72 



PRACTICAL STRUCTURAL DESIGN 



The tensile stress is low and the compressive stress is high, so the 

 height of the tensile force triangle will be greater than the height 

 of the compressive force triangle. Steel is placed near the bot- 

 tom of a reinforced concrete beam to give it increased tensile 

 strength, and this lowers the neutral axis. The concrete is not 

 relied upon to furnish any tensile strength, this being concen- 

 trated in the steel. The concrete below the neutral axis is there- 

 fore used only to furnish shearing strength and protect the steel 

 from corrosion. 



The total tensile stress is considered as being carried by the 

 steel, and the compressive stress is carried by the concrete above 

 the neutral axis, where the variation in fiber stress follows the 



k- 4*--n - >l 



JL. 



^ 



Fig. 57 



straight line law already considered. Actually, the compressive 

 stress varies as a parabola and not as a triangle, but to use the 

 triangle is safe and the " straight-line method " alone is permitted 

 in building ordinances and in all regulations issued by responsible 

 officials in this and other countries. 



To properly treat reinforced concrete design will require a book, 

 and the author has one in preparation which will shortly follow 

 the present book, and replace his " Reinforced Concrete, a Manual 

 of Practice," written in 1907 and now out of print. The method 

 he uses for determining the position of the neutral axis in a rein- 

 forced concrete will be given here, together with his method for 

 determining the percentage of steel. The methods, and the 

 resulting formulas, he believes to be original, as they have never 

 appeared in any book to his knowledge, or to the knowledge of a 

 large number of teachers and consulting engineers to whom he 

 wrote about the matter. 



