66 



ENGINEERING ON THE FARM 



the lower surface have been pulled apart and those near the 

 upper surface have been crushed together. This is the 

 usual way in which a simple beam breaks. In addition to 

 the compression and tension there is a vertical and hori- 

 zontal shear, as mentioned above. The fact that there is a 

 horizontal shear is quite clearly illustrated by the example 

 of a pile of planks placed in the position of a simple beam 

 (Fig. 94). It will be seen that sliding of one surface on 

 another has taken place. This same tendency to slide 

 between horizontal surfaces is present in a solid beam, but 

 is resisted by the strength of the material. For this reason 

 the solid beam will not deflect as much as the pile of planks. 

 The vertical shear is not quite so evident, but examination 

 of Figure 93 will show that on any section of the beam, 

 such as AB, the support below the beam pushes upward 

 on one side and the load on the beam pushes down on the 

 other, tending to move one part of the beam along an 

 imaginary surface dividing it from the other part, thus 

 producing vertical shearing stress. 



Since the stress within the beam varies from compression 

 at the top to tension at the bottom, there must be some 



Compression W 



Tension^ 



Neufrol surface. 



dangerous 

 section 

 No shear here 



Fig. 93. Flexure in a beam 



*l<\j 



surface between the top and bottom surfaces where the stress 

 changes from tension to compression — where there is no 

 stress at all. This surface is called the neutral surface. 



