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

Abstract

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. ). 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.

Collapse of concrete columns during and after the cooling phase of a fire

“…Although this assumption has proved to yield quite accurate results in structures heated by a standard time-temperature curve, its validity for representing the behaviour of concrete during a cooling phase may be questioned because transient creep is, in nature, not reversible. Recent numerical simulations have been made by the authors on the same columns as the one presented in this paper, now with an explicit transient creep constitutive model [15,16]; it appeared that the occurrence of collapse during or after the cooling phase is predicted even more often with an explicit creep model than with the implicit model of Eurocode 2 that has been used for the results presented in this paper.…”

A 2D beam element for the analysis of flexural buckling of steel structures at elevated temperatures

“…The material models given in Eurocode (EC2, 2004b) are adopted for steel and concrete at elevated temperature, including the compressive strength reduction as a function of the concrete class as plotted in Figure 1. Transient creep strain is considered explicitly in the concrete model (Gernay, 2012;Gernay and Franssen, 2012). The columns are subjected to a vertical load at the top and then to ISO fire on their four sides, the load being kept constant.…”

Fire Performance of Columns Made of Normal and High Strength Concrete: A Comparative Analysis

“…The material models given in Eurocode [4] are adopted for steel and concrete at elevated temperature. Yet in the concrete model, the transient creep strain is considered explicitly as this has proven to provide more accurate results [6][7][8]. The columns are subjected to a vertical load at the top and then to ISO fire on their four sides.…”