OVERVIEW

The structures we have considered so far have been subjected to loading systems that were stationary, i.e. the loads remained in a fixed position in relation to the structure. In many practical situations, however, structures carry loads that vary continuously. For example, a building supports a system of stationary loads which consist of its self- weight, the weight of any permanent fixtures (such as partitions, machinery, etc.) and also a system of imposed or live loads which comprise snow loads, wind loads or any movable equipment. The structural elements of the building must then be designed to withstand the worst combination of these fixed and movable loads.

Other forms of movable load consist of vehicles and trains that cross bridges and viaducts. Again, these structures must be designed to support their self-weight, the weight of any permanent fixtures such as a road deck or railway track and also the forces produced by the passage of vehicles or trains. It is then necessary to determine the critical positions of the vehicles or trains in relation to the bridge or viaduct. Although these loads are moving loads, they are assumed to be moving or changing at such a slow rate that dynamic effects (such as vibrations and oscillating stresses) are absent.

The effects of loads that occupy different positions on a structure can be studied by means of influence lines. Influence lines give the value at a particular point in a structure of functions such as shear force, bending...