116 F. M. GUMMOW. 



such a period had never existed. The carrying capacity of 

 the construction however is thereby not affected. Prom 

 this it must be deduced that it is inadmissible to draw con- 

 clusions as to the carrying capacity of a construction from 

 the elongations at the earlier stages of loading. Regard- 

 ing the appearance of cracks before period three is reached 

 it is necessary to distinguish as to their causes, whether 

 they are due to changes of temperature ; to too quick dry- 

 ing out ; to weakness of the construction, or to settlement 

 of the supports and distortion of same. The latter are 

 especially a source of danger and difficult to prove, so that 

 it becomes absolutely necessary on that account to pay 

 particular attention to the foundation of the supports. 



This outlines the theory of concrete-iron constructions, 

 and although the scientific investigations so far made, 

 permit of a fairly correct view of the theory, there are 

 many points yet which require clearing up. Attention is 

 here directed to the indiscriminate use of formulae, since 

 eacli requires data of different permissible stresses for the 

 various mixtures, and the percentages of reinforcement, 

 values which, as already pointed out differ from the results 

 obtained from direct tests of the materials. Should such 

 information be available from abroad, care should be 

 exercised in its use as local conditions and materials con- 

 siderably affect their values, rendering local investigations 

 absolutely necessary. The theory which appears a most 

 simple one when first entering into the subject becomes 

 the more complex the deeper the same is studied as in try- 

 ing to solve one point, other points come to light which 

 also require investigating. 



The theory of concrete-iron having been dealt with a few 

 notes on its application may be of interest. The earliest 

 scientific application of concrete-iron was in the manufac- 

 ture of plates with free ends, these being the most simple 



