Fire Design of Concrete Filled Steel Tube Continuous Column Subjected to Concentric or Eccentric Axial Loading

Abstract

Design recommendations for concrete filled steel tubular (CFST) columns in fire are based on the results of experimental standard fire testing of CFST members where the same temperature is applied to the column over the full column height. However, this is not representative of a CFST column in a typical building especially in seismically active countries, which is continuous between floors and which, in fire, is subjected to severe fire conditions on one floor at a time while the floors above and below remain cooler. In the experimental tests described in this thesis, the columns are of 3.2 m height with the fire applied only to the central 2 m. The tests covered three different types of infill; plain concrete, bar reinforced concrete and steel fibre reinforced concrete, with the columns subjected to compressive load levels ranging from 0.33 to 0.41. End restraint conditions of fixed-fixed (F-F) and pinned-fixed (P-F)) are considered at the ends of the 3.2m length of column . The results show that the significant initial longitudinal expansion of the steel tube relative to the concrete reported by many researchers did not occur, due to the restraining effect of the unheated column between the furnace and the end supports (bottom 500 mm and top 700 mm). The use of steel fibre reinforced concrete or bar reinforced concrete increases the axial capacity of the column when loaded concentrically or eccentrically when compared to steel tubes filled with only plain concrete. This occurs due to an increase in the tensile strength of the concrete cores by the addition of steel reinforcement (bar reinforcement or steel fibre) to the concrete core which reduces the spalling of the concrete core at elevated temperatures. A design procedure to calculate the axial capacity of a CFST column in fire is presented and validated against over 238 laboratory experiments carried out over the last 36 years on CFST columns in fire by various researchers including research carried out by the author. The proposed design procedure is shown to be more accurate when compared to other design equations presented in the literature. More importantly, the proposed design procedure includes the design of the axial capacity of steel tube columns filled with steel fibre reinforced concrete, which has been covered by only a few researchers and which is becoming increasingly popular. The proposed design procedure has been used for the design of Auckland International Airport Phase 4 Pier B Extension concrete filled columns and brief details of which are presented in section 8 of this thesis.