100 MECHANICS 



When a beam is rigidly held, or fixed, at both ends, as 

 shown in Fig. 4, it is called a restrained beam, or, more com- 

 monly, a beam fixed at both ends. 



LOADS ON BEAMS 



The forces due to the weights that a beam supports are 

 known as loads. If the whole load is applied at one point, 

 or practically one point, it is called a concentrated load; if it 

 extends over a portion of the beam, it is called a distributed 

 load; and if it is equally distributed over the beam, so that 

 each unit of length has the same load, it is called a uniform 

 load. 



There are certain methods by which such loads may be 

 represented graphically. These methods may best be illus- 

 trated by referring to Fig. 5, which shows a simple beam. 

 , Starting at the left, the 



point of the arrow 

 shown at RI is the 

 ^4 point of support. The 

 FIG. 5 arrow a represents a 



concentrated load. At bb' is shown a uniformly distributed 

 load, of a certain number of pounds per foot of beam, that 

 extends from b to &'. At c, d, and f are shown other concen- 

 trated loads similar to the one at a. From e to e' is a distrib- 

 uted load, represented by the shaded triangle under ee' , that 

 starts from nothing at e and increases to a given number of 

 pounds per foot of beam at e'. At gg" is a distributed load, 

 starting from nothing at g and increasing to a maximum 

 at g', and then decreasing to nothing at g". At 7?2 is shown 

 the right-hand support. 



REACTIONS 



A beam with any loads it may carry is held up by the 

 supports; that is, the beam presses on its supports at the ends. 

 The supports resist this pressure and prevent the beam from 

 falling. This upward force exerted by each support is 

 known as the reaction. 



There are two facts susceptible to proof in regard to a 

 beam: (1) The resultant of all the forces acting on a body 



