Some Mooted Questions in Reinforced Concrete Design | Page 9

with real strength is wofully lacking in practical foundation.
Even the assumption of reinforcing value to the longitudinal steel rods
is not at all borne out in tests. Designers add enormously to the
calculated strength of concrete columns when they insert some
longitudinal rods. It appears to be the rule that real columns are
weakened by the very means which these designers invest with
reinforcing properties. Whether or not it is the rule, the mere fact that
many tests have shown these so-called reinforced concrete columns to
be weaker than similar plain concrete columns is amply sufficient to
condemn the practice of assuming strength which may not exist. Of all
parts of a building, the columns are the most vital. The failure of one
column will, in all probability, carry with it many others stronger than
itself, whereas a weak and failing slab or beam does not put an extra
load and shock on the neighboring parts of a structure.
In Bulletin No. 10 of the University of Illinois Experiment Station,[C] a
plain concrete column, 9 by 9 in. by 12 ft., stood an ultimate crushing
load of 2,004 lb. per sq. in. Column 2, identical in size, and having four
5/8-in. rods embedded in the concrete, stood 1,557 lb. per sq. in. So
much for longitudinal rods without hoops. This is not an isolated case,
but appears to be the rule; and yet, in reading the literature on the
subject, one would be led to believe that longitudinal steel rods in a
plain concrete column add greatly to the strength of the column.
A paper, by Mr. M.O. Withey, before the American Society for Testing
Materials, in 1909, gave the results of some tests on concrete-steel and

plain concrete columns. (The term, concrete-steel, is used because this
particular combination is not "reinforced" concrete.) One group of
columns, namely, _W1_ to _W3_, 10-1/2 in. in diameter, 102 in. long,
and circular in shape, stood an average ultimate load of 2,600 lb. per sq.
in. These columns were of plain concrete. Another group, namely,
_E1_ to _E3_, were octagonal in shape, with a short diameter (12 in.),
their length being 120 in. These columns contained nine longitudinal
rods, 5/8 in. in diameter, and 1/4-in. steel rings every foot. They stood
an ultimate load averaging 2,438 lb. per sq. in. This is less than the
column with no steel and with practically the same ratio of slenderness.
In some tests on columns made by the Department of Buildings, of
Minneapolis, Minn.[D], Test A was a 9 by 9-in. column, 9 ft. 6 in. long,
with ten longitudinal, round rods, 1/2 in. in diameter, and 1-1/2-in. by
3/16-in. circular bands (having two 1/2-in. rivets in the splice), spaced
4 in. apart, the circles being 7 in. in diameter. It carried an ultimate load
of 130,000 lb., which is much less than half "the compressive resistance
of a hooped member," worked out according to the authoritative
quotation before given. Another similar column stood a little more than
half that "compressive resistance." Five of the seventeen tests on the
concrete-steel columns, made at Minneapolis, stood less than the plain
concrete columns. So much for the longitudinal rods, and for hoops
which are not close enough to stiffen the rods; and yet, in reading the
literature on the subject, any one would be led to believe that
longitudinal rods and hoops add enormously to the strength of a
concrete column.
The sixteenth indictment against common practice is in reference to flat
slabs supported on four sides. Grashof's formula for flat plates has no
application to reinforced concrete slabs, because it is derived for a
material strong in all directions and equally stressed. The strength of
concrete in tension is almost nil, at least, it should be so considered.
Poisson's ratio, so prominent in Grashof's formula, has no meaning
whatever in steel reinforcement for a slab, because each rod must take
tension only; and instead of a material equally stressed in all directions,
there are generally sets of independent rods in only two directions. In a
solution of the problem given by a high English authority, the slab is

assumed to have a bending moment of equal intensity along its
diagonal. It is quite absurd to assume an intensity of bending clear into
the corner of a slab, and on the very support equal to that at its center.
A method published by the writer some years ago has not been
challenged. By this method strips are taken across the slab and the
moment in them is found, considering the limitations of the several
strips in deflection imposed by those running at right angles therewith.
This method shows (as tests demonstrate) that when the slab is oblong,
reinforcement in the long direction rapidly diminishes in usefulness.
When the ratio