Progressive collapse mechanisms investigation of planar steel moment frames under localized fire

•The progressive collapse mechanisms of steel frames under localized fire are studied.•Explicit dynamic method is used for progressive collapse analysis under fire.•Mainly three failure mechanisms exist for planar moment steel frames when one column heated.•Axial force increase of adjacent columns may be even larger than the load carried by the heated column at room temperature.•The most adverse fire scenarios of a planer steel moment frame are identified.

In this paper, the possible progressive collapse mechanisms of planar steel frames when one column failed under elevated temperature was studied through extensive case studies. An explicit dynamic solver was adopted, which could continue beyond local element buckling. The effects of analysis parameters such as mesh size and loading speed were investigated. And the numerical model was validated against experimental data and analysis results of other researchers. The investigated parameters included beam cross-sectional size, load ratio and location of heated column. Three progressive collapse mechanisms were found, namely, cantilever beam mechanism, pull-in force induced mechanism and high load ratio member failure mechanism, of which the last one is a new discovery. To evaluate progressive collapse of planar steel frames under fire, the cantilever beam mechanism and the pull-in force induced mechanism should be checked when the outer columns are heated, and the pull-in force induced mechanism and the high load ratio member failure mechanism need to be checked when the inner columns are heated. And the most adverse fire scenarios of a planer steel moment frame are when one of the following columns is heated: first floor column of the outmost, second and third outmost column lines or top floor column of the outmost, second and third outmost column lines.