FARM STRUCTURES 407 



stress at the wall, would be twice as great for a 20-pound 

 load as for a 10-pound load. It is to be noticed that the stress 

 would be greater at the point where the beam enters the 

 wall than at any other point; or, in other words, the maxi- 

 mum bending moment would exist at that point. 



Expressed in the form of a formula: 

 B M = W L 



where B M is the maximum bending moment, W the weight, and 

 L the length of beam in inches. 



If the beam be supported at ^ --^wfr- --^_- _, 



both ends or extend into the wall 

 at both ends, the maximum bend- 

 ing moment would have an entire- 



fnr a Fig> 259 ' A sketch illustrating 

 lOr a the action of a concentrated load at 



beam resting in supports at both i 

 ends with a load at the center, 



B M = y WL 



If the load be uniformly distributed over the beam, then 

 B M = y 8 W L 



The Modulus of Section. It is generally known that a 

 2x4 piece of wood will support a greater load when placed on 

 edge than when laid flat. The modulus of section is simply 

 a measure of the strength of a beam according to the arrange- 

 ment of the material. Thus, for a beam with a rectangular 

 cross-section, 



bd 2 

 MS- T 



where M S is the modulus of section, b the width of the beam in 

 inches, and d the depth of the beam in inches. 



Thus it is seen that a 2x4-inch beam is twice as strong 

 when set on edge as when laid on the flat ; for, when placed 

 on edge, 



2X(4X4)32 



M 



_ 

 6 66 



