CHAPTER VI 
SIMPLE STRAINS AND STRESSES 
76. Load.—The combination of external forces acting on any piece 
of construction is called the load on that piece. The following are 
examples of forces which may constitute the load on a piece :—(1) Forces 
arising directly from the purpose for which the piece is designed, and 
which constitute the useful load. For instance, the useful load on the 
chain or rope of a crane or hoisting engine is the weight to be lifted. 
(2) Forces due to the weight of the piece, or of pieces connected with it ; 
thus, in the chain or rope mentioned above the load partly consists of the 
weight of the chain or rope, and in winding engines for deep mines the 
weight of the wire rope used forms a considerable part of the load which 
the rope has to carry. (3) Forces due to the inertia of heavy moving 
parts when their velocities vary; thus, the thrust or pull on the piston- 
rod of a steam-engine is not simply that due to the pressure of the steam 
on the piston. When the velocity is increasing the effect of the inertia 
of the piston is to diminish the thrust or pull due to the steam pressure, 
and wice versa. (4) Centrifugal forces, as in the arms and rim of a 
rotating wheel or pulley. (5) Forces due to friction. (6) Forces due 
‘to the unequal expansion or contraction of parts following variations of 
temperature. 
77. Strain and Stress.—The effect of a load acting on any piece of 
construction is a change of form or dimensions of the piece, and this 
change of form or dimensions is called strain. The combination of 
internal forces which are called into play in the material of any piece of 
construction to resist or balance the load is called stress. ° 
There are three kinds of simple strain and stress :—(1) Tensile strain 
and tensile stress. (2) Compressive strain and compressive stress. (3) 
Shearing strain and shearing stress. 
78. Tensile Strain and Tensile Stress.—If a bar AB (Fig. 77) be 
pulled in opposite directions by forces 
PP acting at its ends the bar becomes abet abo > 
longer, and a tensile strain or elongationis |A B} 
produced. If 7 is the length of the un- C a0 
strained bar, and x the increase in length R_IK Qasr B 
produced by the action of the load, then. =[-——— eae oe s 
the tensile strain is measured by the Fic. 77. 
fraction x/. If any imaginary section of 
the bar be taken at right angles to its length, say at C, the internal 
forces Q at this section will balance the force P at B, and the internal 
forces R will balance P at A. These internal forces, which are distributed 
over the whole of the section at C, resist the tendency of the forces 
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