Some Mooted Questions in Reinforced Concrete Design | Page 6

a very much larger moment. Who ever heard of a failure
because of continuous beam action in the stringers of a bridge? Why
cannot reinforced concrete engineering be placed on the same sound
footing as structural steel engineering?
The eighth point concerns the spacing of rods in a reinforced concrete
beam. It is common to see rods bunched in the bottom of such a beam
with no regard whatever for the ability of the concrete to grip the steel,
or to carry the horizontal shear incident to their stress, to the upper part
of the beam. As an illustration of the logic and analysis applied in
discussing the subject of reinforced concrete, one well-known authority,
on the premise that the unit of adhesion to rod and of shear are equal,
derives a rule for the spacing of rods. His reasoning is so false, and his
rule is so far from being correct, that two-thirds would have to be added
to the width of beam in order to make it correct. An error of 66% may
seem trifling to some minds, where reinforced concrete is considered,
but errors of one-tenth this amount in steel design would be cause for
serious concern. It is reasoning of the most elementary kind, which
shows that if shear and adhesion are equal, the width of a reinforced
concrete beam should be equal to the sum of the peripheries of all
reinforcing rods gripped by the concrete. The width of the beam is the
measure of the shearing area above the rods, taking the horizontal shear
to the top of the beam, and the peripheries of the rods are the measure
of the gripping or adhesion area.
Analysis which examines a beam to determine whether or not there is
sufficient concrete to grip the steel and to carry the shear, is about at the
vanishing point in nearly all books on the subject. Such misleading
analysis as that just cited is worse than nothing.
The ninth point concerns the T-beam. Excessively elaborate formulas
are worked out for the T-beam, and haphazard guesses are made as to
how much of the floor slab may be considered in the compression
flange. If a fraction of this mental energy were directed toward a logical
analysis of the shear and gripping value of the stem of the T-beam, it
would be found that, when the stem is given its proper width, little, if

any, of the floor slab will have to be counted in the compression flange,
for the width of concrete which will grip the rods properly will take the
compression incident to their stress.
The tenth point concerns elaborate theories and formulas for beams and
slabs. Formulas are commonly given with 25 or 30 constants and
variables to be estimated and guessed at, and are based on assumptions
which are inaccurate and untrue. One of these assumptions is that the
concrete is initially unstressed. This is quite out of reason, for the
shrinkage of the concrete on hardening puts stress in both concrete and
steel. One of the coefficients of the formulas is that of the elasticity of
the concrete. No more variable property of concrete is known than its
coefficient of elasticity, which may vary from 1,000,000 to 5,000,000
or 6,000,000; it varies with the intensity of stress, with the kind of
aggregate used, with the amount of water used in mixing, and with the
atmospheric condition during setting. The unknown coefficient of
elasticity of concrete and the non-existent condition of no initial stress,
vitiate entirely formulas supported by these two props.
Here again destructive criticism would be vicious if these mathematical
gymnasts were giving the best or only solution which present
knowledge could produce, or if the critic did not point out a substitute.
The substitute is so simple of application, in such agreement with
experiments, and so logical in its derivation, that it is surprising that it
has not been generally adopted. The neutral axis of reinforced concrete
beams under safe loads is near the middle of the depth of the beams. If,
in all cases, it be taken at the middle of the depth of the concrete beam,
and if variation of intensity of stress in the concrete be taken as uniform
from this neutral axis up, the formula for the resisting moment of a
reinforced concrete beam becomes extremely simple and no more
complex than that for a rectangular wooden beam.
The eleventh point concerns complex formulas for chimneys. It is a
simple matter to find the tensile stress in that part of a plain concrete
chimney between two radii on the windward side. If in this space there
is inserted a rod which is capable of taking that tension at a proper unit,
the safety of the chimney is assured, as far as