STKENGTH AND ELASTICITY OF KEINFORCED CONCRETE. 159 



Limit E = f = 42,000 lbs. per square inch. 

 Ultimate strength = 29 tons per square inch. 

 Elongation on 10 inches = 25f u 

 Contraction of area = 69°/° 



The equations become then : — 



x = ± /108p + 576 p* -[9'6 p 



„ = 14000 (2*7 - x)p. 



c = ^=67200 2. 

 5 ^ a? 



From which we obtain : — 



x = 0*41, If- = 576*828 



c = 2950 pounds per square inch. 



It is assumed that the extension of the steel rods is the 

 same as the concrete in which it is embedded, and that 

 consequently there is no slip, and that a stress of 42,000 lbs. 

 per square inch was developed in the steel rods at the 

 moment of fracture. If the length of the beam is insufficient 

 to provide the necessary adhesion area to develop this 

 stress, the beam will fail with a smaller load. The average 

 load causing a crack in the three beams was 8*8 tons, and 

 the bending moment consequently 591*360 inch tons. 



.*. -^ = 591-360 



The mean extension per inch obtained from the mirror 

 extensometers showed that the elastic limit of the steel 

 was just reached at the moment of fracture. The moment 

 of resistance obtained in the foregoing calculations is 

 sufficiently near the mean result obtained in the testing of 

 the three beams to prove the accuracy of the method 

 adopted in the calculations. 



