Effect of Transient Creep Strain Model on the Behavior of Concrete Columns Subjected to Heating and Cooling

In the numerical analysis of structures in fire, the material models that are
used have important implications on the global behavior of the structure. In concrete,
a particular phenomenon appears when subjected to high temperatures: the transient
creep strain. Models integrating explicitly a term for transient creep strain have been
proposed in the literature but, in the current Eurocode 2 model, there is no explicit
term for transient creep strain. This phenomenon is included in the Eurocode 2
model, but it is implicitly considered in the mechanical strain term. A series of experimental
fire tests on axially restrained concrete columns subjected to heating and cooling
has been recently performed at South China University of Technology and
described by Wu et al. (Fire Technol 46:231–249). In the original paper, it was shown
that using the implicit model of Eurocode 2, the behavior of the columns cannot be
simulated properly, especially during the cooling phase. The objective of the present
paper is to perform again the fire tests simulations using a new formulation of the
Eurocode 2 model that contains an explicit term for transient creep. In the first part
of the paper, the explicit formulation of the Eurocode 2 model is presented. In the
second part, the fire tests are modeled with the software SAFIR using, on the one
hand, the implicit Eurocode model and, on the other hand, the new explicit model. It
is shown that the transient creep model has significant implications on the global
behavior of structural concrete members, as the residual axial load sustained by the
columns at the end of the fire can differ by up to 25% of the initial applied load
depending on the transient creep strain model that is used for the calculation. The
experimental behavior is better matched with the new explicit model than with the
current Eurocode model. Particularly, the results given by the Eurocode model during
the cooling phase are unconservative as the residual axial load is overestimated.
Finally, it is explained why, on the basis of an example, in a performance-based
approach, these results can have important implications on the global fire resistance
of a structure.