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Lateral Confinement Needed to Suppress Softening of Concrete in Compression

Suppression of softening in the load-deflection diagram of concrete-filled tubular columns and spiral columns is proposed to serve as a design criterion helping to avoid the size effect and explosive brittle character of collapse. To this end, the recently developed “tube-squash” tests, in which a short concrete-filled steel tube is squashed to about a half of its original length and allowed to bulge, are conducted with tubes of different wall thicknesses. A finite-strain element computer code with a microplane constitutive model is used to simulate the tests. After its verification and calibration by tests, the code is used to analyze nonbuckling concrete-filled tubular columns and spirally reinforced columns. It is found that softening in the load-deflection diagram can be fully suppressed only if the reinforcement ratio (ratio of the tube volume or spiral volume to the total volume of column) exceeds about 14%. If mild softening is allowed, the reinforcement ratio must still exceed about 8%. These ratios are surprisingly high. If they are not used in design, one needs to pay attention to the localization of softening damage, accept the (deterministic) size effect engendered by it, and ensure safety margins appropriate for protecting against sudden explosive brittle collapse. This is of particular concern for the design of very large columns.