In extended experiments on what he has called " hooped concrete," M. Considere 2 has shown that reinforcement is much more important and beneficial in a transverse or circumferential direction than if longitudinal. This may be accounted for by the fact that the natural method of failure of concrete prisms, is by splitting along planes parallel to the direction of pressure, and the ordinary method of failure by shear along inclined surfaces is induced by the friction of the plates transmitting the pressure to the prism. It was also shown that while concrete reinforced by longitudinal bars with the ordinary amount of lateral ties breaks suddenly, hooped concrete fails gradually under a much heavier load.
1 Trans. A. S. C. E., Vol. 1, p. 487.
2 Comptes Rendus de I'Academie des Sciences, 1898-1902. Translation, "Reinforced Concrete," by Armand Considere, translated by Leon S. Mois-seiff, McGraw Publishing Co., New York.
622. M. Considere concluded from his experiments that the circumferential ties should not be farther apart than one-seventh to one-tenth the diameter of the column, even when longitudinals were used to assist in completing the network, and that the results were more successful the nearer together the hoops or ties were placed. He found that spirals were better than individual single ties and that longitudinals were of value chiefly in assisting to confine the concrete, transmitting the bursting pressure at a given plane to the contiguous spirals above and below.
623. M. Considere says1 that the " compressive resistance of a hooped member exceeds the sum of the following three elements : —
"1. Compressive resistance of the concrete without reinforcing.
"2. Compressive resistance of the longitudinal rods stressed to their elastic limit.
"3. Compressive resistance which could have been produced by imaginary longitudinals at the elastic limit of the hooping metal, the volume of the imaginary longitudinals being taken as 2.4 times that of the hooping".
To subject hooped concrete to a practical test, M. Considere constructed, in 1903, a truss bridge of sixty-five foot span with parabolic top chord of seven and one-half feet rise,2 the compression members being of hooped concrete, and the tension members of concrete-steel with longitudinal reinforcement, or concrete protected steel. A central panel of the truss was constructed with a reduced section of top chord about eight inches diameter reinforced by eight longitudinal bars .43 inch in diameter and a helix 6 1/4 inches in diameter of .43 inch metal coiled to a pitch of about one inch. This reduced top chord section showed signs of failure when the computed stress reached about 5,000 pounds per square inch.
1 "Reinforced Concrete," p. 159.
2 Engineering News, May 5, 1904.