48 FARM MACHINERY 



resist compression, so neither of these may be used for 

 transverse stresses. The modulus of rupture is a measure 

 of the transverse stresses necessary to produce rupture 



FIG. 2Q BEAMS OF UNIFORM STRENGTH 



FIG. 30 



and is determined experimentally. It is usually a quan- 

 tity lying between the compressive and tensile strengths 

 of the material. 



81. Section modulus (S.M.) is the quantity represent- 

 ing the ability of the beam to resist transverse stresses. 

 It has been noticed by all that a plank will support a 

 greater load on the edge than on the flat. For a rectan- 

 gular cross section, Fig. 30, if h = depth in inches and 

 b = breadth in inches, the section modulus is 



bh* 

 6 



that is, the strength of a rectangular beam is propor- 

 t.onal to its breadth and to the square of its depth. 



