REINFORCED CONCRETE. 



57 



4 in the plain concrete beam Fig. 4. In Fig. 6 five strain 

 curves are plotted for five corresponding bending moments, 

 and the stress curves derived from the strain curves by 

 means of Fig. 3 are complete on the compression side of 

 the neutral axis. The curves on the tension side are neces- 

 sarily incomplete as Fig. 3 does not furnish the data for 

 continuing the curves beyond the points shown, which is 

 the tensile strength of the plain concrete. The stress in 

 the steel reinforcement is determined from the extensions 

 measured, and the coefficient of elasticity of the steel. 



Fig. 6 — Distribution of Strain, and equivalent Stress, over the Cross Section of 

 Eeinforced Concrete Beams as experimentally determined. 



Extension on 40 in. in inches. Tensile Stress in lbs. per sq. inch. 



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h afb 



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Jfef£f?£ffi£ pi 



/ BMoment =3625 inch fens 



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 o o 6 



Compressions on 40 Compressive Stress 

 inches in inches. in lbs. per sq. inch. 



The curves 1, 2, 3, 4 and 5, in the strain diagram on the 

 left of Fig. 6, show that a plane section before flexure is 

 not a plane section after flexure, and that the deviation 

 from the plane is greater as the bending moment increases. 

 Again the neutral axis moves from the centre of the beam 

 towards the compression side as the bending moment 



