Some Mooted Questions in Reinforced Concrete Design | Page 2

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.
It is true that there is a shear or a diagonal tension in the beam, and the
diagonal portion of the rod is apparently in a position to take this
tension. This is just such a force as the truss-rod in a queen-post truss
must take. Is this reinforcing rod equipped to perform this office? The

beam is apt to fail in the line, A B. In fact, it is apt to crack from
shrinkage on this or almost any other line, and to leave the strength
dependent on the reinforcing steel. Suppose such a crack should occur.
The entire strength of the beam would be dependent on the grip of the
short end of Rod 3 to the right of the line, A B. The grip of this short
piece of rod is so small and precarious, considering the important duty
it has to perform, that it is astounding that designers, having any care
for the permanence of their structures, should consider for an instant
such features of design, much less incorporate them in a building in
which life and property depend on them.
The third point to which attention is called, is the feature of design just
mentioned in connection with the bent-up rod. It concerns the
anchorage of rods by the embedment of a few inches of their length in
concrete. This most flagrant violation of common sense has its most
conspicuous example in large engineering works, where of all places
better judgment should prevail. Many retaining walls have been built,
and described in engineering journals, in papers before engineering
societies of the highest order, and in books enjoying the greatest
reputation, which have, as an essential feature, a great number of rods
which cannot possibly develop their strength, and might as well be of
much smaller dimensions. These rods are the vertical and horizontal
rods in the counterfort of the retaining wall shown at a, in Fig. 2. This
retaining wall consists of a front curtain wall and a horizontal slab
joined at intervals by ribs or counterforts. The manifest and only
function of the rib or counterfort is to tie together the curtain wall and
the horizontal slab. That it is or should be of concrete is because the
steel rods which it contains, need protection. It is clear that failure of
the retaining wall could occur by rupture through the Section A B, or
through B C. It is also clear that, apart from the cracking of the concrete
of the rib, the only thing which would produce this rupture is the
pulling out of the short ends of these reinforcing rods. Writers treat the
triangle, A B C, as a beam, but there is absolutely no analogy between
this triangle and a beam. Designers seem to think that these rods take
the place of so-called shear rods