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 finite 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. DOI: 10.1061/~ASCE!0733-9399~2002!128:12~1304! CE Database keywords: Compression; Concrete; Softening; Collapse.