STRENGTH OF MATERIALS 237 



284. Bending Force. When a beam is bent, the forces 

 at any point tend to pull the fibers apart in the upper part 

 and push them together in the lower part, while the portion 

 between the two is subject to less stress. The nearer the 

 center the force acts, the less becomes the stress, until fi- 

 nally the beam or neutral axis is reached. At this point the 

 bending stress is zero. Accordingly structural steel beams 

 are made with flanges (reinforcements) at the top and bot- 

 tom to take care of the bending stresses. These flanges are 

 connected by a plate called a web. The material of the web is 

 subject to a shearing stress the maximum of which occurs 

 at the support and the minimum where the bending is greatest. 



Wood offers the greatest resistance when placed in an 

 upright position. A short post is stronger than a long one 

 of the same section, since the stress in the short post is due 

 merely to compression, while in the long post there is apt 

 to be bending. By applying a stay or projection to the 

 part about to bend, firmness may be given to the support. 



A fluted column offers a greater resistance to a bending 

 force than a smooth one; therefore it is stronger. When a 

 beam is supported at both ends, it is twice as strong as one- 

 half its length supported only at one end. Of two beams with 

 the same cross-section area, the longer beam is the weaker. 



285. Measurement of Stresses. Stresses are measured 

 in pounds per square inch. 



For example, if we have a bar in tension there is a stress distrib- 

 uted equally all over its cross-section. In other words, if the bar 

 is 1 in. square, each particle of that square inch will bear the same 

 stress or load. If a bar is 2 in. square then its area is 4 sq. in. and 

 each inch of this area has an equal load or stress acting upon it. 

 The pounds of stress per square inch on a piece in tension or com- 

 pression is called the unit stress. 



