Fire induced progressive collapse of steel building structures: The role of interior gravity columns

• Fire effects on the stability behavior of steel buildings were simulated.
• Components were modeled to incorporate temperature dependent nonlinear behavior.
• Gravity columns were the most critical components for fire safety of the structure.
• If a gravity column fails, steel reinforcement in slab helps redistribute the load.

This paper presents a qualitative assessment of the importance of gravity columns on the stability behavior of a typical mid-rise (10 story) steel building subjected to corner compartment fires. Two ten-story steel buildings with composite floor systems were designed following the design practices in the US. One of these buildings had perimeter moment resisting frames (MRFs) to resist lateral loads while the other building had an interior core of RC shear walls. Effects of gravity loads and fire conditions were simulated using the finite element method and numerical analysis techniques.The results from the numerical investigations indicated that gravity columns govern the overall stability of building structures under fire conditions. Gravity columns have the highest utilization ratio, and they are most likely to reach their critical temperatures first. If gravity column failure occurs, the load shed or dropped by the failed column has to be redistributed to the neighboring columns to maintain overall structural stability. This axial load redistribution can occur through the development of alternate load paths including catenary action. Simulation results indicate that the presence of steel reinforcement in the concrete slabs (in addition to the minimum shrinkage reinforcement) facilitates uniform redistribution of the axial load dropped by the failed gravity column to the neighboring columns. The additional steel reinforcement improves the flexural and tensile strengths of the composite floor system, which enhances its ability to develop alternate load paths including catenary action in the slab, and thus maintain structural stability after gravity column failure.