Fire resistance and burnout resistance of reinforced concrete columns

Abstract

This paper quantifies the resistance to complete burnout of reinforced concrete columns under fire exposure. It adopts a new metrics to complement the fire resistance rating that captures the specific effects of the cooling phase and characterizes explicitly the ability of a structural member to survive burnout. The analysis is performed on a dataset of 74 standard fire resistance tests on columns. Finite element modeling is applied first to reproduce the standard tests. The obtained ratio between the computed and experimental fire resistance has an average value of 0.95 with a standard deviation of 0.29. Then, finite element modeling is used to analyze the 74 columns under ‘standardized’ natural fire comprising a cooling phase. The effects of cooling are carefully incorporated in the materials laws including thorough consideration of irreversibility of properties and explicit modeling of transient creep. An iterative computational procedure is applied to subject each column to increasing durations of fire exposure until finding the shortest standardized fire that cannot be survived until burnout, from which the burnout resistance metrics is defined. The duration of this shortest fire increases approximately linearly with the fire resistance of the columns, but with a factor smaller than one, revealing an increased propensity to delayed failure in columns with longer fire resistance. Finally, a simple equation is formulated to estimate the burnout resistance from the fire resistance. Such equation can support design for complete burnout which has benefits for safety of fire brigades and first responders, as well as for property protection and resilience of the built environment.