54 



BROOMALL 



the point considered into this vertically planed horizontal 

 shear. In other words, the unbalanced part of the forces is 

 transmitted, by virtue of the shear, from the outside of the 

 flanges to the web and through the web to the neutral axis. 

 The shearing planes for the horizontal shear in the flanges 

 and the web are indicated in Figure 4. 



1 1 m 1 1 J I N I II 



fmrrr^ 



FIGURE 4- 



The relationship between direct stress and shear in the 

 flange of the I Beam is the same as that existing in the rec- 

 tangular beam and in the web of the I Beam. The law of 

 variation of stress, however, is not the same. In the flange 

 of the I Beam the direct stress varies according to the rectang- 

 ular law and the shear according to the triangular law. In 

 the web, on the other hand, the direct stress varies according 

 to the triangular law, while the shear varies more or less para- 

 bolically. In Figure 5 is indicated, for the upper flange and 

 for the web, the nature of this variation of forces. 



From what has preceded we see that in the flanges of the 

 I Beam the effect of the external loads is to produce horizon- 

 tal shear with vertical shearing planes, together with the com- 

 panion right-angled shear likewise with vertical shearing 

 planes, and also direct stress, compression in upper flange and 

 tension in lower flange. In the web there exist the ordinary 

 horizontal and vertical shear and direct stress. These forces 

 are shown in Figure 5. 



