Practical method for calculating the buckling temperature of the web-post in a cellular steel beam in fire

• Buckling behaviors of the web-post in a cellular steel beam were investigated through using a thermal-mechanical coupled finite element model.
• Three models were used to calculate the compression stresses in the web-post under given load in fire.
• Effective strut width was defined as width of compression stress band.
• A simplified method to calculate effective width of the strut was proposed.

Buckling behaviors of web-posts in a cellular steel beam at elevated temperatures in a fire were studied using the Finite Element Method (FEM) analysis and available analytical models. The buckling temperatures obtained by the analytical models differed greatly to those obtained from the FEM simulation. Among these analytical models, the buckling temperature obtained through the strut model based on BS5950-1 agreed with the FEM result the best. It is more reasonable to take the width of the compression stress band in the web-post as the effective width of the strut. Numerical parametric studies showed that the width of the compression stress band varied with the opening diameter, the opening distance and the web thickness. A simplified method was proposed to calculate the effective width of the strut. The accuracy of the strut model integrating the new effective width was validated against the FEM simulations. The obtained buckling temperature of the web-post using the modified strut model agreed well with the FEM simulation result.