Page:Popular Science Monthly Volume 23.djvu/633

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FIRE-PROOF BUILDING CONSTRUCTION.

The proportions of cement and coarse beach-sand and gravel, used in re-enforcing iron beams for floors and roof-supports, were one part of cement to two parts of sand and gravel. The size of the iron beam, selected for an experimental test, was a four-inch I-beam of lightest pattern, twelve feet long, weighing thirty pounds to the yard, and its safety load was limited to eleven hundred and fifty pounds. A plank mold was made the length of the iron beam, twelve inches deep by five inches wide, in the bottom of which a layer of béton was first moderately tamped down to an inch in thickness; then the iron beam was laid on the course at equal distances from each side of the mold, and settled down on the surface of the course of béton to a good bearing. This brought the top surface of the beam seven inches below the top of the mold. The work of filling and tamping the courses was then continued until the mold was filled.

The reason for placing the iron beam so near the bottom of the mold was to utilize its tensile quality for resisting the strain below the neutral axis when this composite beam was exposed to heavy loads, while the béton above this line was relied on for resisting compression from load-strain. The béton became thoroughly hardened in about thirty days, when the following tests of transverse strength were made: It was placed upon suitable supports, with a bearing of three inches at each end. A lever was adjusted so as to bring the testing-load on a knife-edge bearing at the center of the beam. Weight was then applied to the long end of the lever, until the stress on the center of the beam reached nine thousand five hundred pounds. Under this load there was a deflection at the center of the beam of seven sixteenths of an inch, but not a sign of rupture appeared at any point.

The load was then removed, and the beam returned to the original line it occupied before the test, showing that the combination possesses the essential quality of elasticity in addition to the enormous increase of capacity to resist strain over that which was possible for either material to sustain if used separately, and in the same quantity.

It is suggested that for future construction an inverted ⊥-beam would furnish a more preferable distribution of iron in the composite beams than the I-beams which were used.

The result of this experiment demonstrated the reliability of the composite beam of iron and béton, and showed that the adhesion of the cement to the iron could be depended on under heavy strains. This warranted the adoption of béton, re-enforced with small rods, for the floors and roofs.

The beams for supporting the floors throughout the house were placed at such convenient distances apart as to insure perfect safety to the floors, and at the same time afford ample opportunities for producing the best effects in deep, paneled ceilings.

All the beams were molded in the positions where they belonged,