Thermal-induced restraint forces in reinforced concrete columns subjected to eccentric loads

Analytical and numerical analyses are conducted in this paper to investigate the additional axial forces induced in eccentrically-loaded columns that are restrained from thermal elongation in concrete framed structures when a fire occurs. A simplified analytical model to directly determine these so-called thermal-induced restraint forces is proposed based on the concepts of equivalent distributed temperature as well as eccentricity- and temperature-dependent reduction factor of axial stiffness. The model is validated by fire tests conducted at Nanyang Technological University on twelve restrained concrete column specimens subjected to uniaxial and biaxial bending. Relatively good agreement between the analytical and the experimental results of restraint force development is obtained. Hence, the proposed model can be used to explain the effects of axial restraint ratio, eccentricity, initial load level, concrete strength, as well as uniaxial and biaxial bending on the development of restraint forces. It is also shown that the analyses using material models which implicitly consider concrete transient strain overpredict the restraint forces induced in restrained RC columns under fire conditions.