Some Mooted Questions in Reinforced Concrete Design | Page 3

in a beam, and that the inclined rods
are equivalent to the rods in a tension flange of a beam. It is hard to
understand by what process of reasoning such results can be attained.

Any clear analysis leading to these conclusions would certainly be a
valuable contribution to the literature on the subject. It is scarcely
possible, however, that such analysis will be brought forward, for it is
the apparent policy of the reinforced concrete analyst to jump into the
middle of his proposition without the encumbrance of a premise.
There is positively no evading the fact that this wall could fail, as stated,
by rupture along either A B or B C. It can be stated just as positively
that a set of rods running from the front wall to the horizontal slab, and
anchored into each in such a manner as would be adopted were these
slabs suspended on the rods, is the only rational and the only efficient
design possible. This design is illustrated at b in Fig. 2.
[Illustration: FIG. 2.]
The fourth point concerns shear in steel rods embedded in concrete. For
decades, specifications for steel bridges have gravely given a unit shear
to be allowed on bridge pins, and every bridge engineer knows or ought
to know that, if a bridge pin is properly proportioned for bending and
bearing, there is no possibility of its being weak from shear. The
centers of bearings cannot be brought close enough together to reduce
the size of the pin to where its shear need be considered, because of the
width required for bearing on the parts. Concrete is about one-thirtieth
as strong as steel in bearing. There is, therefore, somewhat less than
one-thirtieth of a reason for specifying any shear on steel rods
embedded in concrete.
The gravity of the situation is not so much the serious manner in which
this unit of shear in steel is written in specifications and building codes
for reinforced concrete work (it does not mean anything in
specifications for steelwork, because it is ignored), but it is apparent
when designers soberly use these absurd units, and proportion shear
rods accordingly.
Many designers actually proportion shear rods for shear, shear in the
steel at units of 10,000 or 12,000 lb. per sq. in.; and the blame for this
dangerous practice can be laid directly to the literature on reinforced
concrete. Shear rods are given as standard features in the design of

reinforced concrete beams. In the Joint Report of the Committee of the
various engineering societies, a method for proportioning shear
members is given. The stress, or shear per shear member, is the
longitudinal shear which would occur in the space from member to
member. No hint is given as to whether these bars are in shear or
tension; in fact, either would be absurd and impossible without greatly
overstressing some other part. This is just a sample of the state of the
literature on this important subject. Shear bars will be taken up more
fully in subsequent paragraphs.
The fifth point concerns vertical stirrups in a beam. These stirrups are
conspicuous features in the designs of reinforcing concrete beams.
Explanations of how they act are conspicuous in the literature on
reinforced concrete by its total absence. By stirrups are meant the
so-called shear rods strung along a reinforcing rod. They are usually
U-shaped and looped around the rod.
It is a common practice to count these stirrups in the shear, taking the
horizontal shear in a beam. In a plate girder, the rivets connecting the
flange to the web take the horizontal shear or the increment to the
flange stress. Compare two 3/4-in. rivets tightly driven into holes in a
steel angle, with a loose vertical rod, 3/4 in. in diameter, looped around
a reinforcing rod in a concrete beam, and a correct comparison of
methods of design in steel and reinforced concrete, as they are
commonly practiced, is obtained.
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