INTERNAL FORCES 



69 



strength of these materials. The material is so disposed that 

 practically all the metal highly stressed is concentrated in the 

 flanges, the web transmitting the stresses and taking care of 

 shear. When a beam of /-section is required having a depth greater 

 than can be properly rolled, one is made of a plate having angles 

 riveted along the edges, this being known as a plate girder. When 

 a still deeper girder is required a lat- 

 ticed girder is used, this being known 

 as a truss. 



Wooden beams are made only in 

 solid form, rectangular or round, for 

 wood is composed of distinct fibers, 

 many of which would be completely 

 detached from the main fibers in 

 shaping tl^ section to provide broad 

 flanges. Steel and iron will transmit 

 stresses equally well in all directions, 

 so while in the filleted section con- 

 necting the flange to the web there is I ig. 55 Comparison of Mo- 

 some concentration of stresses, this 

 has been taken care of in designing 

 the beam. In all solid and rolled 

 shapes the maximum fiber stress is the skin stress. In built-up 

 sections, such as plate or latticed girders, the maximum fiber 

 stress is assumed to cover the flange member and the whole 

 action is on the line passing through the center of gravity of the 

 flange. The stress is transmitted from the web, or the web 

 members, to the flange through rivets, which must be properly 

 proportioned in size and properly spaced to take care of the shear. 



Elastic Limit 



Within the limit of strength known as the " elastic limit," all 

 materials may be stressed a number of times and recover their 

 original dimensions. The elastic limit is a stress where the 

 material is permanently deformed and stress in excess of the 

 elastic limit causes rapid deformation. Up to the elastic limit 

 the stress-strain curve is straight, but it curves after the elastic 

 limit is passed. Fig. 56 is a stress-strain diagram of steel, iron, 

 and wood and gives a good idea of the relative strengths and 

 deformations. The illustration is copied from " Materials of 



AxJsA-A 



1-720 1-2/5.8 



S-120 S- 36.0 



AxisB-B 

 I- 125 1-95 



ments of Inertia and Section 



Moduli in a Rectangular 



Beam and in an I-beam 



