Besides being false theoretically, this method of investing phantom columns 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