110 F. M. GUMMOW. 



concrete, and the percentage of iron used. In this case 

 the initial stresses so produced are of actual benefit to the 

 construction, inasmuch as when the same is subject to 

 loading, the iron being in tension comes at once into action. 

 Further, concrete which has hardened in air will expand 

 when absorbing water, and contract again when drying 

 out. The effects of these hygrometrical alterations on the 

 perfectly hardened concrete are however less than those 

 which result from the gradual hardening of same in air, 

 and the amounts of such volume alterations are inversely 

 proportionate to their coefficients of elasticity. 



Reference has been made to the combined materials 

 being capable of acting as one body in taking up the stresses 

 when subject to bending moments. To understand this it 

 is necessary to study the relations between the length 

 alteration and the stresses or in its graphical representa- 

 tion the so-called lines of form-alteration. POF and HOI 

 represent the lines of form-alteration for the iron and con- 

 crete respectively in compression and tension, and OBOGE 

 the line of form-alteration for concrete-iron in tension. 

 The form-alteration of the latter in compression being the 

 same as for the concrete (only with an extended range) has 

 not again been plotted. The ordinates represent the specific 

 stresses and the abscissae the accompanying shortenings or 

 elongations as the case may be. The compressive stresses 

 are plotted upwards and the tensile downwards, the shorten- 

 ing to the right, the elongations to the left. 



The line of form-alteration for iron is straight, showing 

 that the deformations take place proportionately to the 

 stresses (within its elastic limit), while those for the con- 

 crete and concrete-iron are curved, denoting that the 

 deformations are not proportionate to the stresses. Thus 

 we have two kinds of elastic bodies before us, viz. t^e iron 

 with a constant coefficient of elasticity, and the joncrete 



