Effect of transverse reinforcement spacing on fire resistance of high strength concrete columns

The confinement effect due to the congested transverse reinforcement is a very important feature for reinforced concrete columns subjected to accidental load conditions. The influence of transverse reinforcement spacings on load bearing capacity of high strength concrete (HSC) columns at ambient temperature has been the subject of many research projects, both experimental and theoretical. However, at high temperature the results of research into this subject are scarce and do not provide unambiguous evidence as to the effect of spacing of transverse reinforcement on fire load capacity and fire resistance. The first part of this paper presents an experimental study of the influence of transverse reinforcement spacing on fire resistance of axially loaded, HSC columns with circular cross-section. The results of full-scale tests indicate that columns with spacing of ties recommended by the code provisions for design of concrete structures could suffer a premature failure as a consequence of inelastic buckling of main reinforcing bars between adjacent ties. The next part of the paper concerns the supplementary numerical analysis of tested columns. The columns were modelled in axisymmetry with embedded reinforcement. The applied material model took into account the influence of transient temperature on mechanical properties of concrete and steel. The effect of cracking, development of transient creep strains and plastic strains for concrete were also included in the analysis. The inelastic buckling of main reinforcement was modelled using average stress–strain relationships for steel in compression. The comparison of numerical simulations and experiments shows reasonable agreement. The assessment of failure modes using the numerical simulation is also presented in the paper. The results of calculations indicate that during the whole heating period high thermal gradients generated tensile stresses in the plane of cross-section of the columns. Due to this fact the confinement effect was not observed for the columns with congested spacing of transverse reinforcement.