Structural behaviour and fire-resistant analysis of reinforced concrete columns at elevated temperatures

Abstract

As a constituent of last line of defence against progressive collapse, column forms one of the most important load-bearing elements in building structures. When a fire incident occurs in a concrete framed building, a heated column can be simultaneously subjected to eccentric loads and axial restraint resulted from nonuniform thermal responses of adjacent structural elements. This thesis focuses on the analyses of structural behaviour and fire-resistant design of axially-restrained reinforced concrete (RC) columns under either uniaxial or biaxial bending. The main research objective is to fulfill the gaps in previous research works and in current codes of practice for structural fire design of RC columns subjected to such a critical heating condition, as shown in a comprehensive literature review conducted. Experimental, numerical, and analytical approaches are employed in this study. A test programme is conducted on fifteen full-scale column specimens using a test set-up designed and fabricated for modelling of the simultaneous effects of axial restraint and eccentric loads on heated columns. Axial restraint levels within a practical range are incorporated into the fire tests. Six and nine RC columns are loaded under uniaxial bending and biaxial bending, respectively. New experimental data of structural responses including temperature-dependent axial deformations and mid-height lateral deflections, thermal-induced restraint forces, failure modes and failure times of RC columns are discussed and compared to predictions from numerical analyses using computer program SAFIR. A simplified analytical model is proposed to determine thermal-induced restraint forces based on the concepts of equivalent distributed temperature and a reduction ratio of axial stiffness, which depends on both load eccentricity and fire exposure.