Web-post buckling of fully and partially protected cellular steel beams at elevated temperatures in a fire

• Web-post buckling of protected cellular steel beams in a fire was investigated.
• Temperature gradient in web-post affects behaviors of partially protected beam.
• Principle compression stress in web-post decreased suddenly when web-post buckled.
• Stress at web-post center could be in tension due to large out-of-plan deformation.

Web-post buckling behaviors of fully and partially protected cellular steel beams at elevated temperatures in a fire were investigated through verified finite element models. The partially protected cellular steel beam represented a protected cellular steel beam with the hole edge left unprotected. The temperature distribution in the web-post was non-uniform, even for the unprotected CSB and the fully protected CSB. The fire resistance time of a CSB increased linearly with the increase in fire coating thickness. However, for the partially protected cellular steel beam, the temperature gradient in the web-post becomes greater with the increase in the fire protection thickness. And with the increase in the fire protection thickness, the increment in the fire resistance time decreased. Additional thermal stress occurred due to the non-uniform thermal strain in the web-post. When web-post buckled, the principle compression stress in the web-post decreased suddenly. The stress at the center of the web-post could change from compression to tension due to the membrane action caused by the large buckling deformation.