Axial mode i cracking in core regions of compressed reinforced concrete columns subjected to fire loading

Reinforced concrete columns must withstand extreme scenarios like fires. Understanding their structural behavior during such events is of great interest to structural engineers. The present study is focused on a cylindrical reinforced concrete column subjected to a moderate fire. The anal- ysis combines a Fourier series solution for radial heat ingress into the column and thermo-elastic Bernoulli-Euler beam theory. A temperature history known to be relevant for fire accidents is im- posed as boundary condition at the lateral surface of the column. The resulting radial symmetric temperature field is translated into a field of thermal eigenstrains. The latter is decomposed, in every cross-section, into two portions: a spatially uniform portion representing the eigenstretch of the axis of the column, and a spatially nonlinear portion representing the eigenwarping of the cross-sections. The eigenstretch is constrained by the support conditions at the structural scale (“axial elongation constraint”). The eigenwarping is hindered, because the the cross-sections remain plane also in the deformed configuration. This activates hindered-warping-induced cross-sectional stresses which are self-balanced and, therefore, do not contribute to the normal force. Total thermal stresses are ob- tained from adding the hindered-warping-induced stresses to the normal-force-related stresses. The influence of the axial elongation constraint on the structural behavior is discussed in the context of a sensitivity analysis. Thereby, the compliance of a spring placed on top of the column is varied. The simulations are first performed under consideration of linear-elastic material behavior of steel and concrete. Subsequently, the analysis is extended towards consideration of elasto-brittle material behavior of concrete. It is concluded that the axial elongation constraint significantly influences the structural behavior and the mechanism of damage of the fire-loaded column. For small values of the constraint, concrete will crack in the core of the cross-section, due to tensile stresses reaching the tensile strength. For large values of the constraint, compressive stresses increase particularly close to the heated surface. This may lead to spalling provided that the stresses reach the compressive strength of concrete. 1 ´ Maximilian Sorgner, Rodrigo Dıaz Flores, Hui Wang, Christian Hellmich and Bernhard L.A.