Ferro-Concrete. 387 



-which adjoins the reinforcement, are in the proportion of Ee to Ec. 

 Most of the theorists have accepted this fundamental rule as a 

 basis of their formulae, without investigating the correctness. It 

 is, however, of great value to examine it more closely in order to 

 prove whether this fundamental rule can be accepted or whether it 

 is possible to disregard it completely. 



Experience has not given us many facts to answer these ques- 

 tions. Harel de la Nae, a French Engineer, assumes that in a 

 ferro-concrete body under strain the cross-section shows a point of 

 contraction, but only then when the limit of elasticity of the concrete 

 has been reached. According to this engineer's assertions, the slip- 

 ping of the reinforcement within the body can either be caused by 

 the elongation of the iron when it reaches the limit of elasticity, or 

 by shearing of the concrete. These assumptions are completely 

 proved by loading tests which were carried out to the breaking of 

 test pieces. Should the breaking of such a test piece occur in the 

 centre, the adhesion of the concrete to the iron ceases through the 

 elongation of the iron. Should, however, the test piece break near 

 the supports, the reinforcement is dislocated by the shearing forces. 

 This proves that the adhesion of the concrete to the iron is attacked 

 by external forces which at a certain limit destroy the adhesion. 

 In any case it is most interesting to know whether these forces give 

 such an elastic change to the form of the body before reaching the 

 limit, that both materials cannot be considered as co-operating. 



From the purely theoretical point of view it is evidently inac- 

 curate to assume that the fibres of the concrete and iron lying close 

 together are in the same way pulled or compressed,. 



Even when shearing stresses resulting from the external forces 

 are not appearing, the diversity of the material must necessitate 

 a particular connection between concrete and iron at the point where 

 both materials come into contact with one another. It cannot be 

 disregarded that the theory of elasticity can really only be applied 

 to bodies of the same material. The theory of the strength of 

 materials which does not permit of the consideration of all irregu- 

 larities in the shape of the homogeneous bodies, is more difficult to 

 apply to the elastic change of form of those bodies which are built 

 up of irregular voids, cracks and widely differing materials. Differ- 

 ently composed bodies do not therefore allow a purely scientific 

 investigation. If we now consider the shearing strains which under 

 the influence of the load come into action at the point of contact 

 •of the iron with the enveloping concrete, then we find a fresh reason 

 for the probable disconnection of the concrete and the iron. At 

 another page of my paper I will refer to the fact that the cross- 

 section of a homogeneous body under a bending moment must be 

 subjected to changes of form through shearing forces. In the ferro- 

 concrete body the shearing force, which the iron exercises on the 

 concrete, must have a reaction of the same kind. Experience does 

 not give us any facts regarding the value of the modulus of shearing 

 elasticity, and on the other hand such elastic changes of form are 

 not taken into consideration in the calculation of the homogeneous 



