12 STRESSES. 



abed being A, then the stress, in units of force per 

 unit of area, is 



/"I ; < L > 



Thus we can talk of the force, or load, on a section, but. 

 the stress is the intensity of thejorce or the force per unit 

 area. For instance, a metal bar whose cross-section is 

 4 sq. in. may be subjected to a tensile load of 20 tons ; the 

 load on the bar is 20 tons, but the tensile stress is 



20 



-7- = o tons per sq. in. 



Again reverting to Fig. 1, if the force P, instead of 

 acting away from, acted towards the cube, it would then be 

 called a compressive load, and its tendency would be to 

 cause failure by crushing. In this case also the com- 

 pressive stress is 



/. = I ;. (IL) 



P in this case having the opposite direction. 



Suppose, now, that the two forces S, S, were to act on 

 the cube in such a way as to cause the upper half above 

 the section to slide across the lower half, as shown, these 

 forces would be called shearing forces, and would produce 

 a shearing, or tangential stress on the section in question. 

 The magnitude of this stress would be 



/. =| (I"-) 



These stresses which have been mentioned are termed 

 simple stresses. They are generally expressed as so many 

 tons or pounds per square foot or square inch, the pound 

 weight and its multiple the ton weight being the units of 

 force which are found most convenient and suitable for 

 engineering work. 



5. Compound Stresses. In addition to these simple 

 stresses there are a number of cases of compound stress 

 which occur frequently in parts of structures. Those most 

 often met with are the stresses in bending, torsion, and 

 combined bending and torsion. Bending is exhibited in its 

 simplest form on Fig. 2. A load W is supported in the 



^fr 



FIG. 2. 



