Fire Resistance Design of Cold-Formed Thin-Walled Steel Beams and Beam-Columns using Direct Strength Method

Abstract

The use of cold-formed, thin-walled construction continues to grow owing to their many advantages including high strength to weight ratio, flexibility of manufacture, fast and easy construction as well as low handling and transportation costs. However, the behaviour in fire is complex due to different buckling modes and non-uniform temperature distributions in the steel members. Furthermore, it is common for such members to be subjected to a combination of bending and compression. This thesis presents the results of a comprehensive research leading to the development of a design method using the Direct Strength Method (DSM). DSM was selected because it has advantages compared to the Effective Width Method in being simpler to implement, especially for sections with non-uniform temperature distributions, and being able to include interactions between different components of steel sections. The findings of this research are based on results of systematic and extensive numerical simulations, which cover different dimensions of thin-walled steel sections, different temperature distributions in steel cross-sections, different loading conditions and different steel grades, and with different failure modes. The main findings are that DSM is a suitable method for thin-walled steel members at uniform and non-uniform elevated temperatures. A thin-walled steel member under a combination of axial compression and bending can be converted into an equivalent beam. To calculate cross-sectional resistance at elevated temperatures, plastic analysis can be used. Using these conclusions, DSM equations have been derived for thin-walled steel members under global, local and distortional buckling for the following cases: - beams under transverse load on the top flange with uniform temperature distribution; - beams under transverse load on the top flange with non-uniform temperature distribution and hotter lower flange; - beam-columns under eccentric compression forces at ends under uniform temperature distribution; - beam-columns under eccentric compression forces under non-uniform temperature distribution. For non-uniform temperature distribution, the structural temperatures were obtained by Abaqus modelling for three different cases of interior insulation: no interior insulation, full interior insulation, or half interior insulation. The proposed DSM equations were then used to derive partial safety factors for structural resistance for conditional structural failure after flashover probabilities of 0.1, 0.01 and 0.001. The numerical simulation results were also used to assess the load ratio-limiting temperature method. It has been found the method is applicable to transversely loaded beams with uniform and non-uniform temperature distributions and beam-columns with uniform temperature distribution for all buckling modes, as well as beam-columns with non-uniform temperature distribution under global buckling mode. However, different equations based on steel grade should be used. In most cases, the average temperature of the cross-section should be used as the reference temperature. For beam-columns under local and distortional with non-uniform temperature distribution, a general load ratio-limiting temperature method is not suitable. However, the load ratio-limiting temperature method was found suitable by separating the beam-columns into two different categories with small and large eccentricities. Furthermore, EN 1993-1-3 and EN 1993-1-2 design methods were assessed for members at ambient and uniform elevated temperatures. The results show that the methods underestimate global buckling resistance, but both methods give local and distortional buckling resistances in close agreement with ABAQUS simulation results and DSM.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Meini Su (Supervisor) & Yong Wang (Supervisor)