Fire design of steel columns: Effects of thermal gradients

• Thermal gradient effects on the stability behavior of structural steel columns.
• Numerical and experimental methods are employed for the investigation.
• The assumption of uniform heating is conservative for most of the columns.
• Failure modes and mechanisms were observed for different thermal loading patterns.
• New design guidelines for columns with thermal gradient were proposed.

The behavior and design of steel columns subjected to thermal gradients due to fire loading were evaluated numerically and experimentally. The numerical (FEM) modeling approach was verified using experimental data from large-scale tests. The FEM modeling approach was used to conduct parametric studies to evaluate the effects of different heating configurations on steel column strength, and failure behavior at elevated temperatures. The analyses were conducted by coupling transient heat transfer analysis with implicit dynamic stress analysis. Columns subjected to four sided heating configuration had uniform temperature distributions through the cross-section. The columns were subjected to non-uniform (partial) heating to produce thermal gradients through the cross-section. The analysis results indicated that the column strength and failure behavior depended on the column slenderness, axial loading, and heating configuration. Failure modes included flexural buckling about the weak axis, flexural buckling about the strong axis, and flexural-torsional buckling. The analysis results also indicated that columns subjected to uniform heating had significantly higher heat influx. In most cases, columns subjected to non-uniform heating failed at lower average temperatures than columns subjected to uniform heating. However, the columns subjected to uniform heating reached their failure temperatures faster than the columns subjected to non-uniform heating due to the higher heat influx. The exceptions were very slender columns subjected to axial loads greater than 50% of their ambient load capacity. The results from the parametric studies were used to develop design equations for wide flange steel columns subjected to non-uniform heating resulting in thermal gradients through the cross-section.