Creep modeling for concrete-filled steel tubes

Using the rate of flow method and the double power law function for basic creep of concrete, an algorithm is developed for the time-dependent behavior of concrete-filled steel tube (CFT), with or without the interface bond. The model adheres to geometric compatibility and static equilibrium, and considers the effects of sealed concrete, multi-axial state of stresses, creep Poisson's ratio, stress redistribution, variable creep stress history, and creep failure of the column. The model is verified against previous creep tests for bonded and unbonded specimens. A study is then carried out on the practical design parameters that may affect creep of CFT columns under service loads, or lead to their creep rupture at high levels of sustained load. The study indicates that creep of CFT columns should be considered in the design, however, with creep coefficients much lower than those prescribed in the current ACI. Creep of CFT is shown to be a function of concrete mix, column geometry, and interface bond. Therefore, a single ultimate creep coefficient cannot be used for all concrete mixes, column geometries, and construction types. Bonded tubes curtail creep of concrete much more than the equivalent unbonded ones, mainly because of the stress relaxation phenomenon, which is more pronounced for smaller diameter-to-thickness ratios. For diameter-to-thickness ratios of 40 or less, bonded tubes are more durable in creep rupture than the equivalent unbonded ones. Creep rupture life of 75 years is quite feasible in bonded CFT, with diameter-to-thickness ratio of 40 or less, for sustained loads as high as 65% of the static capacity of the column. (C) 2003 Elsevier Ltd. All rights reserved.