EXTERNAL FORCES 13 



bearing is then 6 in. from the face of the wall. The length used 

 for the beam will be 10 ft. 6 in. (10.5 ft.) and the distance to the 

 concentrated load will be 7 ft. 6 in. (7.5 ft.). 



.U 6 = (400 X 7.5) + f 500 * 10 ' 5 ) = 5625 ft. Ibs. 



Add a load of 200 Ibs. 4 ft. from the wall and a load of 300 

 8 ft. from the wall. 



M 6 = (400 x 7.5) + (200 x 4.5) + (300 x 8.5) + 

 * 10 ' 



' 5 ) - 



9074 ft. Ibs. 



The reader will notice that the distance to the center of gravity 

 has in all cases been measured back from the face of the support, 

 but the length used in computing the weight of the beam was the 

 clear length. In this way all the weights are those clear of the 

 supports, for the portion of the beam resting on the support has 

 no effect on the bending moment. 



It is wrong to use only the distance from the face of the support 

 for cantilever beams, or the clear span between supports for 

 simply supported beams, when figuring a bending moment, as it 

 throws all the bearing on the edge. By using the longer distance 

 in computing bending moments a stiffer beam is secured. Com- 

 mercial designers in competitive work invariably use the distance 

 measured from the face of supports, as they thereby save a little 

 material and are enabled to cut down cost. All designs should be 

 prepared by men who have no other interest than that of securing 

 for the owner a design which is not " skinned." 



Continuous beams with uniform moment of inertia are not 

 subject to all the limitations of simply supported beams. This 

 will be discussed later. 



Reactions 



The loads acting downward exert an action on the beam, 

 which is resisted by the strength of the tearing. Considered theo- 

 retically the bearing exerts an upward force pushing against the 

 downward force and equal in amount. This is an example of the 

 old saying, " Action and reaction are always equal and in opposite 

 directions." 



The reactions being equal to the load, it follows that the reac- 



