These stirrups can take but little hold on the reinforcing rods—­and this must be through the medium of the concrete—­and they can take but little shear.  Some writers, however, hold the opinion that the stirrups are in tension and not in shear, and some are bold enough to compare them with the vertical tension members of a Howe truss.  Imagine a Howe truss with the vertical tension members looped around the bottom chord and run up to the top chord without any connection, or hooked over the top chord; then compare such a truss with one in which the end of the rod is upset and receives a nut and large washer bearing solidly against the chord.  This gives a comparison of methods of design in wood and reinforced concrete, as they are commonly practiced.

Anchorage or grip in the concrete is all that can be counted on, in any event, to take up the tension of these stirrups, but it requires an embedment of from 30 to 50 diameters of a rod to develop its full strength.  Take 30 to 50 diameters from the floating end of these shear members, and, in some cases, nothing or less than nothing will be left.  In any case the point at which the shear member, or stirrup, is good for its full value, is far short of the centroid of compression of the beam, where it should be; in most cases it will be nearer the bottom of the beam.  In a Howe truss, the vertical tension members having their end connections near the bottom chord, would be equivalent to these shear members.

The sixth point concerns the division of stress into shear members.  Briefly stated, the common method is to assume each shear member as taking the horizontal shear occurring in the space from member to member.  As already stated, this is absurd.  If stirrups could take shear, this method would give the shear per stirrup, but even advocates of this method acknowledge that they can not.  To apply the common analogy of a truss:  each shear member would represent a tension web member in the truss, and each would have to take all the shear occurring in a section through it.

If, for example, shear members were spaced half the depth of a beam apart, each would take half the shear by the common method.  If shear members take vertical shear, or if they take tension, what is between the two members to take the other half of the shear?  There is nothing in the beam but concrete and the tension rod between the two shear members.  If the concrete can take the shear, why use steel members?  It is not conceivable that an engineer should seriously consider a tension rod in a reinforced concrete beam as carrying the shear from stirrup to stirrup.