Study On Seismic Performance Of Reinforced Concrete Short Columns After Fire

The high temperature of fire can obviously damage the shear performance ofreinforced concrete column, or even lead to shear failure. The accurate calculation ofthe residual shear resistance after high temperature is of great significance to thedamage evaluation and strengthening of reinforced concrete columns. However, thereare no research reports in terms of the shear performance of the reinforced concretecolumns after fire at present. This paper takes the reinforced concrete short columnsas its object, and makes research on the shear and aseismatic performance of thereinforced concrete columns after fire from the aspects of test, damage analysis andpractical calculation. The main work and conclusions are as follows:1. Taking quasi-static tests for seven fire-heated reinforced concrete shortcolumns and one without fire-heated to analyze the effect of the axial load ratio, spanratio and fire time on the shear performance and hysteresis energy dissipationcapacity of the reinforced concrete after fire. The test results show that:(1) Fire timehas obvious effects on those factors of a short column which are reduced from high tolow in degree for shear resistance, stiffness and energy dissipation capacity after fire.The peak load, yield stiffness and hysteresis energy of a column at the axial load ratioof0.2decrease by19.47%,16.46%and1.78%respectively after being fire-heated60minutes,27.02%,24.05%and12.15%respectively after90minutes, and12.15%,37.50%and13.40%respectively after120minutes.(2) The axial compression ratiocan greatly affect the shear performance of the reinforced concrete short columnsafter fire. The carrying capacity and stiffness first increase with the axial compressionratio, and then decrease slightly when the axial compression ratio is large. As theaxial compression ratio increase from0.1to0.2, the peak load and the correspondingstiffness increase by11.26%and7.16%respectively, then increase to0.3, they dropby3.06%and8.97%respectively.(3) Shear span ratio has obvious effects on thosefactors of a short column after fire, reduce from high to low in degree for the stiffness,energy dissipation capacity and shear bearing capacity. The same fire time, comparedwith specimen of=1.577, the peak load and the corresponding stiffness of specimens of=1.777and=1.977were decreased by5.17%and11.79%,11.54%and30.92%respectively, and hysteresis energy were increased by8.20%and23.47%respectively.2. The influence of the steel ratio of longitudinal reinforcement and shaped steelon the temperature field distribution is investigated by using SAFIR analysis software.The results of numerical examples show that:(1) The influence of reinforcement ratioon the temperature field distribution of concrete section is little and is reasonable tobe ignored. The temperature of cover concrete will be decreased by reinforcementwhile that of core concrete will be increased.(2) The shaped steel ratio has greatinfluence on the temperature field distribution of concrete section. When the fireexposure time is112min, the maximum temperate differences at the center of flangeand the centroid of cross section are17.1℃and98.7℃, respectively. It is necessaryto consider the influence of shaped steel on the temperature field distribution in thefire resistance analysis of steel reinforced concrete (SRC) members under elevatedtemperature.(3) The mechamical behaviour of core shaped steel can mostly berecovered in the residual strength evaluation of SRC members.3. After calculating and analyzing the quasi static test results of reinforcedconcrete short columns after fire, it finds that:(1) As the displacement angle increases,the energy dissipation coefficient of test pieces after fire firstly witness an obviousreduce to the minimum value (displacement angle is approximately1/75), and thenshow an increasing tendency after reaching the first peak (displacement angle isapproximately1/500). However, test pieces without fire-heating is just decreaseslightly after reaching its peak and exhibit a longer stable stage of change. This, to acertain extent, shows that the energy dissipation ability of test pieces withoutfire-heating is more stable than test pieces after fire.(2) When the displacement angleis large, the energy dissipation coefficients of fired and unfired test pieces are bothincreased significantly, especially the unfired ones. There are more remarkablegrowth rate and trend for them. This suggests that, test pieces have a greater plasticitydevelopment and a strengthened energy dissipation capacity in a relatively shortperiod of time prior to their failure.4. According to the shortcomings of the applicability of the classic Park injury model to the test, puts forward the improved Park injury model, coordinates themaximum deformation relationship of the members under the conditions ofmonotonic load and cyclic load, and defines the injury evaluation criterion bycombining the component failure state. And the results show that the ultimatedisplacement of RC short columns under low-reversed cyclic loading after fire is0.924times as much of those under monotonic loading at room temperature.5. After calculating the average reduction coefficient of the concrete strengthsubjected to different fire time of square columns at cross sections of300×300afterfire, recommendations for the practical shear bearing capacity calculation method ofconcrete columns at such sectional dimension were put forward, which can providereference for the safety identification of reinforced concrete columns after fire.