Art. 4. 



KINDS OF LOADS. 



The internal forces produced by the loads are called stresses. 

 The stresses are the internal resistances offered by the material 

 to the deformations caused by the loads. 



4. Kinds of Loads. The dead load consists of the weight 

 of the structure itself, together with such accessories as are 

 permanently fixed in position. 



A LIVE LOAD is a moving load, or one that moves over the 

 structure, and may have any practical points of application. It 

 is assumed that a railway train moves over a bridge on the track 

 and a horse on the roadway. Pedestrians, vehicles, the wind 

 blowing against a moving train, and traveling cranes are exam- 

 ples of live loads 1 . 



A CONCENTRATED LOAD is one that is treated as being applied 

 at a single point or along a line, such as a wheel load. Prac- 

 tically, every load is applied over an area although this area 

 may be very small. 



A DISTRIBUTED LOAD is a continuous load applied over an area 

 and is usually uniformly distributed. A structure's own weight 

 is usually taken as uniformly distributed, and live loads are 

 often assumed to be uniformly distributed. The pressure of 

 water against a dam, or of earth against a retaining wall, are 

 examples of non-uniformly distributed loads. 



5. Manner of Application of Loads. Loads are applied 

 gradually, more or less suddenly or with impact. 



A dead load, like the structure's own weight or the weight 

 of snow, is usually gradually applied but not necessarily so. 



Theoretically the effect of an instantaneously applied load, 

 if it is not applied with a blow, is double that of a gradually 

 applied load (19), but the effect of a suddenly applied load is 

 usually much less, unless it is applied with a blow. 



A LIVE LOAD may be gradually applied, but is usually applied 

 more or less suddenly and with some impact. The weight of a 

 railway train is applied to a bridge more or less suddenly, de- 

 pending on its speed; it is also applied with impact on account 

 of some of the moving parts of the engine being unbalanced, on 

 account of "flat wheels," "low joints," etc. The striking of a 

 horse's hoofs upon the floor of a bridge, and the jolting of a 

 vehicle due to inequalities of the floor, are examples of impact. 



ir Tlie definitions of dead and live loads here given do not correspond 

 with those given in mechanics but with the usage of engineers. 



