JOHN C. OSTRUP, M. AM.  SOC.  C. E.—­The author has rendered a great service to the Profession in presenting this paper.  In his first point he mentions two designs of reinforced concrete beams and, inferentially, he condemns a third design to which the speaker will refer later.  The designs mentioned are, first, that of a reinforced concrete beam arranged in the shape of a rod, with separate concrete blocks placed on top of it without being connected—­such a beam has its strength only in the rod.  It is purely a suspension, or “hog-chain” affair, and the blocks serve no purpose, but simply increase the load on the rod and its stresses.

The author’s second design is an invention of his own, which the Profession at large is invited to adopt.  This is really the same system as the first, except that the blocks are continuous and, presumably, fixed at the ends.  When they are so fixed, the concrete will take compressive stresses and a certain portion of the shear, the remaining shear being transmitted to the rod from the concrete above it, but only through friction.  Now, the frictional resistance between a steel rod and a concrete beam is not such as should be depended on in modern engineering designs.

The third method is that which is used by nearly all competent designers, and it seems to the speaker that, in condemning the general practice of current reinforced designs in sixteen points, the author could have saved himself some time and labor by condemning them all in one point.

What appears to be the underlying principle of reinforced concrete design is the adhesion, or bond, between the steel and the concrete, and it is that which tends to make the two materials act in unison.  This is a point which has not been touched on sufficiently, and one which it was expected that Mr. Beyer would have brought out, when he illustrated certain internal static conditions.  This principle, in the main, will cover the author’s fifth point, wherein stirrups are mentioned, and again in the first point, wherein he asks:  “Will some advocate of this type of design please state where this area can be found?”

To understand clearly how concrete acts in conjunction with steel, it is necessary to analyze the following question:  When a steel rod is embedded in a solid block of concrete, and that rod is put in tension, what will be the stresses in the rod and the surrounding concrete?

The answer will be illustrated by reference to Fig. 10.  It must be understood that the unit stresses should be selected so that both the concrete and the steel may be stressed in the same relative ratio.  Assuming the tensile stress in the steel to be 16,000 lb. per sq. in., and the bonding value 80 lb., a simple formula will show that the length of embedment, or that part of the rod which will act, must be equal to 50 diameters of the rod.

[Illustration:  FIG. 10.]