CHAPTER LXIV 



MECHANICS OF MATERIALS AND MATERIALS OF 

 CONSTRUCTION 



The Strength of Beams. The strength of a beam or its 

 ability to support a load depends upon three principal factors : 

 (1) The way the beam is stressed, or the way the load is 

 applied or distributed and the beam supported; (2) the way 

 the material is arranged ; and (3) the kind of material. These 

 factors are represented by the maximum bending moment, 

 the modulus of section, and the modulus of rupture. 



The Bending Moment. The bending moment is a meas- 

 ure of the stresses acting on a beam. Suppose a beam to be 

 fixed solidly at one end, as would be the case if it extends 

 into a solid wall, and a load or a weight to be suspended at the 

 extreme end, as shown in Fig. 258. It is to be noted that the 



greatest stress in the beam would 

 be at the point where it enters 



L $ the wall. The force would tend 



to rotate the beam about a point 

 in the beam where it enters the 

 wall. The stresses produced would 

 tend to pull the material in two 

 at the upper side and to crush it 



on the lower. If the weight be placed somewhere between 

 the wall and the end, the stress on the beam would be less 

 than in the first instance; in fact, the stress would be in direct 

 proportion to the distance from the wall to the weight. The 

 stress would also be in direct proportion to the size of 

 the weight. Thus the tendency to break the beam, or the 



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