The first point to which attention is called, is illustrated in Fig. 1.  It concerns sharp bends in reinforcing rods in concrete.  Fig. 1 shows a reinforced concrete design, one held out, in nearly all books on the subject, as a model.  The reinforcing rod is bent up at a sharp angle, and then may or may not be bent again and run parallel with the top of the beam.  At the bend is a condition which resembles that of a hog-chain or truss-rod around a queen-post.  The reinforcing rod is the hog-chain or the truss-rod.  Where is the queen-post?  Suppose this rod has a section of 1 sq. in. and an inclination of 60 deg. with the horizontal, and that its unit stress is 16,000 lb. per sq. in.  The forces, a and b, are then 16,000 lb.  The force, c, must be also 16000 lb.  What is to take this force, c, of 16,000 lb.?  There is nothing but concrete.  At 500 lb. per sq. in., this force would require an area of 32 sq. in.  Will some advocate of this type of design please state where this area can be found?  It must, of necessity, be in contact with the rod, and, for structural reasons, because of the lack of stiffness in the rod, it would have to be close to the point of bend.  If analogy to the queen-post fails so completely, because of the almost complete absence of the post, why should not this borrowed garment be discarded?

If this same rod be given a gentle curve of a radius twenty or thirty times the diameter of the rod, the side unit pressure will be from one-twentieth to one-thirtieth of the unit stress on the steel.  This being the case, and being a simple principle of mechanics which ought to be thoroughly understood, it is astounding that engineers should perpetrate the gross error of making a sharp bend in a reinforcing rod under stress.

The second point to which attention is called may also be illustrated by Fig. 1.  The rod marked 3 is also like the truss-rod of a queen-post truss in appearance, because it ends over the support and has the same shape.  But the analogy ends with appearance, for the function of a truss-rod in a queen-post truss is not performed by such a reinforcing rod in concrete, for other reasons than the absence of a post.  The truss-rod receives its stress by a suitable connection at the end of the rod and over the support of the beam.  The reinforcing rod, in this standard beam, ends abruptly at the very point where it is due to receive an important element of strength, an element which would add enormously to the strength and safety of many a beam, if it could be introduced.

Of course a reinforcing rod in a concrete beam receives its stress by increments imparted by the grip of the concrete; but these increments can only be imparted where the tendency of the concrete is to stretch.  This tendency is greatest near the bottom of the beam, and when the rod is bent up to the top of the beam, it is taken out of the region where the concrete has the greatest tendency to stretch.  The function of this rod, as reinforcement of the bottom flange of the beam, is interfered with by bending it up in this manner, as the beam is left without bottom-flange reinforcement, as far as that rod is concerned, from the point of bend to the support.