THEORY AND PRACTICE OF CONCRETE-IRON CONSTRUCTIONS. Ill 



and concrete-iron with variable coefficients under increased 

 loading. 



The lines of form-alteration for the concrete and concrete- 

 iron indicate : 



1. That the coefficients of elasticity decrease much 



more rapidly in tension than in compression, under 

 increased loading. 



2. That the material in the vicinity of the neutral axis 



is better utilised than in bodies with a constant 

 coefficient of elasticity. 



The leading formulse for calculating concrete-iron con- 

 structions subject to bending moments express the above 

 data of the distribution of the stresses in a more or less 

 simplified manner, as shown by the two types 1 and 2. 

 These formulae although utilising as far as possible all the 

 scientific data to hand are unable (and it is probably safe 

 to predict will remain so) to give results which will cor- 

 respond with direct compressive and tensile stresses. Con- 

 cerning the compressive stresses in concrete-iron construc- 

 tions, those containing l-J°/° or more iron (instances where 

 the destruction of the concrete-iron body is dependent on 

 the compressive strength of the concrete) differ in this 

 respect most markedly, from the results obtained from 

 direct compression tests. A poor concrete of 1 part cement, 

 3 of sand, and 3 of broken metal, at 3 months old, had a 

 compressive strength as a cube under direct compression 

 of 1710 flbs. per square inch, whereas the calculated com- 

 pressive bending stress of similar concrete in a concrete- 

 iron construction was 2838 lbs. per square inch. Richer 

 concretes show less difference than poorer ones. The 

 question, therefore, arises what circumstances cause these 

 differences. To determine this, it is necessary to compare 

 the two methods of testing. On the one hand a cube is 

 very soon destroyed if the pressure faces are not truly 



