced in the structure are more than if the loads are applied gradually.
Since the live load may or may not be there and it may take any pattern on the structure, so the question arises is that where to place live load so that its effect is most critical. For this, consider a unit concentrated load measure of weight such as 1.0 kN. Then the effect of this unit concentrated load moving about the structure is investigated. For example the effect of this unit load on the shear at any chosen point or its effect on one of the reactions of the beam at any chosen point may be investigated.
If the magnitude of the effect of a unit concentrated moving load is plotted right at the position where the concentrated unit load is moving, the result is an influence line.
Thus, an influence line is the graphical display showing the effect on a chosen function of a concentrated unit moving load.