KEIXFUHCED CONXRETE STRENGTH OF BEAMS. 



651 



The results of these computations are uiven in the foUowing 

 table, and plotted in Fig. -i : — 



It Avill be noticed that from 3 '29 to •5'52 ft. from centre the 

 neutral axis doubles back, so that we have an apparent ambiguity — 

 that i.'^ to say, the moment of resistance at any intermediate point 

 may be due to either one of the t-v\'o diagrams or stresses in Fig. 3, 

 one having a higher compressive stress and a higher level for tlie zone 

 of fracture; the other a lower compressive stress and no zone of 

 fracture. 



A reasonable conclusion to draw would be that the latter diagram 

 would be correct so long as the stres.ses represented by it were not 

 exceeded, but if any slight extra load were added the concrete would 

 immediateh' fail, and the former diagram would represent the 

 stresses. 



An examination of the diagiam of stress in the steel leads one 

 to doubt whether the stresses arrived at in the above t;^ble are 

 possible; from the diagram we would conclude that the stress in the 

 steel rises rapidly towards the centre from about the ambiguous 

 region referred to above. This increase of stress can only be imparted 

 to it by means of the shear in the concrete; now the concrete in this 

 very region will be cracked, as shown by dotted line in figiu'e, and con- 

 sequently incapable of delivering any stress. 



If the coneiete in the cracked zone were stripped away, we would 

 have less difficulty, becatise the remaining concrete would tlien be an 

 arch of which the thrust would be supplied by the reinforcement. 



In Fig. i I have shown at a — a the pressure line which would 

 be proper to such an assimiption. It is appropriate here to remark 

 that tests have been made at the Universities of Illinois ;nd "Wis- 

 consin, in which the rods have been exposed for a considerable distance 



