MECHANICS 87 



It should be understood that each column of a building 

 supports a given floor area, and that the load coming on each 

 column will depend on the extent of this area multiplied by 

 the live load, in pounds per square foot of floor. Each 

 column carries not alone this load, but also the loads trans- 

 mitted directly from column to column. Thus, the column 

 supporting the fifteenth floor supports also four other 

 columns above with all their loads. 



While this system of graduating the live loads on the 

 columns from floor to floor is generally practiced, the amount 

 of reduction at each floor is a matter that depends on the 

 judgment of the designer. The percentage of reduction is 

 often fixed by city building laws, with which the designer 

 must comply. 



MECHANICS 



FORCES 



Two forces may be compared when the three following 

 facts about each force are known : (1) The point of applica- 

 tion, or point at which the force acts; (2) the direction of 

 the force or line along which it acts; and (3) the magnitude 

 of the force when compared with a given standard. 



In engineering work in America, the unit of force is always 

 taken as the pound. 



Representation of a Force. A force may be represented 

 by a line. Thus, in Fig. 1, let A be the point of application 

 of the force, let the length of the line AB . 

 represent its magnitude and line of action A ' 

 to any convenient selected scale, as, for FIG. 1 



instance, 1 in. equals 10 lb., and let the arrowhead 

 indicate the direction in which the force acts. Then the 

 line AB fulfils the three required conditions in regard to 

 point of application, direction, and intensity, and the force is 

 fully represented. 



