74 REINFORCED CONCRETE CONSTRUCTION 



the design of beams having web reinforcement in the form of 

 bent rods. It is evident, however, that the above formula for 

 bond stress along horizontal rods can in no way apply to the bond 

 stress along vertical or inclined stirrups. The method of obtain- 

 ing the necessary diameter of stirrups, and the spacing of stirrups 

 and bent rods will be explained later. 



39. Tests. It would be poor judgment to base design of rein- 

 forced concrete beams on theory alone. Seeing is believing and 

 if tests could not be reconciled with theory, there are few engi- 

 neers who would have much dependence upon theoretical for- 

 mulas. The reason for making tests, in the first place, is to 

 see if the theoretical formulas deduced from the principles of 

 mechanics give results closely agreeing with the actual conditions 

 in practice. 



A reinforced concrete beam will usually fail in one of three ways: 



(a) By the yielding of the steel at, or near, the section of 

 maximum bending moment. 



(b) By the crushing of the concrete at the same place. 



(c) By a diagonal-tension failure of the concrete at a place 

 where the shear is large. 



Methods (a) and (b) are called moment failures. Method (c) 

 is sometimes improperly called a shear failure. To be sure, shear 

 is the chief factor in the cause of such a failure, but the true reason 

 for the destruction of the beam in such a case is the cracking 

 of the concrete due to inclined tensile stresses. 



We have already spoken of the fact that the bond stress be- 

 tween concrete and steel may be exceeded and that the beam 

 may fail by the slipping of the bars. This, however, is not 

 possible if the beam is designed properly and if a fairly wet 

 mixture of concrete is employed and carefully placed around the 

 steel. In an important series of tests on beams by Bach in 

 which the primary cause of failure was the slipping of the bars, 

 the bond stress along the horizontal bars at the end of beam was 

 calculated by the preceding formula. The average maximum 

 result for the rectangular beams with straight rods only was 291 

 Ib. per square inch, and with stirrups 330 Ib. per square inch. 

 With a number of bent rods, the average value was 493 Ib. per 

 square inch. 



Failure by the shearing of the concrete near the support is 

 possible where the load is very close thereto, but as the shearing 

 strength of concrete is at least one-half the crushing strength, 



