50 REINFORCED CONCRETE CONSTRUCTION 



Turning our attention to the principles of mechanics, in order 

 to determine which ones may be used to find the resisting moment 

 in a plain concrete beam, we find we have three, as follows: 



1. For beams rectangular in section, the average unit tensile 

 and compressive fiber stresses on any cross-section are represented 



FIG. 23. 



by the average abscissae in the tensile and compressive parts of 

 the combined stress-deformation diagram respectively. Also, 

 the total tension T } Fig. 26, and the total compression C on a 

 cross-section are proportional respectively to the areas XOX" 



A Compression 



Neutral plane 



Tension ' 

 FIG. 24. 



and AOA"] hence, according to some scale, the areas represent 

 T and C respectively. 



2. The resultant tension T and resultant compression C act 

 through the centers of gravity of the tensile and compressive 

 areas in the combined stress-deformation diagram. 



3. When all the forces (loads and reactions) applied to the 

 beam act at right angles to it, then the resultant tension T equals 



