Slab deflection and subsidence of column supports in a floor test of International Hall, Chicago, made September, 1913
✍ Scribed by Henry T. Eddy
- Publisher
- Elsevier Science
- Year
- 1916
- Tongue
- English
- Weight
- 446 KB
- Volume
- 182
- Category
- Article
- ISSN
- 0016-0032
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✦ Synopsis
REINFORCED concrete floor slabs are somewhat imperfectly elastic, and in testing them an allowance is commonly made for this fact by the requirement that in case of any excessive deflection at least 80 per cent. of it shall disappear within a week after the removal of the load which produced the deflection. This requirement recognizes the imperfection of the elastic properties of the slab in two ways, since first, it does not forbid some residual permanent deflection, and, further, it does not require immediate recovery, neither of which concessions could be made in case of a perfectly elastic structure.
Few materials of construction are so perfectly elastic as actually to make an immediate and complete recovery, but a much larger margin is allowed to reinforced concrete than to most other materials. It should be noticed that on account of its being a composite structure, composed partly of steel and partly of concrete, of which the steel is more nearly perfectly elastic than the concrete, a floor slab with steel massed in the column heads will have a more prompt and complete recovery than one with so-called drop heads of concrete in place of the heavy reinforcement in the column heads.
But in the retardation or time lag of slab recovery a phenomenon is exhibited which is not found to any perceptible extent in other kinds of materials of Construction in ordinary use. Since recovery from slab deflection is gradual and may never be complete, and since a corresponding gradual increase of deflection must occur under a load, it has been argued by some that final stability under a load is impossible. The permanence, however, of ancient concrete structures seems to show that there must be a limit to the deformations and deflections that will take place --* Communicated bythe Author.