Numerical investigation of axial restraint imposed by surrounding structure on reinforced concrete columns exposed to fire

Purpose During building fires, the thermal expansion of reinforced concrete (RC) columns is restrained by adjacent cooler structural members, inducing restraint forces that can significantly influence structural response and fire performance. This study investigates the axial restraint imposed on RC columns by surrounding structural systems under fire conditions. Design/methodology/approach A three-dimensional thermo-mechanical finite element model developed in InfoGraph® was used for the study. A systematic parametric study was conducted, including isolated columns under different load utilization factors, beam–column joint substructures with varying beam geometries and fire exposure conditions and a four-to six-storey building frame subjected to localized and global fire scenarios. The resulting structural interaction was quantified in terms of restraint force and axial displacement, from which an equivalent axial spring stiffness was derived. Findings The results demonstrate that the axial restraint is strongly influenced by the stiffness and fire exposure of the surrounding structural members, as well as by the fire scenario and column location. Longer and more flexible surrounding beams provide lower restraint, whereas increased beam stiffness results in greater restraint. Localized fire scenarios generate substantially higher restraint than global fire exposure because the surrounding unheated structural members retain greater stiffness. The restraint response also varies with the number of storeys above the heated column. Originality/value This study provides insight into the development of axial restraint in RC columns within a realistic multistorey structural system under different fire scenarios. The equivalent spring stiffness provides a simplified representation of surrounding structural restraint for analysis and design of structures in fire.