Comparison of Existing Time-Equivalence Methods and the Minimum Load Capacity Method

Abstract

A fire resistance rating (FRR) is the minimum required ability of a building element to resist a fire. It is quantified as the time for which the element survives the exposure to the standard fire. One way of determining FRR is the time-equivalence (TE) approach which relates the destructive potential of a post-flashover fire to an equivalent duration under standard fire exposure. Many existing TE approaches use empirical correlations which account for fuel load, ventilation conditions, compartment size, lining materials and structural materials. Whilst they ease the determination of FRR, many parameters also affecting the structural failure are not explicitly considered such as load ratio, member size and reinforcement size in reinforced concrete (RC) members. A change in any of these will alter the survival duration of a member, however it is not reflected in the existing empirical correlations. Increased understanding of fire behavior and structural response has made it possible to better analyze the behavior of structures at elevated temperatures and determine the minimum fire resistance using fundamental approaches. As part of a new research to redevelop TE methods in New Zealand, this paper presents an analytical comparison of selected TE methods and the minimum load capacity method. A close inspection of the sensitivities of varying ventilation, glazing fallout, load ratio, member size and reinforcement size in RC members to the prediction of FRR has been undertaken. The paper identifies limitations in the current New Zealand verification method and provides recommendations for improvement of the TE approach. Keywords Fire resistance rating • FRR • time-equivalence • C/VM2 • minimum load capacity