Effect of thermal restraint on reinforced concrete columns

Internal stresses can occur in building components due to their external constraint, which hinders the elongation. If a component is prevented from expanding freely in a fire, additional "external constraint" occurs. This type of constraint is caused, for example, by non-displaceable or only partially yielding structural components that are adjacent to the fire-stressed component and remain cool. The resulting restraint stresses are usually compressive, they can lead to a higher load utilization by superimposing with load induced stresses from mechanical action. A positive effect can occur when these restraint stresses counteract the stress caused by mechanical action.

Due to preventive structural and system-related fire protection measures, fires are often localised or spatially limited. The components located directly in the fire area are therefore primarily affected by the fire load and the associated heating. Structural components located outside the fire area or separated from it by components such as slabs and walls generally experience no or only very low temperature stress. This circumstance can lead to the thermally induced expansion of the structural component exposed to fire being impeded by the neighbouring structural components, which form a non-displaceable or only partially yielding boundary. As a result of this restraint to expansion, constraint arises which causes additional stresses both in the fire-stressed component and in the neighbouring or surrounding structural components. This can result in a change in internal forces for the load-bearing structure, which influences the load- bearing and deformation behaviour. In general, constraining forces are increased by a rigid or non-displaceable supporting structure, a lack of expansion possibilities between the components, and the use of monolithic, fixed or non- displaceable component connections. These factors are typically found in reinforced concrete structures. The restrained longitudinal expansion of reinforced concrete slabs leads to overpressing of the concrete tension zone, which can have a positive effect on the load-bearing capacity and deflection of the component [1, 2]. In the case of reinforced concrete columns, the concrete compressive strength or the maximum column buckling load might be exceeded due to the additional force induced by the external constraint. This might lead to the column failing earlier than it would with conventional testing methods. Martins et al., considered the constraining forces from restrained thermal expansion in fire tests on reinforced concrete columns and concluded that increasing restraint from the surrounding structure slightly reduces the fire resistance of the column [3,4]. Therefore, it is necessary to study the restraint caused by the surrounding structural elements on the reinforced concrete column during the fire.

This paper addresses the numerical simulation of a reinforced concrete column exposed to fire, where thermal expansion induced by fire-exposure is constrained by surrounding structural components such as beams and slabs which remain cool. The main objective is to study how the vertical expansion of the column and the internal forces develop during a fire with different surrounding structural elements and to calculate the restraint as spring stiffness, which can be used in hybrid fire testing methods, such as substructuring technique to simulate external constraints [5,13].