Time-dependent behaviour of concrete-filled steel tubular columns: analytical and comparative study

Concrete-filled steel tubes (CFSTs) have become popular in both building and bridge applications over recent decades, and an accurate prediction of their time-dependent response is necessary. Different models are currently available to describe the time-dependent behaviour of concrete and these have been shown to lead to very different predictions. In the particular case of CFST members, there is no recommended method for service calculations for routine design. The particularity of the long-term behaviour of CFSTs relies on the fact that no moisture exchange occurs between the core concrete and the surrounding environment due to the presence of the steel section. In this context, this paper recommends a suitable concrete model using 81 tests for benchmarking purposes. An extensive parametric study is carried out to evaluate the influence of time effects for geometric cross-sections commonly used in real bridge applications. Finally, different algebraic methods suitable for design calculations are applied to the long-term modelling of CFST members and their accuracy is investigated based on the results obtained using the step-by-step procedure. The results indicate that, by considering a nil exposed perimeter, Eurocode 2 performs well in predicting the long-term behaviour of CFST members. The creep effect has a considerable influence on the static response of CFST members even when actions are applied at a very great concrete age, while shrinkage in the core concrete has a quite limited contribution to the overall deformation if the core concrete is of normal strength. The mean stress method is recommended for simple design calculations.