STRENGTH AND ELASTICITY OF REINFORCED CONCRETE. 155 



teristic, and is seen more or less well defined in all load 

 deflection curves with reinforced concrete beams. The 

 curves after passing this point become much straighter 

 and resemble those obtained in direct tension tests. 



Comparing Fig. VI. d. with Fig. VI. e. it will be seen how 

 greatly the reinforced beam differs from the plain beam, 

 in the increased loads it is able to carry, and the enorm- 

 ous increase in the deflections sustained before fracture 

 compared with a plain beam. 



The experiments show that the extensions increase in 

 a reinforced beam from the point where the maximum 

 tensile strength of the plain concrete has been attained, 

 to the point where fracture occurs, where it may be ten 

 times as great as in a plain concrete beam. The tensile 

 coefficient of elasticity in a reinforced beam becomes less 

 in proportion to the greater extension, since the tensile 

 strength of the concrete remains constant during the 

 period included, between the point where the fracture 

 would occur in a plain beam, to the actual fracture in the 

 reinforced beam. 



The equations for calculating the position of the neutral 

 axis and the moment of resistance for a reinforced concrete 

 beam may be found in the following manner (Figs. 10, 11, 

 and 12) :— 



Let h x — the distance from the compression face to the 

 neutral axis of the beam. 

 h u = the distance from the compression face to the 



centre of gravity of the reinforcement. 

 h (1 — x) = the distance from the neutral axis to the 



tension face, 

 /i = the total depth of the beam. 

 E s E c jG c — the coefficient of elasticity of the metal 

 reinforcement, the concrete in compression, and 

 the concrete in tension respectively. 



