600. There are several methods of failure of concrete-steel beams other than those considered above, direct tension in the steel or direct compression in the concrete due to the bending moment. These other methods of failure are popularly called failures in shear, although some of them cannot properly be so classed.
601. We have seen that the shearing stress of concrete is usually considered to be somewhat in excess of the tensile strength (§458) and that the latter is one-fifth to one-tenth the compressive strength. With a beam having only a normal amount of reinforcement, then, there is little danger to be feared from simple vertical shear, and as a matter of fact, tests have not developed instances of such weakness. In comparatively short spans, however, failures have occurred near the quarter points, in cracks starting at the under side of the beam and extending upward in a direction inclined toward the center. This method of failure has the appearance of being due to a combination of shear and tension in the lower section of the beam, since the cracks are approximately at right angles to the theoretical " lines of direct tension." Such failures, however, are almost always accompanied by a slipping of the steel bar in the concrete, and may frequently be prevented by taking proper precautions against such slipping.
602. A more frequent cause of failure is a longitudinal shear in the plane near the steel reinforcement and on that side of it lying nearer the concave side of the beam.
It is evident that a failure caused by slipping of the bar in the beam, although caused primarily by shearing forces, is really a failure in adhesion, yet the two forms of weakness are so closely connected that it is simpler to consider them together.
In a steel plate girder the lower flange is considered to carry the tension, the upper flange the compression; the web connects the two flanges, causing them to act together as one beam, and we may think of the web as preventing the ends of the compression flange sliding beyond the ends of the tension flange. When the web is not able to accomplish this without buckling, it is stiffened by vertical angles.
In a concrete steel beam we have considered the entire tension to be carried by the steel reinforcement, and the entire compression to be carried by the concrete on the other side of the neutral axis. The connecting web is also concrete. This web is thick and not liable to buckle, but it may shear in a longitudinal plane as a wooden beam may do when short and deep. All of the tension in the steel reinforcement must be transmitted through the surrounding concrete. If there are no projections on the steel bar, the adhesion of the concrete to it may, under certain circumstances, be not strong enough to safely carry this stress; and if the adhesion is sufficient, then the shearing strength of the concrete may be too low to transmit the stress to contiguous fibers or layers.