Fire behavior of axially loaded slender high strength concrete - filled tubular columns

This paper describes sixteen fire tests conducted on slender circular hollow section columns filled with normal and high strength concrete, subjected to concentric axial loads. The test parameters were the nominal strength of concrete (30 and 80 MPa), the infilling type (plain concrete, reinforced concrete and steel fiber reinforced concrete) and the axial load level (20% and 40%). The columns were tested under fixed-pinned boundary conditions and the relative slenderness at room temperature was higher than 0.5 in all of the cases. A numerical model was validated against the tests, in order to extend the results and understand the failure mode of such columns. It is the aim of this paper to study the influence in a fire situation of the use of high strength concrete, as opposed to normal strength concrete. The results have shown that for slender columns subjected to high temperatures, the behavior of high strength concrete was different than for stub columns, spalling not being observed in the experiments. Furthermore, the addition of steel fibers was not found very advantageous in slender columns, since no increment in terms of fire resistance was obtained for the columns which used this type of reinforcement. However, the addition of reinforcing bars seems to be the solution in some cases, where the use of external fire protection wants to be avoided in the design of HSS structures, since the reinforcing bars allow the tube to resist a higher axial load.