2017 Volume 82 Issue 731 Pages 145-154
To estimate collapse temperature of a steel fame subjected to fire, collapse mechanisms of heated members in the fire compartment room must be clarified. It is well known that there is a possibility that column buckling at elevated temperature triggers overall frame collapse, which must be avoided at fire resistant design. In accordance with the past studies using fire response analyses for the overall frames, an axial load that the buckled column in the fire compartment room has sustained can be redistributed to the peripheral members at the ambient temperature, through shear force of the beams arraigned at upper stories than the fire compartment room. The load redistribution capacity for the overall frame at the fire is defined as 'the fire redundancy', and the redundant frame can avoid the overall frame collapse even if the heated column loses resistant strength. On the other hand, developments of the collapse mechanisms are influence on variations of steel strength of the members, it is, for the reason, possible that unexpected overall frame collapse is triggered, because of changes of the member collapse mechanisms.
The main purpose of this study is to quantify the variations in the collapse temperatures and probability of occurrence on the collapse modes for the overall steel frames in case of considering the variations in the steel strength. In particular, relationships between the fire redundancy, which is given by a ratio of the shear strengths of the beams at the upper stories to the axial forces of the heated columns, and the probability of occurrence on the overall frame collapse are clarified, by numerical analyses using Monte Carlo (referred to as MC) method. In the MC analyses, the steel strength samples including characteristics of the past coupon test results at the elevated temperature are used. They has been proposed by the previous study (Ozaki et. al., 2015).
From minute investigations on the MC analytical results, the following 1)～3) were clarified.
1) The overall frame collapse occurs at the same time that the heated column at the fire compartment room buckles in case of the small redundant frames. By increasing the fire redundancy, the collapse mode changes to the heated beam collapse, which is the member collapse localized in the fire compartment room. The variation of the collapse temperature for the large redundant frame is equal to that for the beam.
2) The full redundant frame, which is the stable frame after the buckled columns lost their resistant strengths at the high temperature, is developed in the case of the large fire redundant frame. The probability of occurrence depends on the variation in the steel strength of the beams at the upper stories than the fire compartment room.
3) It is possible that the collapse modes of the heated members in the fire compartment room are changed by the degree of variation of steel strength at high temperature. Controlling those collapse modes at the fire resistant design is difficult, particularly, in the case when the collapse temperatures among the heated members are close. On the other hand, by increasing the fire redundancy, the probability of occurrence of the overall frame collapse becomes small, and the controlling collapse mode design to avoid the overall frame collapse becomes easy.
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