Structural Behavior of Concrete Columns under Natural Fires including Cooling Down Phase

This study deals with the structural behavior of concrete columns subjected to
natural fire, with special focus on the cooling down phase. As the reliability of the numerical
simulations largely depends on the validity of the constitutive models that are used, this issue
is addressed in details in the first part of the paper. It is found that the current concrete model
of Eurocode may not be adapted for representing the behavior of concrete during a cooling
phase because the effects of transient creep are incorporated implicitly in the mechanical
strain term. A formulation of the Eurocode concrete model that contains an explicit term for
transient creep strain has been developed and, after implementation in the software SAFIR, it
was used for the numerical simulations presented in this paper. In its second part, the study
gives an insight into the possibility of collapse of reinforced concrete columns during and
after the cooling phase of the natural fire. It is found that failure during the cooling phase of a
fire is a possible event and, more dangerous, that a failure of the concrete columns is still
possible when the fire has been completely extinguished. The main mechanisms that lead to
this type of failure are found to be the delayed increase of temperature in the central zones of
the element and the additional loss of concrete strength during the cooling phase of the fire. A
parametric study shows that the most critical situations with respect to delayed failure arise
for short fires and for columns with low slenderness or massive sections. The numerical
analyses have been performed also with the concrete model of Eurocode and the results also
show a risk of delayed collapse. Yet, the occurrence of collapse during or after the cooling
phase is predicted more often with the explicit creep model than with the implicit model of
Eurocode, which suggests that the way the transient creep effects are handled in the Eurocode
model may lead to unsafe results when considering the cooling phase of a fire.