RECTANGULAR BEAMS 75 



such failures are exceedingly unlikely and need rarely be con- 

 sidered. The usual so-called shear failures are in reality diagonal 

 tension failures. 



When a beam is to be tested to destruction in order to investi- 

 gate any particular type of failure, the steel and concrete used 

 are tested separately. In this way, the properties of the mate- 

 rials are known when separately subjected to stress. 



When a beam begins to fail by the yielding of the steel, any 

 further load rapidly increases the deformation, large cracks open 

 up in the concrete on the tension side, the neutral axis rises on 

 this account, and the ultimate failure soon occurs by the crushing 

 of the concrete. A steel tension failure is found to occur when 

 the amount of steel used is less than the amount determined by 

 theoretical formulas which makes the beam of equal strength in 

 tension and compression. This result agrees, then, with what is 

 expected. Likewise it is found that with a larger amount of steel 

 than is theoretically required, the yield point of steel is not 

 reached and the beam fails directly by crushing of the concrete. 

 Beams with no web reinforcement and with the existence of 

 large shearing and moment stresses, fail by inclined cracks open- 

 ing up in the concrete, thus substantiating to a considerable 

 degree the theoretical deductions regarding the internal stresses 

 in beams. 



The results of breaking tests on reinforced beams with different 

 percentages of steel reinforcement compare well with the results 

 derived from theoretical formulas. Considering the nature of 

 the material, the calculations by the two assumptions of stress 

 variation are found to agree sufficiently with the experimental 

 results to justify their use in problems of design. 



Another method of testing reinforced concrete beams is by 

 the use of extensometers to measure distortions, so that the 

 deformation of the steel and of the extreme fiber of the con- 

 crete may be calculated and the neutral axis determined. In 

 making beam tests it is customary to place equal loads at points 

 dividing the length into three equal parts. (See illustration 

 of beam marked Cl.) The advantage of this arrangement lies 

 in the fact that the bending moment is practically uniform be- 

 tween the loads and, if measuring devices are attached, the de- 

 formations of the fibers at the top and at the bottom may be 

 easily determined. If in Fig! 40 the deformations be aa' and 

 66', the neutral axis is located by connecting a' and &' with a 



