CEMENT CONSTRUCTION 187 Fig. 42 shows the failure of the same type of beam as previously shown, but provided with reinforcing bars with fixed stirrups. From the cracks shown near the bottom of beam, this seems to be a well-balanced reinforcement, with the main stress occurring at the center of the horizontal rods.
While these different forms of reinforcement seem to be favored by many, they are criticised by some investigators who claim to have records of tests showing that the additional strength of the stirrup construction does not make up for the additional cost. The method of reinforcing shown in Fig. 43 has been successfully used in deep beams, and also in beams which are "con tinuous"—that is, which extend over more than one span between columns.
In Fig. 43, the method of fastening the ends of the reinforcement is a point to be noticed. In case of a continuous beam, the reinforcement would simply extend upwards toward the end of the column, and over into the next span. It is claimed by many authors that the ends of all rods for reinforcement should be either bent over and embedded in the concrete, or fitted with some kind of expanded end, to prevent the ends slipping when the beam is bent.
The most important principle in placing re inforcement in concrete beams is to place the steel so that it will relieve the concrete from all tensile stresses if possible, and thus aid in devel oping the high compressive strength of the material.
Every ounce of tension in the steel is only effective as it is transferred to the concrete. In the case of a plain beam with free ends, there is no stress in the steel at the ends, while the maxi mum tension is usually at or near the center of the beam. The entire amount of this tension must be gradually transferred from the steel to the concrete.
While the adhesion or sticking of the con crete to the steel is relied on to permit the trans fer of this stress from one material to the other in much of the reinforced concrete work now being done, it is realized that this adhesion is not always permanent. Failures of floors have already occurred, due to loss of the adhesion, after they have successfully supported heavy loads for many years; the adhesion being greatly reduced with age and under certain unfavorable conditions, such as continued soaking of the con crete in water, long-continued vibration, etc.
Experience has demonstrated that beams may fail in other ways than by the pulling in two of the reinforcing steel, as, for example, by shearing across a vertical plane, by tension along a diagonal plane, or by slipping of the rods through the concrete.