Preliminary Study On Fire Performances Of Seismic Damaged Square CFST Columns

Post-earthquake fire is a kind of common disaster and threaten structures significantly.Nowadays square concrete-filled steel tube(CFST)is widely applied in engineering,but quantity of research on its post-earthquake fire performance is few.Experimental studies include quasi-static test and fire test,with numerical and parametric analysis,were carried out to determine fire performance of seismic damaged square CFST and its thermal field:1.Quasi-static tests of five CFST double-column specimens with fire-protection coating(FPC)were carried out,revealing effect of maximum drift ratio and axial load ratio on behavior of fire-protection coating and residual deformations.In addition,additive deformation of specimens in transportation process was collected.It is found that:(1)with the increasing of maximum drift ratio and axial load ratio,cracking pattern of FPC at ends of columns is more severe.Facial spalling of and regional steel expose occur successively when maximum drift ratio is 0.33.Cracking and spalling pattern of FPC is more severe on actuating faces than non-actuating faces;(2)with the increasing of maximum drift ratio and load ratio,residual deformation increases by varying degree.For specimens with axial load ratio of 0.4,maximum drift ratio has limited influence on recovery of horizontal deformation(difference of maximum deformation and residual deformation);(3)It is considered that additive deformation during transportation process has no effect on fire performance of seismic-damaged CFST in subsequent fire test.2.Fire tests of five seismic damaged square CFST double-column specimens and one undamaged specimen were carried out.The thermal profiles at ends and at mid-section of columns were determined,and the effects of maximum drift ratio and axial load ratio on the fire performances of seismic damaged columns were analyzed.Test results reveal that:(1)setting steel wire mesh in fire-protection coating could prolong fire endurance of seismic damaged columns effectively;(2)for specimens with axial load ratio of 0.4,maximum drift ratio has limited influence on fire endurance of damaged columns when varying from 0 to 1/50;but when maximum drift ratio reach 1/33,fire endurance of seismic damaged column decrease by 50%;(3)for specimens with maximum drift ratio under 1/50,axial load ratio has more significant influence on fire endurance of columns than maximum drift ratio;(4)horizontal additive deformations of seismic damaged columns follow the direction of residual deformations caused by quasi-static test,and with the increase of axial load ratio,this additive deformation is generally greater.3.The thermal fields of seismic damaged columns in fire were simulated in ABAQUS.A parametric study including 49 columns with different degree of seismic damaged conditions was carried out,to determine the effect of depth of spalling,height of spalling,position of spalling and time of fire exposure on thermal fields of seismic damaged columns.The modeling calculation reveals that:(1)after 150 min heat exposure,at faces where FPC spalling occurs,the edge of spalling influencing region of steel was vertically 300～350 mm away from the edge of spalling,and the edge of spalling influencing region of concrete at depth of 50 mm was vertically 200 mm away from the edge of spalling;at faces where FPC spalling doesn’t occur,influencing region of steel was vertically 250 mm away from the edge of spalling,and concrete at 60 mm depth is almost not influenced;(2)remaining fire-protection coating can still provide good protection.