392 Report S.A.A. Advancement of Science. 



an S-shaped curve of the sections. With equal force such an assump- 

 tion can be applied to the deflection of reinforced concrete bodies. 

 The question whether the tensile strength of the concrete shall be 

 taken into account with the deflection caused some diversity of 

 opinion. By practical engineers the point at issue was settled from 

 the beginning, with the result that the tensile strength of the con- 

 crete is not considered at all, and the full permissible tensile strength 

 of the reinforcement at the side of the tension is used. On that 

 sound basis the theory since then has still further been developed, 

 and greatly assisted by the elasticity experiments. This theory, in 

 which the tensile strength of the concrete remains unconsidered, is 

 supported by such authorities as Emperger, Christophe, and 

 Considere. 



It is mainly owing to the endeavour to theoretically explain the 

 favourable adaptability of ferro-concrete that many methods of 

 calculation were brought into existence, mostly by theorists. The 

 oldest methods take the tensile and compressive elasticity of the 

 concrete as equally great, subsequently the modulus of elasticity for 

 concrete was taken smaller, then parabolic curves were taken for 

 the elongation curves, and at last, through Considere's experiments, 

 the line of tension of the concrete under strain was represented by 

 a straight line being parallel to the cross-section. It must be 

 observed that with such assumptions formulae must be obtained, the 

 length of which may be regarded by the authors to reflect the most 

 minute accuracy and reliability ; for the constructing engineer, 

 however, these intricate formulae offer little inducement for their 

 employment. Further, the substitution of the elongation curve is 

 less correct, than the substitution by a straight line, because for the 

 law of power the exponent m is nearer to i than to 2, and the elonga- 

 tion curves would have to be forced into the shape of a parabolic 

 curve. But apart from all that, these methods of calculation do 

 not afford the desired degree of safety ; and they can become some- 

 what dangerous if the percentage of reinforcement in the concrete 

 chosen is too small. Considering that the concrete is liable to crack, 

 caused either through faulty preparation or interruption during the 

 concreting or by drying too quickly, one cannot reckon on a uniform 

 tensile strength of the concrete. Under the circumstances, there is, 

 therefore, no protection against cracks in the concrete under strain, 

 and, for instance, in loading tests small cracks occur frequently at 

 an early stage, originating from the tension, in the body, the cause 

 whereof is as yet unknown. At any rate, the exact time when these 

 cracks appear on the side of tension cannot be predicted with cer- 

 tainty. Further, considering that the object of every statical 

 calculation is less the precise demonstration of the strains in 

 a structure under load than it is proof of a sufficiently great factor 

 of safety in the structure, the tensile strains of concrete must not 

 be considered, for the reason that its tensile strength cannot surpass 

 the limit of elasticity of the iron, thus already giving way before 

 the point of rupture is reached. 



