Structural behaviour and design of cold-formed steel hollow section columns under simulated fire conditions

This research study was undertaken to study the structural performance of high-strength cold-formed steel square and rectangular hollow section (SHS and RHS) columns under fire conditions, and to establish suitable methods to determine their FRR to verify the adequacy of the sections for use as columns.

Firstly, an experimental investigation was conducted to study the level of deterioration of mechanical properties and the stress-strain characteristics of steel in high-strength cold-formed steel hollow sections. The local and global buckling behaviour and strength of high-strength cold-formed steel hollow sections were the investigated by conducting compression tests of grade 450 SHS stub and slender columns at elevated temperatures up to 700. Finite element models of these tested columns were developed by using the measured ambient temperature mechanical properties and the proposed elevated temperature mechanical property predictive equations, along with the developed stress-strain models. These finite element models were validated using the stub and slender column test results. Thereafter, parametric studies were carried out to study the effects of various parameters on the section and member capacities of SHS and RHS columns subject to local and global buckling effects at ambient and elevated temperatures.

The FEA predictions were compared with the currently available design rules, and suitable modifications were proposed for design rules that did not produce safe predictions. An innovative method to predict the critical temperatures for SHS and RHS columns of varying slenderness was developed as a function of the load ratio in fire conditions. The developed method enables accurate FRR predictions of both protected and unprotected high-strength cold-formed steel SHS and RHS columns of various dimensions and lengths, when used with suitable time-temperature profiles of the columns. Structural and fire engineers can use the design methods developed in this research to decide where bare steel columns can be used without any fire protection, but also design the required fire protection.