Mechanical Property Research On Slab-column Connections Under The Post-earthquake Fire

The secondary fire after the earthquake is the secondary disaster with the highest frequency of occurrence.Once it occurs,it will cause the deterioration of structural mechanical properties,increase the damage degree of the structure itself and even cause the structure to collapse.However,only a few people have studied the performance of building structures under the action of post-earthquake fires.Therefore,through the research in this area,the design reference basis for structural earthquake resistance and fire resistance is provided.This pair of building structures has a dual theoretical and practical significance in the emergency after the earthquake caused a secondary fire.Based on the analysis of the damage form of the seismic behavior of reinforced concrete slab-column structure and the failure mechanism of slab-column joints affected by unbalanced bending moments,this paper summarizes the damage model with crack as the main damage form.And representing the expansion of cracks by controlling the relative crack depth.Based on heating curve ASTME-119 and thermal-stress sequence coupling method,the fire resistance performance of the interior joints for R.C.slab-column structure was analyzed by the finite element analysis software ABAQUS.The influence of crack on the temperature field distribution,ultimate bearing capacity and fire endurance of the slab-column joints is obtained.The specific conclusions obtained by numerical simulation are as follows:(1)As the relative crack depth increases,the high temperature region at the crack gradually expands toward the pressure side of the slab,and the temperature of the plate near the pressure side column gradually increases.When the crack penetrates through the thickness direction,the crack is completely high temperature,and the temperature of the plate between adjacent cracks is about 150℃-200℃ higher than other crack depths.The crack depth has little effect on the temperature of the fire side protection layer of the plate,but the temperature of the steel bar at the crack is higher than that of the non-crack.(2)With the increase of the relative crack depth,the ultimate bearing capacity of the column joints in the reinforced concrete slab column structure under normal temperature is decreasing continuously,and the decreasing rate is increasing.When the relative crack depth is 1.0,that is,the crack penetrates completely,the ultimate bearing capacity of the plate-column joint is reduced by more than 80%,and it can be considered that the member has lost the bearing capacity at this time.(3)The ultimate bearing capacity of the slab-column joint is decreasing with the increase of temperature,and the ultimate bearing capacity change curve is gradual with the increase of the heating time,indicating that the decreasing rate is decreasing.When the temperature rise time exceeds 4h,regardless of the relative crack depth of the damage component,the ultimate bearing capacity at normal temperature is reduced by more than 50%.(4)As the relative crack depth increases,the ultimate bearing capacity of the plate column is declining.The quantitative relationship between the ultimate bearing capacity reduction coefficient and the relative crack depth and time is obtained,and the relationship between the ultimate bearing capacity reduction factor and the relative crack depth and heating time is fitted.(5)The fire resistance limits of the seismic damage plate joints with relative crack depths of 0,0.25,0.5,and 0.75 are 278 min,229 min,177 min,and 124 min,respectively.As the relative crack depth increases,the temperature of the steel at the crack increases,resulting in the deterioration of the mechanical properties of the steel and the rapid reduction of the fire resistance.