Concrete by Edward Godfrey

Author:Edward Godfrey
Language: eng
Format: epub
Publisher: Author
Published: 1908-03-25T05:00:00+00:00

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Fig. 1.

Tests have shown that rods of a greater diameter than about 1-200 of the span are apt to pull out of the concrete without breaking, on account of the lack of length in con-crete to grip the metal effectually. Other tests have shown that beams having a greater depth than one-tenth of the span and containing horizontal rods at the bottom, or having rods turned up at the support and not anchored at the ends, will fail when loaded to destruction, approximately as per sketch Fig. i. This demonstrates the. inability of the concrete in beams so designed to carry the end shear, vertical and horizontal (which combine in .the diagonal line in the figure), when the depth exceeds, one-tenth of the span.

On the location of the neutral axis, theoretical methods of determining its position by the relative moduli of elasticity of steel and concrete are unsatisfactory, on account of the wide variation in the modulus of elasticity of concrete as determined by compression tests. There is* however, a more direct method of locating this axis, and one which gives more concordant results, namely, by measuring the relative extension and shortening of the lower and upper fibres in a beam under test. In a letter replying to a critic on this phase of the subject the writer gave, the

following defense of his position, whkh is here repeated to make this paper complete for reference:

"In Engineering News, July-December, 1904, pp. 124-125, Prof. Talbot gives plottings of the position of the neutral axis. For 1.39 per cent reinforcement the distance from top of beam is almost exactly 50 per cent of the depth from top of concrete to steel. For 0.97 per cent it is about 0.42 of the same depth. Prof. Talbot gives this formula for finding the position: k = 0.26 -f 0.18 p, where k is the fraction of the depth from top to neutral axis and p is the percentage of steel. For 1% per cent this would be 0.485 of the depth from top of concrete to steel. Prof. F. E. Ttirneaure in Engineering News ? July-December, 1904, p. 215, says: The diagrams show the neutral axis to lie at first very near the center of the concrete beam. As the cracks develop it moves gradually nearer to the compression side.'"

On the proper percentage of steel the writer's paper was also criticised by the person to whom the reply in the last paragraph was directed. This critic stated that 1% per cent is too large a percentage of steel. This criticism is correlative with the one on the location of the neutral axis. A higher location of that axis means a less percentage of steel, since it affords a larger lever arm or effective depth in the resisting moment. The writer's reply to this criticism was as follows: "As to 1% per cent being too high, I quote from Prof. A. N. Talbot, in Engineering News, July-December, 1904, page 125: Tor 1:3:6 concrete, reinforcement as high as i 1 /* per cent for steel of 33,000 lbs.

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