42— THEORETICAL INVESTIGATIONS REGARDING 



FERRO-CONCRETE 



By H. Kestner, C.E. 



There can be no question that the use of reinforced concrete 

 as a building material has made steady progress in England, 

 America, and on the Continent during the last few yeiirs, so that 

 one is justified in asking, Why has this building system been so 

 much neglected in South Africa? 



To arouse some interest and to draw the attention of engineers 

 to this system of construction will be the object of this paper. 



The practical use of ferro-concrete is of course known to every 

 engineer, and it is only the theory which has not kept pace with 

 the progress of this form of construction, about which there still 

 exist very vague and erroneous views, especially with regard to 

 the statical action of this method of construction. The reason that 

 the theory has not kept pace is on account of the rapid development 

 of ferro-concrete building. 



A method of calculation conformable for the purpose must, as 

 far as possible, correspond with the existing statical action of both 

 materials forming the ferro-concrete, i.e., concrete and iron. Only 

 under these conditions can safety and economy be gained. Very 

 much depends on the kind of calculation adopted, and as to whether 

 the formulae are very long and intricate, or simple and easy of 

 comprehension. It must at once be confessed that the various theories 

 about ferro-concrete in the vain hope of obtaining complete accuracy, 

 make the calculations so intricate that they are completely useless 

 for the purpose they are intended. Every engineer who is a friend 

 of this new building system, and is desirous of assisting its progress, 

 should avoid these faults and only choose the simplest formulae. 



The most accurate theories which always consider the elastic 

 qualities of both constituent materials, concrete and iron, in every 

 point, have their fundamental rules always connected with certain 

 presuppositions, and this disqualifies them of the term accurate, 

 but on account of the care which has been applied in the solution 

 of the question they are the most meritorious theories at present 

 we possess. 



In the present state of the theoretical investigations, it is in 

 my opinion more justifiable to adhere to a theory connected with 

 presuppositions instead of adopting a fallacious accuracy. It 

 naturally depends entirely on the more or less careful investigation 

 of the basis of this theory. This will be the object of my paper, and 

 I will make use of the facts which experience has taught us. 



Co-operation of Concrete and Iron. 



In the theoretical investigation of ferro-concrete it is generally 

 accepted that the concrete is in a position to follow the iron in its 

 changes of form through tension, compression or other strains. If 

 we place instead of the modulus of elasticity of the iron the letter 

 Ee, and of that of the concrete the letter Ec, we assume that the 

 elastic strains of the reinforcement, and of the concrete, 



