Evaluating Critical Temperatures of Axially Loaded I-shaped Steel Members Using ANSI/AISC-360 Appendix 4 and Finite Element Model

Stability is paramount to the load-carrying capacity of structural steel members subjected to fire. Actual buckling strengths of steel members in fire become lower than that at ambient temperature since modulus of elasticity (E) and yield strength (Fy) significantly diminish with increasing temperatures. Appendix 4 of the ANSI/AISC-360 specification provides an equation for calculating the flexural buckling stress (Fcr) of columns at temperatures greater than 200 °C. However, finite element analysis showed that columns can fail by buckling at temperatures below 200 °C if the applied axial load is greater than 80 % of its ambient compressive strength. This paper presents: (i) critical temperatures estimated using both the Appendix 4 equations and finite-element analysis and (ii) parametric study results showing effects of applied load level, member slenderness, and steel grades on the critical buckling temperature. Closed form equations are developed and presented along with limitations for use in design practice.

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