Post-Earthquake Fire Resistance of Ductile Concrete Filled Double-Skin Tube Columns
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Fire following an earthquake has been a major cause of damage in a number of historic seismic events. Considering the amount of damage and statistics showing a high probability for the occurrence of post-earthquake ignitions, there is a need to study the effects of seismic damage on the fire resistance of structures. Concrete Filled Double-Skin Tube (CFDST) columns, a special composite structure which has been shown to have satisfactory performance under separate seismic loading and fire conditions, were studied in this research when subjected to post-earthquake fire scenarios. Experimental studies were conducted to examine the behavior of concrete filled double-skin tube (CFDST) columns exposed to fire after being subjected to simulated seismic loads. The experiments were conducted in two separate phases, consisting of the quasi-static cyclic tests followed by fire tests. Three nominally identical column specimens were constructed for these studies. One of the specimens was directly tested under fire to quantify its resistance in an undamaged condition. The other two specimens were first subjected to quasi-static cyclic lateral loads, imposing varying degrees of lateral drift to simulate two different seismic events with moderate and high damage levels before being exposed to fire. Both of the specimens were pushed to the maximum drift of 6-6.5% with different residual drifts of 1.4% and 3.9% for moderate and high damage levels, respectively. The undamaged and damaged columns were then subjected to the same fire tests following the standard ASTM E119 (ASTM 2012) temperature-time curve while sustaining an axial load until the column failed due to global buckling. Local buckling of the tubes was also observed in the specimens due to the thermal expansion and separation from the concrete. Overall, the results showed marginal differences in the fire resistance of the three specimens, providing evidence for the resilient performance of these columns under post-earthquake fire scenarios. An additional quasi-static cyclic loading test was conducted on the specimen that had been exposed to fire without any prior damage, to investigate the behavior of the column subjected to seismic loads after the fire test. Again, differences in behavior were modest, except for a 5.7% drop in strength attributed to permanent degradation in material properties due to the fire test. In addition to the experimental studies, detailed finite element analyses were conducted using ABAQUS and LS-DYNA to simulate the behavior of CFDST columns subjected to post-earthquake fires. The models were shown to be capable of replicating the experimental results with sufficient accuracy. A simplified step by step analytical procedure was proposed for calculation of the axial load capacity of CFDST columns subjected to fire. The procedure was defined based on an analytical solution to the heat transfer problem and calculation of axial load capacity using the fire-modified material properties. A number of design recommendations, based on the knowledge gained from the experimental and analytical studies, were proposed for CFDST columns subjected to fire.