12 TENSILE AND COMPRESSIVE STRESSES, Art. 14. 



(f) The form of the cross section has some influence upon 

 the distribution of stress and deformation, because the fibers do 

 not act independently as is ordinarily assumed ; that is, a bar of 

 circular cross section will not offer exactly the same resistance 

 as one of rectangular cross section having the same area; nor 

 will one bar, whose area is equal to the total area of four bars, 

 offer the same resistance as the four bars. 



Since the parts of a solid, body can not be deformed inde- 

 pendently, there is, even within the elastic limit, a constant ten- 

 dency to equalize the stress on any section. As soon as one part 

 is deformed more than another it throws more stress upon adja- 

 cent parts, but abrupt changes of section should always be 

 avoided in order that the stress may be uniformily distributed. 



14. Tensile and Compressive Stresses. Tensile and com- 

 pressive stresses are alike except that they act in opposite direc- 

 tions; they are normal stresses. Forces which elongate a body in 

 the direction of their action, produce tensile stresses in it, while 

 those which compress it, produce compressive stresses. The in- 

 tensity of stress in either case is determined by equation (1), 

 page 6. It is important to note, however, that when the ratio 

 of the length to the least lateral dimension of a piece in com- 

 pression, exceeds a not well defined amount when the piece 

 is a column and not a block there will be buckling stress; this 

 is treated under columns in Chapter IX. 



15. Shearing Stresses. Forces which deform a body by 

 moving parallel surfaces past each other, produce shearing 



stresses in it; these are tangential 

 *\ stresses. Scissors and shears pro- 



duce shearing stresses in a single 



Fig. 8. plane as nearly as may be. In 



Fig. 8, there are shearing stresses 

 in the bolt along the planes AB 

 and CD. 

 - i A weight on a beam as in 



Fig. 9, produces shearing stresses 

 at every cross section of the beam 

 ** because its effect must be trans- 

 mitted to the supports, producing 



