THEORY AND PRACTICE OF CONCRETE-IRON CONSTRUCTIONS. ] 15 



each other, and that they take up the loads in the propor- 

 tion of their coefficients of elasticity. 



At point B, where there is a decided change in the direc- 

 tion of the line of form-alteration, we enter the second stage 

 or period, during which the stress on the concrete remains 

 with small variations the same throughout, although the 

 concrete-iron body continues to elongate at a much more 

 rapid rate than in the first stage. The coefficient of 

 elasticity of the concrete in tension sinks rapidly, while 

 the diagram of form-alteration of concrete in compression 

 (O H) shows that the. coefficient of elasticity in compression 

 experiences but slight alterations in comparison. As soon 

 as these alterations in compression, however, become more 

 conspicuous by increasing loads, or when the elastic limit 

 of the iron has been reached, we enter the third period. 



The coefficient of elasticity in tension sinks to nought, 

 indications of the destruction of the body become apparent, 

 and the theory of elasticity can no longer be applied. The 

 end of the second period most probably represents the limit 

 of elasticity of the concrete-iron construction. 



The qualification of the concrete to elongate beyond the 

 point B is instilled into the same by the following circum- 

 stances, viz.: — 



1. Its association with the iron and through it the 



uniform distribution of the tensile stresses over 

 its sectional area. 



2. The decrease of its coefficient of elasticity in tension. 



3. The stressless displacement of its molecules. 

 Concrete-iron bodies of green concrete with a high per- 

 centage of iron show when tested that they are frequently 

 quite devoid of that portion of the line of form-alteration 

 known as the first period, and constructions which have 

 already been subjected to repeated loadings are similarly 

 affected, so that if tested later on, it would appear as if 



